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We present various geological models to examine the geothermal energy potential at the Rahat volcanic field, Saudi Arabia. The method involves the application of geophysical techniques of gravity and seismic tomography results to derive a 3D subsurface geological structure representing possible subsurface structural information beneath the northern Rahat volcanic field. Results show a secluded subsurface structure within the basement crust with low densities (< 2500 kg / m 3 ) and a slightly high P-wave velocity, enclosed by a high density (> 2800 kg / m 3) and low P-wave velocity structure. Identified secluded subsurface structure could depict a magma intrusion within the basement and its depth proximity to the basalt deposition in the region signals the viability of this location for geothermal exploration. Deductions also show that the geothermal reservoir could be reached at starting depths of ~ 1.3 km, whereas its effect could be accessed at depths < 1 km. Within the geothermal active region of northern Harrat Rahat, this study has suggested possible locations with high potential for further geothermal exploration. [ABSTRACT FROM AUTHOR]

Seismic tomography has shown that the shear wave velocities (Vs) under continents, especially under cratons, are extremely fast at 100–200 km depth, which is difficult to explain by low temperatures or high Mg#. Alternatively, delaminated eclogitic lower continental crust has been proposed to account for these fast seismic anomalies. However, the thermoelastic properties of jadeite which constitutes up to 60–80 mol% of clinopyroxene in the potentially delaminated lower continental crust are not well constrained. In this study, we measured the single‐crystal elasticity of jadeite by Brillouin spectroscopy under simultaneous high pressure and temperature conditions for the first time. We found that the temperature dependence of Vs of jadeite is extremely small if not negligible. The seismic velocities of the potentially delaminated lower continental crusts were subsequently modeled and found to match the widely observed fast seismic anomalies under cratons between 100 and 200 km depth. Plain Language Summary: The seismic wave velocity variation images show the potential composition and temperature heterogeneities inside the Earth. Fast shear wave velocities (∼7% higher than the global average) have been observed under continents at 100–200 km depths. A candidate explanation of this fast shear wave velocity anomaly is the existence of delaminated eclogitic lower continental crust. However, due to the lack of knowledge of the thermoelastic properties of clinopyroxene, which is the dominant mineral phase (up to 60 vol%) in delaminated eclogitic lower continental crust, evaluation of this hypothesis is difficult. Clinopyroxene in the potentially delaminated lower continental crust is jadeite‐rich (up to 60–80 mol%) due to its high Na content (2.5–3.5 wt%). In this study, we report single‐crystal elasticity of jadeite at high pressure‐temperature conditions. We found the Vs of jadeite is much higher than all the other major upper mantle minerals under upper mantle conditions. The calculated seismic velocities of the potentially delaminated lower continental crusts could easily account for the fast shear wave anomalies observed under cratons. Key Points: High pressure‐temperature single‐crystal elasticity measurements of jadeite are conducted by Brillouin spectroscopyJadeite is among the seismically fastest phases in the Earth's upper mantleDelaminated lower crust can help explain the fast seismic anomalies under cratons [ABSTRACT FROM AUTHOR]

Machine learning (ML) has been used to study the predictability of laboratory earthquakes. However, the question remains whether or not this approach can be applied in a tectonic setting where one may have to rely on sparse earthquake catalogs, and where important timescales vary by orders of magnitude. Here, we apply ML to a synthetic seismicity catalog, generated by continuum models of a rate‐and‐state fault with frictional heterogeneities, which contains foreshocks, mainshocks, and aftershocks that nucleate in a similar manner. We develop a network representation of the seismicity catalog to calculate input features and find that the trained ML model can predict the time‐to‐mainshock with great accuracy, from the scale of decades to minutes. Our results offer clues as to why ML can predict laboratory earthquakes and how the developed approach could be applied to more complex problems where multiple timescales are at play. Plain Language Summary: Accurately forecasting the timing of an impending large earthquake is one of the main but elusive goals of seismology. Recent studies show that machine learning (ML), a powerful tool that can draw inferences from hidden patterns in data, can predict the timing of laboratory earthquakes that mimic faulting in nature. However, the question remains whether or not an ML approach can be applied to natural faults where one may have to rely on sparse earthquake catalogs, and where important timescales vary by orders of magnitude. In this study, we develop and apply an ML approach to a synthetic earthquake catalog generated by physics‐based computer simulations. We find that the trained model can predict the time remaining before the mainshock with great accuracy, from the scale of decades, long before the upcoming earthquake, down to the scale of hours to minutes, right before the mainshock. Our results provide clues as to why ML can predict laboratory earthquakes and how to apply an ML‐based approach to real earthquakes where multiple timescales are at play. Key Points: We developed a network representation of a synthetic earthquake catalog to train a machine‐learning Machine learning (ML) modelThe trained ML model can predict the time remaining before simulated mainshocks with great accuracyThe network representation of seismic moment and event interval enables us to accurately predict the timing of mainshocks [ABSTRACT FROM AUTHOR]

The increased velocity associated with decaying seismicity in the subducting oceanic crust usually has been attributed to its eclogitization. However, the degree of the eclogitization of subducting oceanic crust at depth remains unclear due to the lack of velocity data for eclogite at pressures of >3 GPa. In this study, the P- and S-wave velocities of eclogite aggregates composed of omphacite and garnet (Omp100, Omp80Grt20, and Omp- 50Grt50) were measured simultaneously at pressures of up to 8 GPa using ultrasonic interferometry. The velocities of the eclogite aggregates increased with rising pressure and garnet content. By determining the velocities at different pressures using the finite strain equation, the elastic moduli of the eclogites and their pressure derivatives were determined to be KS0 = 119-134 GPa, KS0‰ = 5.2-5.3, G0 = 73-80 GPa, and G0‰ = 1.5-1.6. The high-wave velocity and low VP/VS ratio of the eclogites, combined with the seismic tomography and seismicity distribution, jointly constrain the depth and composition of eclogitization in the subducted oceanic crust of Northeast Japan, and a new 1-D velocity structure is proposed. We also compiled and calculated the depth of the eclogitization and the garnet content of the subducting oceanic crusts in 27 typical locations around the Pacific Ocean. The depth of the eclogitization was positively correlated with the age of the subduction zone, and the garnet content was estimated to be 12%-32% in our model. [ABSTRACT FROM AUTHOR]

Within seismology, geology, civil and structural engineering, deep learning (DL), especially via generative adversarial networks (GANs), represents an innovative, engaging, and advantageous way to generate reliable synthetic data that represent actual samples' characteristics, providing a handy data augmentation tool. Indeed, in many practical applications, obtaining a significant number of high-quality information is demanding. Data augmentation is generally based on artificial intelligence (AI) and machine learning data-driven models. The DL GAN-based data augmentation approach for generating synthetic seismic signals revolutionized the current data augmentation paradigm. This study delivers a critical state-of-art review, explaining recent research into AI-based GAN synthetic generation of ground motion signals or seismic events, and also with a comprehensive insight into seismic-related geophysical studies. This study may be relevant, especially for the earth and planetary science, geology and seismology, oil and gas exploration, and on the other hand for assessing the seismic response of buildings and infrastructures, seismic detection tasks, and general structural and civil engineering applications. Furthermore, highlighting the strengths and limitations of the current studies on adversarial learning applied to seismology may help to guide research efforts in the next future toward the most promising directions. [ABSTRACT FROM AUTHOR]

West Texas has been a seismically active region in the past decade due to the injection of industrial wastewater and hydrocarbon exploitation. The newly founded Texas seismological network has provided a catalog that characterizes the intense seismicity down to a magnitude of 1.5 Ml. However, there are numerous small-magnitude events (Ml < 1.0) occurring every day that are not analyzed and reported, due to the prohibitively high workload to manually verify the picks from automatic picking methods. We propose to apply an advanced deep learning method, the earthquake compact convolutional transformer (EQCCT), to unleash our power in analyzing hundreds of small earthquakes per day in West Texas. The EQCCT method is embedded in an integrated-detection-and-location framework to output a highly complete earthquake catalog, given a list of available seismic stations, in a seamless way. The EQCCT has enabled us to detect and locate 50-times more earthquakes (mostly smaller than magnitude 1) than we previously could. We applied the EQCCT-embedded detection and location workflow to the Culberson and Mentone earthquake zone (CMEZ) in West Texas and detected thousands of earthquakes per month for consecutively three months. Further relocation of the new catalog revealed an unprecedentedly high-resolution and precise depiction of shallow and deep basement-rooted faults. The highly complete catalog also offers significant insights into the seismo-tectonic status of the CMEZ. Association with nearby injection activities also revealed a strong correlation between the rate of injected fluid volume and the number of small earthquakes. [ABSTRACT FROM AUTHOR]

The investigation of upper mantle structure beneath the US has revealed a growing diversity of discontinuities within, across, and underneath the sub‐continental lithosphere. As the complexity and variability of these detected discontinuities increase—for example, velocity increase/decrease, number of layers and depth—it is hard to judge which constraints are robust and which explanatory models generalize to the largest set of constraints. Much work has been done to image discontinuities of interest using S‐waves that convert to P‐waves (or top‐side reflected SS waves). A higher resolution method using P‐to‐S scattered waves is preferred but often obscured by multiply reflected waves trapped in a shallower layer, limiting the visibility of deeper boundaries. Here, we address the interference problem and re‐evaluate upper mantle stratification using filtered P‐to‐S receiver functions (Ps‐RFs) interpreted using unsupervised machine‐learning. Robust insight into upper mantle layering is facilitated with CRISP‐RF: Clean Receiver‐Function Imaging using Sparse Radon Filters. Subsequent sequencing and clustering organizes the polarity‐filtered Ps‐RFs into distinct depth‐based clusters. We find three types of upper mantle stratification beneath the old and stable continental US: (a) intra‐lithosphere discontinuities (paired or single boundary), (b) transitional discontinuities (single boundary or with a top layer), and (c) sub‐lithosphere discontinuities. Our findings contribute a more nuanced understanding of mantle discontinuities, offering new perspectives on the nature of upper mantle layering beneath continents. Plain Language Summary: Early investigations of the mantle rocks in the US indicate intricate layering. However, uncertainties remain regarding the origins of these structures. Here, we re‐examine mantle rock stratification using a fine‐resolution approach. We use short waves that improve our ability to identify the depth of thin layers and sharp transitions in rock properties. Until now, these methods haven't been used due to interference with waves trapped in the near‐surface layers. We address this problem with machine learning and the CRISP‐RF (Clean Receiver Function Images Using Sparse Radon‐Filters) method. CRISP‐RF filters out the waves trapped in the crust and machine learning reveals spatially coherent patterns. Underneath the stable continents, we find evidence for different types of rock layering: (a) reflectors within cold stiff rocks (b) reflectors at depth ranges where the rocks become warmer and flow more readily, and (c) reflectors at depths farther down in the upper mantle. Our approach enables the test of hypotheses about the origins of upper mantle layering beneath continents. Key Points: Upper mantle stratification is constrained using Clean Receiver‐function Imaging using Sparse Radon Filters and machine learningStratification is classified into intra‐lithospheric, transitional and sub‐lithosphericHigh‐resolution constraints allow the evaluation of different causal models [ABSTRACT FROM AUTHOR]

[Volcanic eruptions generate continuous or episodic tremor, which can provide unrivaled information about activity changes during eruption. However, the wealth of information in episodic tremor patterns is often not harvested and transitions between patterns remain obscure. The 2021 Geldingadalir eruption, Iceland, is an exceptional case, where the lava effusion caused continuous tremor, and 8698 tremor episodes spanning two orders of magnitude in duration and repose. Based on seismometer and video camera data, we associate several‐minute‐long, symmetrical episodes with an open vent system, where lava remains in the crater bowl during repose, connected to a shallow magma compartment. Ramp‐shaped episodes, lasting several hours, are associated with a temporary closure of the vent system, where no lava remains in the crater bowl during repose and more time is required to resume effusion. The transition from continuous to episodic effusion is related to the cumulative time spent in effusion and repose, and to external factors like crater wall collapses. The range of observed eruption styles and shifts between them, took place at unchanged magma supply rate. This underpins the importance of processes, geometry and evolution of the shallow conduit with time.] Plain Language Summary: Volcanic eruptions can provide unrivaled information about the subsurface structure and processes driving it. Often effusion happens continuous over a time span. However, sometimes the effusion starts and stops and is hence episodic. During the 2021 Geldingadalir eruption, Iceland the lava flowed continuously from the vent but later stopped 8698 times. Some of these episodes where a few minutes long, while others lasted several hours. We interpret several‐minute‐long episodes as an open system, where lava remains in the crater during repose. This system can restart fast and easy. During several hour‐long episodes, no lava remains in the crater during repose. The system needs more time to reopen the vent. The system also reacts depending on the time it is in repose or effusion. And finally, external factors like crater wall collapses modify the episodic pattern. The most interesting feature is the range of eruption styles and transitions between them, that occurred at constant magma inflow rate and are hence caused by the shallow conduit. Key Points: The 2021 Geldingadalir eruption, Iceland, featured 8698 tremor episodes of minute to week durationAn open vent system with lava residing in the crater during repose featured minute‐long lava effusion with bell‐ or rectangle‐shaped tremorA semi‐closed vent system with no lava residing in the crater featured hour‐long lava effusion with ramp‐shaped tremor [ABSTRACT FROM AUTHOR]

Seismic source locations are fundamental to many fields of Earth and planetary sciences, such as seismology, volcanology and tectonics. However, seismic source detection and location are challenging when events cluster closely in space and time with signals tangling together at observing stations, such as they often do in major aftershock sequences. Though emerging algorithms and artificial intelligence (AI) models have made processing high volumes of seismic data easier, their performance is still limited, especially for complex aftershock sequences. In this study, we propose a novel approach that utilizes three‐dimensional image segmentation—a computer vision technique—to detect and locate seismic sources, and develop this into a complete workflow, Source Untangler Guided by Artificial intelligence image Recognition (SUGAR). In our synthetic and real data tests, SUGAR can handle complex, energetic earthquake sequences in near real time better than skillful analysts and other AI and non‐AI based algorithms. We apply SUGAR to the 2016 Kaikōura, New Zealand sequence and obtain five times more events than the analyst‐based GeoNet catalog. The improved aftershock distribution illuminates a continuous fault system with extensive fracture zones beneath the segmented, discontinuous surface ruptures. Our method has broader applicability to non‐earthquake sources and other time series image data sets. Plain Language Summary: Detecting and locating earthquakes is fundamental to seismology, volcanology, and tectonics. A number of emerging algorithms, including some based upon artificial intelligence (AI), have made processing large volumes of seismic data much easier. However, their performance is still limited, especially in clustered aftershock sequences whose signals overlap at observing seismographs. We propose a new, AI computer vision‐based approach to this problem, and develop it into a complete earthquake detection and location workflow, named SUGAR. Tests on synthetic and real earthquake data sets show that SUGAR characterizes complex earthquake sequences better than other AI and non‐AI algorithms or professional analysts. We apply SUGAR to the complex aftershock sequence of the 2016 Mw 7.8 Kaikōura, New Zealand earthquake, detecting five times more events than the analyst‐based GeoNet catalog. Whereas surface breaks of the Kaikōura earthquake are highly discontinuous, our improved aftershock distribution supports a continuous fault system surrounded by extensive fracture zones at depth. Our method has broader potential for other types of seismic sources and image series. Key Points: We propose a new seismic source detection and location approach based on the source‐scanning algorithm and 3D image segmentationThis approach outperforms human analysts and popular artificial intelligence (AI) and non‐AI based methods in characterizing intense aftershock sequencesThe resulting catalog of the 2016 Kaikōura earthquake sequence suggests a continuous fault system surrounded by extensive fracturing [ABSTRACT FROM AUTHOR]

Context. Kink oscillations are ubiquitously observed in solar coronal loops, and understanding them is crucial in the contexts of coronal seismology and atmospheric heating. Aims. We studied kink modes supported by a straight coronal loop embedded in an asymmetric environment using 3D magnetohydrodynamic simulations. Methods. We implemented the asymmetric effect by setting different exterior densities below and above the loop interior and initiated the simulation using a kink-like velocity perturbation perpendicular to the loop plane, mimicking the frequently measured horizontally polarized kink modes. Results. We find that the external velocity fields show fan-blade structures propagating in the azimuthal direction as a result of the successive excitation of higher azimuthal Fourier modes. Resonant absorption and phase-mixing can still occur despite an asymmetric environment, leading to the development of small-scale structures at loop boundaries. These small-scale structures nonetheless develop asymmetrically at the upper and lower boundaries due to the different gradients of the Alfvén speed. Conclusions. These findings enrich our understanding of kink modes in coronal loops embedded within an asymmetric environment, providing insights that will be helpful for future high-resolution observations. [ABSTRACT FROM AUTHOR]

High-quality datasets are critical for the development of advanced machine-learning algorithms in seismology. Here, we present an earthquake dataset based on the ChinArray Phase I records (X1). ChinArray Phase I was deployed in the southern north-south seismic zone (20° N-32° N, 95° E-110° E) in 2011-2013 using 355 portable broadband seismic stations. CREDIT-X1local, the first release of the ChinArray Reference Earthquake Dataset for Innovative Techniques (CREDIT), includes comprehensive information for the 105,455 local events that occurred in the southern north-south seismic zone during array observation, incorporating them into a single HDF5 file. Original 100-Hz sampled three-component waveforms are organized by event for stations within epicenter distances of 1,000 km, and records of = 200 s are included for each waveform. Two types of phase labels are provided. The first includes manually picked labels for 5,999 events with magnitudes = 2.0, providing 66,507 Pg, 42,310 Sg, 12,823 Pn, and 546 Sn phases. The second contains automatically labeled phases for 105,442 events with magnitudes of -1.6 to 7.6. These phases were picked using a recurrent neural network phase picker and screened using the corresponding travel time curves, resulting in 1,179,808 Pg, 884,281 Sg, 176,089 Pn, and 22,986 Sn phases. Additionally, first-motion polarities are included for 31,273 Pg phases. The event and station locations are provided, so that deep learning networks for both conventional phase picking and phase association can be trained and validated. The CREDIT-X1local dataset is the first million-scale dataset constructed from a dense seismic array, which is designed to support various multi-station deep-learning methods, high-precision focal mechanism inversion, and seismic tomography studies. Additionally, owing to the high seismicity in the southern north-south seismic zone in China, this dataset has great potential for future scientific discoveries. [ABSTRACT FROM AUTHOR]

Large earthquakes frequently occur along complex fault systems. Understanding seismic rupture and long-term fault evolution requires constraining the geometric and material properties of fault zone structures. We provide a comprehensive overview of recent advancements in seismological methods used to study fault zone structures, including seismic tomography, fault zone seismic wave analysis, and seismicity analysis. Observational conditions limit our current ability to fully characterize fault zones, for example, insufficient imaging resolution to discern small-scale anomalies, incomplete capture of crucial fault zone seismic waves, and limited precision in event location accuracy. Dense seismic arrays can overcome these limitations and enable more detailed investigations of fault zone structures. Moreover, we present new insights into the structure of the Anninghe-Xiaojiang fault zone in the southeastern margin of the Qinghai-Xizang Plateau based on data collected from a dense seismic array. We found that utilizing a dense seismic array can identify small-scale features within fault zones, aiding in the interpretation of fault zone geometry and material properties. [ABSTRACT FROM AUTHOR]

The technique referred as ray approximation treats wave propagation in a heterogeneous medium at the infinitely small wavelength limit. This classic simplification allows useful approximate analytical results to be obtained in cases where complete description of the waveform behaviour is virtually unattainable, hence its wide use in physics. In seismology, this approximation has been widely applied. This paper presents an application in one-dimensional site response (1DSR) analysis: it is used herein, first to explain and elucidate the generality of some previous observations as to the use of the harmonic mean of a shear-wave velocity profile to represent the global behaviour of a site; and second to partially settle an open question in 1DSR, namely 'What are the equivalent homogeneous properties that yield the same response, in terms of natural frequencies and resonance amplitude, for a given inhomogeneous site?', providing a few assumptions are met – chiefly, that excitations of sufficiently high frequency are considered. [ABSTRACT FROM AUTHOR]

In seismic tomography practices the Earth's surface is sometimes assumed to be either spherical or flat for convenience in forward modelling calculations. The effect of irregular surface topography on seismic wave propagation is thus ignored, resulting in biases in the phases and amplitudes of synthetic seismograms, which contribute to the residuals that are mapped into velocity structures in tomography inversions. In this study, we conduct a series of inversion experiments based on the adjoint waveform tomography method to quantitatively assess the topography effect on waveform-based inversion results. We first employ models with simplified topography to better highlight and quantify the topography effect. Results show that when topography effect is ignored in the forward modelling, it is mapped into velocity perturbations, leading to spurious velocity anomalies in tomography models. The strength of the spurious velocity anomaly is quasi-linearly related to locally averaged topography gradient. Our inversion experiments demonstrate that in places of strong topography variation, such as the Longmenshan Fault Zone region where the 2008 M w 7.9 Wenchuan earthquake occurred, topography effect can lead to spurious relative velocity anomalies of up to 10 per cent, which cannot be ignored in waveform-based tomography inversions. [ABSTRACT FROM AUTHOR]

Seismic anisotropy is key to constrain mantle flow, but it is challenging to image and interpret it. Existing large-scale tomography models of seismic anisotropy typically show large discrepancies, which can lead to completely distinct geodynamical interpretations. To better quantify the robustness of anisotropy tomography, we create a 2-D ridge-to-slab geodynamic model and compute the associated fabrics. Using the resulting 21 elastic constants, we compute seismic full waveforms, which are inverted for isotropic and radially anisotropic structure. We test the effects of different data coverage and levels of regularization on the resulting images and on their geodynamical interpretation. Within the context of our specific imposed conditions and source–receiver configuration, the retrieved isotropic images exhibit substantial artificial slab thickening and loss of the slab's high-velocity signature below ∼100 km depth. Our results also show that the first-order features of radial anisotropy are well retrieved despite strong azimuthal anisotropy (up to 2.7 per cent) in the input model. On the other hand, regularization and data coverage strongly control the detailed characteristics of the retrieved anisotropy, notably the depth–age dependency of anisotropy, leading to an artificial flat depth–age trend shown in existing anisotropy tomography models. Greater data coverage and additional complementary data types are needed to improve the resolution of (an)isotropic tomography models. [ABSTRACT FROM AUTHOR]

For a more quantitative discussion of slow earthquake activity, we evaluated the detectable limits of very low frequency earthquakes (VLFEs), which are seismic slow earthquakes observed in very low-frequency (< 0.05 Hz) bands in the Nankai subduction zone. We performed numerical simulations using a local 3-D model and used the observed noise level of permanent broad-band seismometers. First, we investigated the effects of the source-time functions on the maximum amplitudes of the VLFE signals at a certain station. The maximum amplitudes of the VLFE signals were controlled by the VLFE moment rate. The detectable limit of VLFEs at each source location can be defined as the lowest moment rate of detectable VLFEs, which radiate signals larger than the noise levels of any component at ≥ 3 stations. For inland seismometers only, the detectable limits of VLFEs at deep (30–40 km) and shallow (≤ 10 km) depths were 1012–1012.3 and 1012.7 N·m s−1, respectively. Due to the geometrical spreading of VLFE signals and large noise levels in horizontal components, offshore seismometers improved the detectability of shallow VLFEs in regions where seismometers were densely deployed. Based on our detectability and published catalogues, shallow slow earthquakes are less active south-southwest off the Kii Peninsula, where geodetic studies expect mechanical coupling. [ABSTRACT FROM AUTHOR]

One of the major advances in mineral physics and seismology is the realization that different olivine fabric types are functions of temperature, shear stress, and water content in oceanic subducting systems. The distribution of different olivine fabric types and geodynamic processes in the mantle wedge above a subducting continental slab remain poorly understood. Here, based on splitting analysis of shear waves recorded by 46 stations recently deployed in central Myanmar, we reveal trench-perpendicular fast orientations between the 80 and 100 km slab contours sandwiched between trench-parallel fast orientations from the mantle wedge tip to the backarc. The dramatic change in fast orientations indicates the transition of olivine fabric types in the mantle wedge combined with corner flow. Cold continental subduction and shear stress reduction caused by partial melting favor B-type and C- or E-type olivine fabrics, respectively. [ABSTRACT FROM AUTHOR]

Heliotropes are passive solar hot-air balloons that are capable of achieving nearly level flight within the lower stratosphere for several hours. These inexpensive flight platforms enable stratospheric sensing with high cadence enabled by the low cost to manufacture, but their performance has not yet been assessed systematically. During July–September of 2021, 29 heliotropes were successfully launched from Oklahoma and achieved float altitude as part of the Balloon-based Acoustic Seismology Study (BASS). All of the heliotrope envelopes were nearly identical with only minor variations to the flight line throughout the campaign. Flight data collected during this campaign comprise a large sample to characterize the typical heliotrope flight behavior during launch, ascent, float, and descent. Each flight stage is characterized, dependence on various parameters is quantified, and a discussion of nominal and anomalous flights is provided. [ABSTRACT FROM AUTHOR]

Human beings have encountered numerous natural disasters over time, andthe phenomenon of earthquake is one of the most destructive of them. In recent years, the complex behavior of earthquakes in terms of time and space has been investigated using complex networks, which has enabled us to know the global characteristics of this phenomenon. Studies on the earthquakes using the network method are based on how the network is constructed. In this paper, by constructing the network, the spatio-temporal recurrence of earthquakes which occurred in the Zagros region and were recorded at all broadband and short-period stations of the Iranian Seismological Center (ISC, http://irsc.ut.ac.ir) are subject to statistical review. The results showed that in the Zagros area the spatial recurrenceis about 0.020 km, and the time recurrence for all magnitudethresholds issimilar topower relationship (1/Tα) with α = -0.9. [ABSTRACT FROM AUTHOR]

This study integrates geophysical‐geochemical data to investigate the thermochemical structure of the lithosphere and sublithospheric mantle, along the Southern Tyrrhenian Basin, Apennines, Adriatic Sea, Dinarides, and Carpathians‐Balkanides. We present the lithospheric structure of the Adria microplate and the two opposing mantle slabs along its NE and SW margins. The modeling shows the presence of two asthenospheric mantle wedges aligning with the Apenninic and Dinaric continental mantle slab rollback, along with cold (−200°C) sublithospheric anomalies beneath Adria's NE and SW margins. In the northern Adria region, the lithosphere undergoes synchronous thinning in the Tyrrhenian domain and thickening toward the forefront of the northern Apennines. This is associated with the northeastward rollback of the SW Adriatic slab, leading to subsequent delamination of the continental mantle. In the southern Adria region, the complex deep structure results from the variably oriented lithospheric slabs, and nearly 90‐degree shift of the tectonic grain between the southern Apennines and the Calabrian Arc. At the SW Adria margin, beneath the northern Apennines, the thermal sublithospheric anomaly is attached to the shallower lithosphere, while a slab gap is modeled in the southern Apennines. One possibility is that the gap is due to a recent horizontal slab tear. Along the NE margin of Adria, the thermal anomaly penetrates to depths of about 200 km in the northern Dinarides and 280 km in the southern Dinarides, shallower than the SW Adria anomaly, which extends to at least 400 km depth. Plain Language Summary: We integrate geological and geophysical data (e.g., elevation, gravity, geoid, seismic tomography) to investigate the density and temperature distribution of the crust and lithospheric mantle down to 400 km, along a profile that extends from the Southern Tyrrhenian Basin and Apennines in Italy to the Southern Dinarides and Carpathians‐Balkanides in southern Europe. Taking advantage of our previous research in this region, we provide an integrated view of the lithospheric structure and the uppermost mantle of the Tyrrhenian Basin, Apennines, Adriatic Sea and Dinarides. Our results show notable variations in both the crust and the base of the lithosphere. These variations are observed to be shallower beneath the basins and deeper beneath the mountain belts, particularly in areas with elevated topography. Additionally, our observations highlight the presence of two cold and dense zones situated in the distal margins of the Adria microplate, specifically beneath the Apennines and Dinarides. The thermal anomalies indicate the presence of two oppositely subducting slabs with different geometries and depth of penetration. Their current location, depth and shape have largely controlled the geodynamic evolution of the study region in the last 30 My. Key Points: We unveil the thermochemical structure of the lithosphere and uppermost mantle of the Adria Microplate and its marginsResults reveal mantle wedges aligning with Apenninic and Dinaric slabs, and cold sublithospheric anomalies beneath NE and SW Adria marginsIn N Apennines the slab is attached while a gap is observed in S Apennines. At Adria's NE margin, the slab penetrates deeper southward [ABSTRACT FROM AUTHOR]

The mutual gravitational interaction of binary asteroids, which make up approximately 15 per cent of the near-Earth asteroid (NEA) population, provides a continuous tidal force, creating ground motion. We explore the potential of kilometre-sized binary asteroids as targets for seismological studies of their interior structure. We use a numerical model wherein each body is constructed of discrete particles interacting via gravity and contact forces. The system's orbital properties are modelled based on those of typical binary NEAs: a secondary body orbits a primary body at a distance of a few to 10 primary radii, resulting in orbital periods of a few tens of hours. We varied the elastic moduli (stiffness) of the constituent particles and measured a strain of a few micrometres caused by the orbiting satellite. Over eight orbital periods, the acceleration of the strain vector along the primary body's equatorial axis indicates that tidally induced ground motion generated by a binary asteroid system is detectable by modern seismometers, like the instruments deployed on the InSight mission to Mars. Owing to the relatively short orbital period of the satellite – a mean of 25.8 h for known binary NEAs – only a modest mission lifetime would be required for a seismometer to adequately characterize an asteroid's interior through tidally induced deformation. Future deployment of seismometers on binary asteroids will allow for a detailed characterization of the structure of these objects. [ABSTRACT FROM AUTHOR]

Spectral transform, known as a curvelet, enables sparse representations of complex data. Denoising wave propagation in disordered media and pattern recognition are just a few of the numerous domains in which denoising has a potential application. Based on directional basis functions, this spectral method represents objects with discontinuities along a smooth curve. In this study, we used this technique to eliminate ground roll, an unwanted feature signal that can be seen in seismic data obtained by sonating the earth's geological formations. Additionally, we improved the curvelet transform threshold denoising method combined with a fast non-local mean to remove seismic random noise. First, cyclic translation and block complex domain threshold methods were introduced into the curvelet transform threshold denoising, and the traditional curvelet threshold denoising method was improved to obtain the best denoising results; Subsequently, the removed noise was filtered using a fast non-local mean method to obtain a valid signal. Finally, the data obtained in the above two steps were added to obtain the final denoising result. The results of the model tests and actual seismic data denoising showed that the denoising results obtained using this method had a higher signal-to-noise ratio and fidelity than that using the other methods. [ABSTRACT FROM AUTHOR]

This paper presents a case study of the "electric hypothesis" of the causes of earthquakes, which emerged in the second half of the eighteenth century as part of the first studies of seismology. This hypothesis was related to Franklin's views on atmospheric electricity and developed in a period when electric phenomena were widely studied, and was essentially based on solid empirical evidence and confirmed by model experiments. Even though it resulted from scientific reasoning, the theory remained strongly empirical, and was supported by Italian scholars who were familiar with seismic events. Among these, Giuseppe Saverio Poli, a follower of Franklin, was able to provide a careful and comprehensive explanation of the disastrous earthquake of 1783, which occurred in Calabria, a region of southern Italy, and the St. Anne earthquake of 1805, by drawing not just upon the electric evidence, but all the relevant phenomenology available. We outline here the emergence, the development, and the later evolution (up to the beginning of the nineteenth century) of the "electric earthquake" paradigm by focusing on different works by Poli, including a previously unknown manuscript containing a thorough account of the Calabria earthquake prepared by the Neapolitan scholar for the Royal Society. The present case study therefore offers the opportunity to illustrate how electrical science shaped earthquake science to a degree not usually appreciated in the literature, and is also supported to some extent by the transition from Enlightenment scientific ideals to the Romantic conception of unity in the natural world, in search of common causes among phenomena belonging to different fields. [ABSTRACT FROM AUTHOR]

In our study, we estimated earthquake source parameters and spectral decay characteristics of 378 seismograms corresponding to 80 small earthquake events with magnitudes ranging from 1.5 to 3.5. These earthquakes occurred between July 2011 and May 2012 in the Siang region of the Arunachal Himalaya which is not well-studied. To estimate source parameters, the classical Brune model is employed and through the analysis, a scaling relationship is established between the estimated corner frequency ( f c ) and seismic moment ( M 0 ), which can be expressed as M 0 = 1 × 1 0 22 f c - 3.18 . This relationship is in close agreement with previous studies conducted in the Arunachal and Himachal Himalaya regions. It provides support for deviations from self-similarity in the study area, offering valuable insights into the tectonics and structural heterogeneity beneath the Arunachal Himalaya. Our analysis revealed variations in source radius, ranging from 154.4 m to 312.6 m, and seismic moment, spanning from 2.37 × 1011 N-m to 9.32 × 1013 N-m. Interestingly, we observed an increasing trend in stress drop, ranging from 0.013 MPa to 3.26 MPa, within the same range of seismic moment. This significant variation in stress drop primarily occurs in the upper 10–15 km of the Earth's crust, indicating shear brittle failure in this upper crustal region. Furthermore, we conducted an in-depth examination of spectral parameters, including f c high-cut frequency (fmax), and high-frequency spectral decay parameter kappa (κ). Our study highlighted the dependence of κ and fmax estimates on both source characteristics and propagation path, with the source having the most substantial influence. This observation was substantiated through statistical analyses. Additionally, we explored the effect of recording site characteristics on κ and observed a significant contribution of shallow geology at the recording site. This was evident through a negative correlation between the site component of κ (κ0) and VS30, indicating that the local geology of the recording site plays a significant role in spectral parameter estimation. Based on our comprehensive data analysis and statistical observations, we conclude that the variations in source and decay parameters for earthquakes of different magnitudes are attributed to the diverse structural heterogeneities and complex seismotectonic processes underlying the Arunachal Himalaya region. [ABSTRACT FROM AUTHOR]

We conduct an exploration-scale ambient noise tomography (ANT) survey over the Hillside Iron Oxide–Copper–Gold (IOCG) deposit in South Australia, leveraging Fleet's direct-to-satellite technology for real-time data analysis. The acquisition array consisted of 100 sensors spaced 260 m apart which recorded continuous vertical-component seismic ambient noise for 14 days. High quality Rayleigh wave signals, with a mean signal-to-noise ratio (SNR) of 40, were recovered in the frequency band 1–4 Hz after processing the recorded data between 0.1–9 Hz. Our modelling results capture aspects of the deposit's known geology, including depth of cover, structures linked to mineralisation, and the mineralised host rock, down to approximately 1 km depth. We compare our velocity model with existing magnetic, gravity, induced polarisation and drilling data, showing strong correlation with each. We identify several new features of the local geology, including the behaviour of key structures down to 1 km, and highlight the significance of a Cambrian-age dolomite that cuts across the main structural corridor that hosts the Hillside deposit. An analysis of model convergence rates with respect to Rayleigh wave SNRs shows that real-time data analysis can reduce recording duration at the site by 65% compared to traditional deployment durations, from ∼14 days to ∼5 days. Finally, we conclude by commenting on the efficacy of the ANT technique for the exploration of IOCG systems more broadly. [ABSTRACT FROM AUTHOR]

The southeastern margin of the Tibetan Plateau has undergone complex deformation since the Cenozoic, resulting in a high level of seismicity and seismic hazard. Knowledge about the seismic anisotropy provides important insight about the deformation mechanism and the regional seismotectonics beneath this tectonically active region. In this study, we conduct fullwave multi‐scale tomography to investigate the seismic anisotropy in the southeastern margin of the Tibetan Plateau. Broadband records at 111 permanent stations in the region from 470 teleseismic events are used to obtain 5,216 high‐quality SKS splitting intensity (SI) measurements, which are then inverted in conjunction with 3D sensitivity kernels to obtain an anisotropic model with multi‐scale resolution. Resolution tests show that our data set recovers anisotropy anomalies reasonably well on the scale of 1° × 1° horizontally and ∼100 km vertically. Our result suggests that in the southeastern margin of the Tibetan Plateau the deformation in the lithosphere and asthenosphere are decoupled. The anisotropy in the lithosphere varies both laterally and vertically as a result of dynamic interactions of neighboring blocks as well as lithospheric reactivation. The anisotropy in the asthenosphere largely follows the direction of regional absolute plate motion. The SKS splittings observed at the surface are shown to be consistent with the vertical integral of our depth‐dependent anisotropy model over lithospheric and asthenospheric depths. Plain Language Summary: The southeastern margin of the Tibetan Plateau has undergone significant deformation since the Cenozoic due to the collision between the Eurasian and Indian plates in the south and interactions with the Yangtze Craton in the east. Knowledge about the upper mantle seismic anisotropy helps us understand the deformation and dynamic evolution. In this study, we conduct fullwave multi‐scale anisotropy tomography for the southeastern margin of the Tibetan Plateau using 5,216 high‐quality SKS splitting intensity measurements obtained from the broadband records of 470 teleseismic events at 111 stations. Our result shows a decoupling between the lithosphere and asthenosphere deformations in the southeastern margin of the Tibetan Plateau. The anisotropy in the lithosphere varies both laterally and vertically as a result of the dynamic interactions of neighboring blocks as well as lithospheric reactivation. The anisotropy in the asthenosphere is largely parallel to the regional absolute plate motion, and the SKS splitting observed at the surface is the result of vertical integration of the anisotropy effect through the lithosphere and asthenosphere. Key Points: A 3D shear‐wave anisotropy model for the SE margin of the Tibetan Plateau is obtained by fullwave SKS splitting intensity tomographyResult shows a decoupling of lithosphere and asthenosphere deformations in the region, but a coupling under Tibetan PlateauLithospheric anisotropy has a complex pattern, whereas asthenospheric anisotropy follows the absolute plate motion [ABSTRACT FROM AUTHOR]

The top of the subducting plate is a thick, complex zone where the heterogeneous structure likely controls earthquake rupture processes. Imaging this heterogeneous channel typically involves active‐source methods with limited depth penetration, or low‐resolution teleseismic methods. To access short wavelengths at greater depth, we use high‐frequency P‐to‐S (PS, 1–15 Hz) mode‐converted arrivals from nearby earthquakes >50 km deep to image the plate interface at vertical scales <1 km. We use 37 broadband stations in southcentral Alaska between 2007 and 2008 at 10–15 km spacing, spanning the great 1964 earthquake rupture zone and adjacent deeper slow slip and tremor regions. The central 21 stations record high‐amplitude PS arrivals converting from the megathrust region, at depths corresponding to the top of a prominent low‐velocity zone (LVZ) in receiver function images. The PS/P amplitude ratio (APS_P) varies along strike and with depth of the conversion point but is independent of earthquake location and varies slowly between adjacent stations. APS_P changes with slab depth, indicating changes in lithology or fluid content of the plate interface, consistent with transitions in slip behavior from locked to slow slip. High APS_P cannot be explained by a velocity step or a single low‐velocity, high Vp/Vs layer, but requires several alternating high and low‐velocity layers. These observations indicate that the LVZ is a highly heterogeneous channel at multiple scales, resembling a subduction channel or sheared zone of metasediment and altered crust as observed in many exhumed subduction zones. Plain Language Summary: The largest earthquakes in the world occur in subduction zones. The contact between the overriding and subduction plates, the megathrust, hosts great earthquakes and less hazardous slow slip and tremor events. The location of these depends on the fault's physical properties, but there is a discrepancy between the structures different seismic imaging techniques observe. To address this issue, our study uses a previously underutilized P‐to‐S (PS) signal from local in‐slab earthquakes, which start as a P wave and converts to an S wave at the plate interface. The conversion occurs in the megathrust fault zone, and the arrival amplitude depends on the properties across it. The high amplitudes of PS compared to P across the array favor a model that represents the interface as a broad shear zone, where the velocity and number of layers create variability in the amplitudes. Mapping the amplitudes shows high variability in the fault properties at a finer scale than typical passive seismic imaging techniques. Key Points: Southcentral Alaskan stations record high amplitude arrivals converting from P to S at the megathrust for local earthquakes >50 km deepThe timing of P‐to‐S conversions varies between stations requiring a conversion from top of the subducting plateArrival amplitudes change coherently across the megathrust, indicating lithology or fluid changes at tens of km spatial scale [ABSTRACT FROM AUTHOR]

Given the recent developments in machine-learning technology, its application has rapidly progressed in various fields of earthquake seismology, achieving great success. Here, we review the recent advances, focusing on catalog development, seismicity analysis, ground-motion prediction, and crustal deformation analysis. First, we explore studies on the development of earthquake catalogs, including their elemental processes such as event detection/classification, arrival time picking, similar waveform searching, focal mechanism analysis, and paleoseismic record analysis. We then introduce studies related to earthquake risk evaluation and seismicity analysis. Additionally, we review studies on ground-motion prediction, which are categorized into four groups depending on whether the output is ground-motion intensity or ground-motion time series and the input is features (individual measurable properties) or time series. We discuss the effect of imbalanced ground-motion data on machine-learning models and the approaches taken to address the problem. Finally, we summarize the analysis of geodetic data related to crustal deformation, focusing on clustering analysis and detection of geodetic signals caused by seismic/aseismic phenomena. [ABSTRACT FROM AUTHOR]

A geodynamic model of the modern Earth is constructed based on the SMEAN 2 global seismic tomography model with an emphasis on the Arctic region. For a spherical Earth model, a solution of the Stokes equation for a viscous fluid was obtained based on seismic tomography data using the CitcomS code. The resulting distributions of temperature anomalies and velocity fields of mantle flows explain the main features of the modern geodynamics of the Arctic region. The temperature difference in the subcrustal mantle between the relatively "cold" western Arctic shelf (Barents and Kara seas) and the "warmer" eastern Arctic shelf (from the Laptev Sea to the Bering Strait) reaches 100 degrees, which correlates with the observed intense methane emission from the shallow shelf of the Eastern Arctic caused by permafrost degradation and destruction of gas hydrates against the background of elevated environmental temperatures. The greenhouse effect of methane in the atmosphere, in turn, contributes to climate warming in the Arctic. The region of Iceland and eastern part of Greenland, under the influence of the mantle upwelling, is characterized by a hot subcrustal mantle and increased heat flow at the surface, causing instability and melting of the Greenland ice sheet from below. [ABSTRACT FROM AUTHOR]

Recently, the eastern margin of the Tibetan plateau has experienced three moderate earthquakes. Deep crustal structure is a critical factor for understanding the seismotectonic environment and deformations in this region. Although several multidisciplinary geophysical approaches have been used to develop crustal structure models for this area, substantial inconsistencies still persist in the results due to the intricate structural properties of crust. In this study, we introduce a novel approach to construct a 3-D crustal model with assimilated density structure (MADS) at a resolution of 0.2° × 0.2° × 5 km. This model is built by integrating data from various source, including multisource gravity anomalies and diverse available crustal structure reference models. We use a model-assimilated gravity inversion process with Bayesian parameter optimization. First, we combine the terrestrial gravity profiles data set with published global gravity field models to create a data set rich in reliable high-frequency anomaly information. Secondly, we incorporate three reference models derived from different seismological methods as prior constraints for our model. These models encompass seismic tomography, surface wave dispersion and receiver function data. We optimize the hyper-parameters of these constraints using the Bayesian criterion. The results demonstrate that the MADS not only captures significant changes in the crustal density but also discerns subtle variations in the upper and middle crust, thereby providing detailed insights into the morphologies of major faults. For instance, the central section of the Longmenshan fault is revealed as a high-angle deep thrust feature, while the frontal section of the Longmenshan fault appears as a low-angle mid-deep thrust feature, and the Xianshuihe fault exhibits a vertical deep subduction feature. Additionally, our findings indicate a correlation between the locations of moderate-to-large earthquakes in this region and the high density-gradient zones or asperities with high density within MADS. We believe that the insights into density characteristics offered by the new MADS model can shed light on the study of asperities associated with recent moderate earthquakes and enhance our understanding of deformation in this region. [ABSTRACT FROM AUTHOR]

Polar regions and Greenland in particular are highly sensitive to global warming. Impacts on Greenland's glaciers may be observed through the increasing number of calving events. However, a direct assessment of the calving activity is limited due to the remoteness of polar regions and the cloudy weather which makes impossible a recurrent observation through satellite imagery. To tackle this issue, we exploit the seismological network deployed in Greenland which actively records seismic signals associated with calving events, hereinafter referred to as glacial earthquakes. These seismic signals present a broad frequency range and a wide diversity of waveform which make them difficult to discriminate from tectonic events as well as anthropogenic and natural noises. In this study, we start from two catalogues of known events, one for glacial earthquake events which occurred between 1993 and 2013 and one for earthquakes which occurred in the same time period, and we implement a detection algorithm based on the STA/LTA method to extract signals' events from continuous data. Then, we train and test a machine learning processing chain based on the Random Forest algorithm which allows us to automatically associate the events respectively with calving and tectonic activity, with a certain probability. Finally, we investigate 844 selected days spanning time of continuous data from the Greenland regional seismic network which results in a new, more exhaustive, catalogue of glacial earthquakes expanded of 1633 newly detected glacial events. Moreover, we extensively discuss the choice of the features used to describe glacial earthquakes, in particular the 39 new features created in this study which have drastically improved our results with 7 of the 10 best features being in the added set. The perspective of further expansion of the glacial earthquake catalogue applying the processing chain discussed in this paper on different time spans highlights how combining seismology and machine learning can increase our understanding of the spatio-temporal evolution of calving activity in remote regions. [ABSTRACT FROM AUTHOR]

Southwest China is a tectonically and seismically active region, witnessing strong deformation due to the collision between the Indian and Eurasian plates. Constraining the subsurface velocity structure of this region is thus important in understanding the tectonics and geodynamic processes of continental collision and in mitigating seismic hazards. Numerous studies have provided various 3D seismic velocity models in southwest China. However, discrepancies exist among these models, and less effort has been made to quantify the reliability and accuracy of these existing velocity models. In this study, we use regional 3D waveform simulation to evaluate the performance of various regional crustal 3D velocity models in reproducing observed seismograms. We particularly focus on two recent earthquake sequence in the region, the 2021 Yunnan Yangbi MS6.4 earthquake sequence and the 2022 Sichuan Luding MS6.8 earthquake sequence. The tested 3D velocity models include the Southwest China Community Velocity model V1.0, the Unified Seismic Tomography Models for Continental China Lithosphere V2.0, the adjoint full waveform tomography model of the crustal and upper mantle beneath Eastern Tibetan Plateau, and the shallow seismic structure model beneath continental China. Our results show that the tested 3D velocity models generally capture well long-period (<0.2 Hz) waveforms, indicating that the 3D models adequately resolve overall large-scale subsurface structures. However, the 3D synthetics show discrepancies in higher frequencies (0.05–0.3 Hz) and the performance of the 3D velocity models varies from region to region, suggesting that smaller scale heterogeneities are not well constrained. Including shallow velocity structures (<10 km) can improve the waveform fitting, emphasizing the importance of incorporating shallow structures in waveform modeling. The full-waveform tomography model shows a slighter better performance than the other models, especially for the body-waves, highlighting the advantages of full-waveform method in achieving sub-wavelength resolution despite the usage of very long-period waveforms. In light of these comparison results of model performance, we identify the advantages and limitations of different seismic tomography models and methods, and we propose to incorporate different tomography methods and datasets to better constrain subsurface structures. While our target region in this study is southwest China, the analysis that we have conducted can be applied to other regions of various scales and tectonic settings for quantitative seismic model evaluation. [ABSTRACT FROM AUTHOR]

Heliotropes are passive solar hot air balloons that are capable of achieving nearly level flight within the lower stratosphere for several hours. These inexpensive flight platforms enable stratospheric sensing with high-cadence enabled by the low cost to manufacture, but their performance has not yet been assessed systematically. During July to September of 2021, 29 heliotropes were successfully launched from Oklahoma and achieved float altitude as part of the Balloon-based Acoustic Seismology Study (BASS). All of the heliotrope envelopes were nearly identical with only minor variations to the flight line throughout the campaign. Flight data collected during this campaign comprise a large sample to characterize the typical heliotrope flight behavior during launch, ascent, float, and descent. Each flight stage is characterized, dependence on various parameters is quantified, and a discussion of nominal and anomalous flights is provided. [ABSTRACT FROM AUTHOR]

The Pieniny Klippen Belt (PKB) is located between the Central and the Outer (Flysch) Carpathians and forms a narrow zone with a complex structure, often described as a mélange. It is composed of numerous tectonic elements of different size including strike-slip-bounded tectonic blocks, thrust units, toe thrusts and olistostromes combined together and representing different lithologies. To aid the geological interpretation of the mélanges of the PKB in the Spiskie Pieniny Mountain Region (South Poland), seismic refraction tomography was conducted. Isolated limestone outcrops consist of Jurassic limestones that stand out in the landscape. They form the horizontal narrow belt. In this belt, limestone olistoliths are surrounded by a matrix consisting of sandstones, mudstones and marl sequences forming a sedimentary mélange. The seismic refraction tomography measurements conducted along three profiles across this belt showed significant horizontal and vertical seismic velocity variations, which revealed the complex geological structure of this area. The Złatne, Branisko and Hulina Units were distinguished within the PKB structure. The high-velocity objects detected within the Hulina Unit were found to correspond to limestone outcrops and form isolated blocks surrounded by flysch deposits representing a chaotic sedimentary complex. [ABSTRACT FROM AUTHOR]

The Hawaiian Ridge, a classic intraplate volcanic chain in the Central Pacific Ocean, has long attracted researchers due to its origin, eruption patterns, and impact on lithospheric deformation. Thought to arise from pressure‐release melting within a mantle plume, its mass‐induced deformation of Earth's surface depends on load distribution and lithospheric properties, including elastic thickness (Te). To investigate these features, a marine geophysical campaign was carried out across the Hawaiian Ridge in 2018. Westward of the island of O'ahu, a seismic tomographic image, validated by gravity data, reveals a large mass of volcanic material emplaced on the oceanic crust, flanked by an apron of volcaniclastic material filling the moat created by plate flexure. The ridge adds ∼7 km of material to pre‐existing ∼6‐km‐thick oceanic crust. A high‐velocity and high‐density core resides within the volcanic edifice, draped by alternating lava flows and mass wasting material. Beneath the edifice, upper mantle velocities are slightly higher than that of the surrounding mantle, and there is no evidence of extensive magmatic underplating of the crust. There is ∼3.5 km of downward deflection of the sediment‐crust and crust‐mantle boundaries due to flexure in response to the volcanic load. At Ka'ena Ridge, the volcanic edifice's height and cross‐sectional area are no more than half as large as those determined at Hawai'i Island. Together, these studies confirm that volcanic loads to the west of Hawai'i are largely compensated by flexure. Comparisons to the Emperor Seamount Chain confirm the Hawaiian Ridge's relatively stronger lithospheric rigidity. Plain Language Summary: The Hawaiian Ridge, a seafloor volcanic chain in the Pacific, has long fascinated scientists for its origin, eruptions, and impact on Earth's surface layers. It is believed to form from melted rock originating in the Earth's mantle. As lava builds up each volcanic edifice, it presses down on the Earth's surface, deforming it by a few kilometers vertically, with the degree of deformation depending on its weight and the properties of the Earth's outer layers. Our geophysical study has imaged the interior of the volcanic ridge and the underlying crust and mantle. The volcano contains an interior core of highly crystalline rocks draped by volcanic material, and contributes to a broad lithospheric flexural response of up to ∼3.5 km, vertically. This study sheds light on volcano formation, load‐induced lithospheric flexure, and their implications for understanding Earth's subsurface processes. Key Points: The Hawaiian Ridge at Ka'ena consists of volcanic edifices ∼7 km thick, sitting on Pacific oceanic crust ∼6 km thickSeismic and gravity data reveal an inner volcanic core draped by layers of lava flows and volcanic debrisBroad lithospheric flexure (peak deflection ∼3.5 km) suggests a flexural rigidity of the 90 Myr old lithosphere equivalent to Te = ∼25.6 km [ABSTRACT FROM AUTHOR]

Understanding the scaling laws of source parameters is a fundamental subject in seismology. This study conducts spectral ratio analysis to estimate the source parameters for 409 shallow crustal earthquakes with Mw 3.2–6.0 in Japan. Subsequently, the source parameter scaling relations are investigated for a wide magnitude range by combining the results of this and previous studies. The spectral ratio method applied in this study provides the finite source properties of the localized area with large slip. The single asperity model, a simple heterogeneous source model with a single localized area with large slip, is used to estimate the stress drop since it can be more suitable to characterize earthquake sources than the standard homogeneous circular source. This investigation calibrates the constant to link the corner frequency and the source radius by comparing the corner frequency and source area estimated by the spectral ratio analyses. This calibrated constant is used to re‐calculate the stress drop of small earthquakes from the corner frequency estimated in previous studies. The stress drop and apparent stress increase with increasing magnitude up to Mw ∼ 5 and become magnitude‐independent for larger earthquakes. The radiation efficiency ranges typically from 0.1 to 1.0 and is independent of magnitude. The slip dependences of the stress drop, apparent stress, radiation efficiency, and fracture energy observed in this study are explained by the ones predicted from the slip‐weakening model incorporating the thermal pressurization effect. Thermal pressurization is one of the possible mechanisms explaining the observed source parameter scaling relations. Plain Language Summary: This study investigates the relationship between earthquake sizes and parameters characterizing the earthquake sources (source parameter scaling relations) to understand the difference between small and large earthquakes. The source parameters are estimated for shallow crustal earthquakes in Japan. Then, the source parameter scaling relations are obtained over a broad magnitude range by combining the results of this and previous studies. This investigation is based on a simple heterogeneous source model rather than the homogeneous circular source model, which has been widely used. A familiar hypothesis is that the sources of small and large earthquakes are physically similar (earthquake similarity). The results suggest that the earthquake similarity holds for earthquakes with a magnitude larger than about 5.0 but does not for small earthquakes. This study demonstrates that the observed source parameter scaling relations can be explained by the source parameter scaling relations predicted from the stress relaxation model incorporating the effect of the pore pressure rise within the fault core by frictional heating during seismic faulting (thermal pressurization of pore fluids). Thermal pressurization of pore fluids can be one of the significant mechanisms explaining the differences in the source parameter scaling relations for small and large earthquakes. Key Points: The single asperity model, a simple heterogeneous source model, is used rather than the standard homogeneous circular source modelThe apparent stress and stress drop increase with magnitude up to Mw ∼ 5 and become magnitude‐independent for larger earthquakesThermal pressurization is one of the possible mechanisms explaining the observed source parameter scaling relations [ABSTRACT FROM AUTHOR]

Template matching has proven to be an effective method for seismic event detection, but is biased toward identifying events similar to previously known events, and thus is ineffective at discovering events with non‐matching waveforms (e.g., those dissimilar to existing catalog events). In principle, this limitation can be overcome by cross‐correlating every segment (possible template) of a seismogram with every other segment to identify all similar event pairs, but doing so has been previously considered computationally infeasible for long time series. Here we describe a method, called the 'Matrix Profile' (MP), a "correlate everything with everything" calculation that can be efficiently and scalably computed. The MP returns the maximum value of the correlation coefficient of every sub‐window of continuous data with every other sub‐window, as well as the best‐correlated sub‐window location. Here we show how MP methods can obtain valuable results when applied to months and years of continuous seismic data in both local and global case studies. We find that the MP can identify many new events in Parkfield, California seismicity that are not contained in existing event catalogs and that it can efficiently find clusters of similar earthquakes in global seismic data. Either used by itself, or as a starting point for subsequent template matching calculations, the MP is likely to provide a useful new tool for seismology research. Plain Language Summary: Detecting and cataloging earthquakes through analysis of seismic data—the shapes of seismic waves, recorded by seismometers—is foundational to our understanding of Earth's interior structure and processes, as well as geological hazards such as earthquakes. Methods to improve the efficiency and sensitivity of earthquake detection while maintaining accuracy are critical, as seismic data volumes have grown exponentially in recent decades. Recently, methods that use recorded earthquakes as template patterns to identify in seismic data, have proven capable of detecting several times more earthquakes than traditional methods. However, such methods require knowledge of the template earthquakes ahead of time, and are best at identifying earthquakes whose waveforms are similar to the templates. We present here a new method, called the 'Matrix Profile' (MP), that takes short windows of data from a seismic data stream, and compares them to all other parts of that data stream, identifying parts of the data that are highly similar. The MP effectively identifies earthquakes, even those which are hidden within noise, does not require any templates to be provided upfront, and can be efficiently calculated. We demonstrate its success at detecting earthquakes in different types of seismic data, and provide practical guidelines for applying the method and interpreting the results. Key Points: The matrix profile finds the maximum autocorrelation of every subsequence in a time seriesPeaks of high similarity in the matrix profile can be used to identify seismic events and similar event pairsThe matrix profile method has similar sensitivity to template matching but does not require a priori templates [ABSTRACT FROM AUTHOR]

Foreshocks are spatially clustered seismic events preceding large earthquakes. Since the dawn of seismology, their occurrence has been identified as a possible mechanism leading to further crustal destabilization, hence, to major failures. However, several cases occurred without any previous anomalous seismic activity, so that the hypothesis of foreshocks as reliable seismic precursors fails to pass statistical tests. Here, we perform an all‐round statistical comparative analysis of seismicity in Southern California to assess whether any differences can be identified between swarms and foreshocks. Our results suggest that extremely variable seismic patterns can forerun mainshocks, even though they tend to be preceded by clusters with more numerous events spread over larger areas than swarms and with a wider range of magnitudes. We provide a physical explanation of such dissimilarity and conclude, despite it, that foreshocks can hardly be reliable short‐term precursors of large earthquakes in California. Plain Language Summary: Large earthquakes can be preceded by a wide range of different seismic anomalies. Among these, seismicity has been reported to increase both in magnitude and frequency, but, on the other hand, it can also undergo a short period of reduced intensity before major events. The first pattern corresponds to the occurrence of foreshocks, that is, small to moderate quakes forewarning an upcoming larger one, while the second behavior is called seismic quiescence. In our research, we focus on foreshock activity. We perform an analysis of seismicity in Southern California, for which a well‐provided relocated earthquake catalog is available. While several studies have been conducted so far about what happens before large earthquakes after their occurrence and also there are some works about foreshocks discrimination, a systematic analysis comparing properties of clusters of "swarms", seismicity without a major event, and "foreshocks" before their mainshock is missing. Are foreshocks different from swarms before the occurrence of the main event? Are foreshocks fore‐shocks? Our results suggest that foreshocks can hardly be distinguished from swarms until the largest event takes place. On the base of this analysis and theoretical modeling, we think that foreshocks have limited reliability, if considered alone, for short‐term forecasts. Key Points: The occurrence of mainshocks can be preceded by a wide range of seismic patternsForeshocks slightly differ from swarms in Southern California, but they can be hardly distinguished before mainshocksForeshocks are not reliable precursors of large seismic events [ABSTRACT FROM AUTHOR]

Ground-based monitoring of seismicity and modulation by external forces in the field of planetary seismology remains equivocal due to the lack of natural observations. Constrained by the natural observations (including Earthquakes, Moonquakes, and Marsquakes) and theoretical models, we present the variation in gravitational acceleration "g" of different solar system objects, combined with external harmonic forcings that are responsible for seismicity modulation on the planetary bodies and their natural satellites. From the global diversity in seismicity modulation, it has been observed that the plate-boundary regions on the Earth exhibit both short and long-period seismicity modulation. In contrast, the stable plate interior regions appear to be more sensitive to long-period seismicity modulation, however, lacking in short-period modulation. The deep Moonquakes are susceptible for both the lunar tidal period (13.6 days and 27 days) and long-period pole wobble modulation (206 days), whereas shallow emergent type moonquakes show a seismic periodicity at the lunation period (29.5 days). Further, the seasonal variation with an annual seismicity burst and seismic periodicity at polar wobble periods for high-frequency Marsquakes captured by InSight lander indicate a natural origin. Whereas diurnal and semi-diurnal periodicity along with Phobos' tidal period, indicate possible artifacts due to different detection probabilities and non-seismic noise in the Martian environment. We argue that, in the context of rate-state-dependent fault friction, the gravity-induced resonance destabilization model appears to be better agreement with the contrast and relative diversity in seismicity modulation linked to the Earth, Moon, and Mars. [ABSTRACT FROM AUTHOR]

The reliable estimation of earthquake magnitude and stress drop are key in seismology. The novel technology of distributed acoustic sensing (DAS) holds great promise for source parameter inversion owing to the measurements' high spatial density. In this study, I demonstrate the robustness of DAS for magnitude and stress drop estimation using the empirical Green's function deconvolution method. This method is applied to nine co‐located earthquakes recorded in Israel following the 2023 Turkey earthquakes. Spectral ratios were stacked along the fiber, and fitted with a relative Boatwright source spectral model. Excellent fits were obtained even for similar sized earthquakes. Stable seismic moments and stress drops were calculated assuming that the moment of one earthquake is known. DAS derived estimates were found to be more stable and reliable than those obtained using a dense accelerometer network. The results demonstrate the great potential of DAS for source studies. Plain Language Summary: Estimating earthquake magnitude and stress drop, the two most fundamental source parameters, is key for various seismological investigations, including earthquake self‐similarity and hazard mitigation. The extraction of source parameters requires that path and site effects are reliably deconvolved from source contributions. One approach is to deconvolve a small earthquake from a co‐located larger one in the frequency domain such that common path and site functions cancel and relative source function, between both earthquakes, remain. In this study, I apply the deconvolution approach to nine earthquakes that followed the 2023 Turkey earthquakes, recorded using distributed acoustic sensing (DAS) in Israel. DAS converts standard optical fibers into dense arrays, with seismic measurements every few meters along tens‐of‐kilometers long fibers. The deconvolution was calculated for different earthquake pairs and excellent fits were obtained with a relative source spectral model. Stable seismic moments and stress drops were calculated. These parameters were found to be more reliable than those obtained using a dense standard‐accelerometer network. The results demonstrate the great potential of DAS for source studies. Key Points: The spectral ratios approach was applied to nine earthquakes in Israel recorded by distributed acoustic sensing (DAS) and accelerometersAccurate relative magnitudes and stress drops were calculatedDAS can provide reliable source parameter estimates that may outperform accelerometers [ABSTRACT FROM AUTHOR]

Ambient noise seismic tomography has proven to be an effective tool for subsurface imaging, particularly in volcanic regions such as the Reykjanes Peninsula (RP), SW Iceland, where ambient seismic noise is ideal with isotropic illumination. The primary purpose of this study is to obtain a reliable shear wave velocity model of the RP, to get a better understanding of the subsurface structure of the RP and how it relates to other geoscientific results. This is the first tomographic model of the RP which is based on both on- and off-shore seismic stations. We use the ambient seismic noise data and apply a novel algorithm called one-step 3-D transdimensional tomography. The main geological structures in the study area (i.e. covered by seismic stations) are the four NE–SW trending volcanic systems, orientated highly oblique to the plate spreading on the RP. These are from west to east; Reykjanes, Eldvörp-Svartsengi, Fagradalsfjall and Krýsuvík, of which all except Fagradalsfjall host a known high-temperature geothermal field. Using surface waves retrieved from ambient noise recordings, we recovered a 3-D model of shear wave velocity. We observe low-velocity anomalies below these known high-temperature fields. The observed low-velocity anomalies below Reykjanes and Eldvörp-Svartsengi are significant but relatively small. The low-velocity anomaly observed below Krýsuvík is both larger and stronger, oriented near-perpendicular to the volcanic system, and coinciding well with a previously found low-resistivity anomaly. A low-velocity anomaly in the depth range of 5–8 km extends horizontally along the whole RP, but below the high-temperature fields, the onset of the velocity decrease is shallower, at around 3 km depth. This is in good agreement with the brittle–ductile transition zone on the RP. In considerably greater detail, our results confirm previous tomographic models obtained in the area. This study demonstrates the potential of the entirely data-driven, one-step 3-D transdimensional ambient noise tomography as a routine tomography tool and a complementary seismological tool for geothermal exploration, providing an enhanced understanding of the upper crustal structure of the RP. [ABSTRACT FROM AUTHOR]

We use continuous data from more than 200 vertical-component broad-band seismic stations for the years 2012 and 2013 to model the crustal and lithospheric structure of the southeastern United States (SEUS). Seismic interferometry via cross-coherence is used to retrieve the surface wave Empirical Green's function (EGF) between station pairs. We mitigate the problem of non-stationary sources contributing to the extracted EGF by applying double beamforming to pairs of subarrays of stations. The recovered Rayleigh waves are used to compute group velocity dispersion maps which are then inverted to find shear wave velocity profiles using a Markov Chain Monte Carlo technique. EGFs are computed for more than 80 000 station pairs after filtering to the period band 7–60 s. Results reveal tectonic features in the SEUS that support recent claims that the northernmost extent of the Suwannee suture is located near the boundary of the Carolinia and Charleston terranes and that the Charleston and Suwannee terranes are not seismologically distinct within the resolution limits of the data set. [ABSTRACT FROM AUTHOR]

The Eastern-Europe region (EER), is a complex geotectonic area that captures part of the Alpine-Himalayan Orogen, the subduction of multiple NeoTethys Branches and part of the East European Craton. It is one of the most exciting geological areas in Europe due to a diversity of tectonic processes acting within it: extensional basin evolution, oceanic subduction, post-collisional volcanism, as well as active crustal deformation associated with the push of the Adria plate or the pull of the actively detaching Vrancea slab. This makes EER an excellent natural laboratory to study the behaviour of the lithosphere–asthenosphere system in a heterogeneous tectonic setting. To investigate the lateral heterogeneity and physical properties of the crust in the EER, we use ambient seismic noise data recorded by the vertical components of broad-band stations that have been operational between 1999 and 2020 in Eastern Europe and surrounding regions. We used this significant amount of data and the latest processing techniques of the ambient seismic noise field based on the continuous wavelet transform to compute cross-correlations between various station pairs, turning every available seismic station into a virtual source. The coda of the interstation cross-correlograms were used to determine coda quality factors (Qc) of Rayleigh waves in four different period ranges (3.0–5.0, 5.0–10.0, 10.0–20.0 and 20.0–30.0 s) and to invert them in the 2-D space, constructing the highest resolution attenuation tomography of the region. Our results reveal high attenuation features throughout the northeast Pannonian region, the Bohemian Massif, the East Carpathians and the Moesian Platform. Nevertheless, our findings do not emphasize a close correlation between the depth of sedimentary basins and attenuation features identified at longer periods. In addition, Qc variations are larger at short periods, indicating higher heterogeneity in the uppermost crust of Eastern Europe. Our findings demonstrate the higher efficiency of noise correlation approaches relative to earthquake data analyses investigating Qc at low frequencies. [ABSTRACT FROM AUTHOR]

First-arrival picking is a fundamental and challenging task in seismic data processing. Existing algorithms such as the cross-correlation algorithm (CCT) struggle to achieve satisfactory noise immunity while maintaining picking efficiency. In this paper, we propose the first-arrival picking through mathematical morphology and edge detection (FPME) algorithm with three steps. The converting step transforms the original data into a binary image though a filtering technique, where the binary image represents the global energy distribution of high-energy and low-energy samples. The rendering step delimits the signal zone in the binary image through morphological operations, where the signal zone eliminates the noise in the upper part. The picking step detects first arrivals in the signal zone through the edge detection methods, where an evaluation function is designed to select the optimal detection results as first arrivals. Experiments were performed on four field data sets. The results demonstrated that FPME is more stable and reliable than five classic and popular algorithms. [ABSTRACT FROM AUTHOR]

In seismology and earthquake engineering, it is fundamental to identify and characterize the pulse-like features in pulse-type ground motions. To capture the pulses that dominate structural responses, this study establishes congruence and shift relationships between response spectrum surfaces. A similarity search between spectrum surfaces, supplemented with a similarity search in time series, has been applied to characterize the pulse-like features in pulse-type ground motions. The identified pulses are tested in predicting the rocking consequences of slender rectangular blocks under the original ground motions. Generally, the prediction is promising for the majority of the ground motions where the dominant pulse is correctly identified. [ABSTRACT FROM AUTHOR]

To enhance our comprehension of the dynamic processes associated with complex plate subduction, volcanic magmatism, and lithosphere deformation beneath the Ryukyu–Taiwan–Philippines region, we utilized nonuniform inversion grids for Pn velocity and anisotropy tomography, and obtained the uppermost mantle structure of this area. The results demonstrate remarkable characteristics: cold oceanic subducting plates display high Pn velocities, whereas volcanic arcs, the extinct mid‐ocean ridge in the South China Sea, and the Palawan region exhibit low Pn velocities. The extinct mid‐ocean ridge also displays a low‐velocity anomaly, indicating that residual heat persists even after approximately 15 million years since seafloor spreading. The occurrence of a discontinuous low‐velocity beneath the Ryukyu arc supports the presence of a slab window at approximately 123°E. The volcanic arcs of all subduction zones within the study area displayed trench‐parallel Pn anisotropy. The observed Pn fast directions beneath the Taiwan orogenic belt are consistent with crustal anisotropy, providing evidence for the crust–mantle coupled deformation. Moreover, our results shed light on the deep structural characteristics of the complex subduction zones beneath the Philippines region. Plate subduction causes partial melting due to dehydration; then, the melt ascends and accumulates at the uppermost mantle, showing low Pn velocities. However, the low‐velocity anomaly is not widely distributed but corresponds to a narrow band. In particular, the east–west bidirectional subduction of the Philippines and Negros formed separate low‐velocity anomalies. Low Pn velocities and trench‐parallel anisotropy indicate the location of different subduction zones and finely characterize their impact on the uppermost mantle structure. Plain Language Summary: The Ryukyu–Taiwan–Philippines region serves as a convergence zone for multiple subduction systems and has undergone a complex tectonic evolution. In this study, we utilized an improved uppermost mantle seismic tomography method to investigate the precise Pn velocity and anisotropy structure of this area. The results reveal distinct patterns, wherein high Pn velocities are observed beneath the Eurasian plate, the South China Sea plate, and the Philippine Sea plate. In addition to the volcanic arcs, an extinct mid‐ocean ridge also shows low Pn velocity. The identification of discontinuous low velocities at approximately 123°E beneath the Ryukyu arc supports the existence of a slab tear. Pn fast directions, which align with the orogenic belt's trend, are observed beneath Taiwan Island. This observation concurs with surface and crustal deformation, suggesting that the deformation of the Taiwan orogenic belt is at a lithospheric scale. Low Pn velocities observed beneath the multiple subduction zones in the Philippines region indicate that the magma generated by this process is confined to a narrow band parallel to the subduction zone, instead of being widely distributed. The study precisely depicts the impact of plate subduction on the uppermost mantle structure of the Philippines region. Key Points: We employed a nonuniform grid Pn anisotropic tomography to obtain the uppermost mantle structure in the Ryukyu–Taiwan–Philippines regionThe consistency of Pn and the crustal anisotropy beneath the Taiwan orogenic belt supports crust–mantle coupled deformationThe results beneath the Philippines region indicate the influence of subduction in different directions on the uppermost mantle structure [ABSTRACT FROM AUTHOR]

We compute stress drops for earthquakes in Northern Chile recorded between 2007 and 2021. By applying two analysis techniques, (a) the spectral ratio (SR) method and (b) the spectral decomposition (SDC) method, a stress drop map for the subduction zone consisting of 51,510 stress drop values is produced. We build an extended set of empirical Green's functions (EGF) for the SR method by systematic template matching. Outputs are used to compare with results from the SDC approach, where we apply cell‐wise obtained global EGF's to compensate for the structural heterogeneity of the subduction zone. We find a good consistency of results of the two methods. The increased spatial coverage and quantity of stress drop estimates from the SDC method facilitate a consistent stress drop mapping of the different seismotectonic domains. Albeit only small differences of median stress drop, strike‐perpendicular depth sections clearly reveal systematic variations, with earthquakes at different seismotectonic locations exhibiting distinct values. In particular, interface seismicity is characterized by the lowest observed median value, whereas upper plate earthquakes show noticeably higher stress drop values. Intermediate depth earthquakes show comparatively high average stress drop and a rather strong depth‐dependent increase of median stress drop. Additionally, we observe spatio‐temporal variability of stress drops related to the occurrence of the two megathrust earthquakes in the study region. The presented study is the first coherent large scale 3D stress drop mapping of the Northern Chilean subduction zone. It provides an important component for further detailed analysis of the physics of earthquake ruptures. Plain Language Summary: Stress drop is the released stress on a fault during a seismic rupture. We compute stress drops for earthquakes in northern Chile recorded between 2007 and 2021. By applying two different analysis techniques, (a) the spectral ratio method and (b) the spectral decomposition method, a stress drop map for the subduction zone consisting of 51,510 stress drop values is produced. We find a good consistency of results of the two methods. The good spatial coverage and quantity of stress drop estimates allow a consistent stress drop mapping of the seismically active domains. Although differences of median stress drops are relatively small, the depth section clearly reveals systematic variations, with earthquakes of different classes. In particular, interface seismicity is characterized by the low median values, whereas upper plate earthquakes show noticeably higher stress drops. Intermediate depth earthquakes show comparatively high average stress drop and depth‐dependence. Additionally, we observe variability of stress drops related to the occurrence of the two megathrust earthquakes in the study region. The here presented study is the first large scale 3D stress drop mapping of the Northern Chilean subduction zone. It provides an important component for further analysis of the physics of earthquake ruptures. Key Points: A comprehensive stress drop distribution with more than 51,000 stress drop estimates for the Northern Chilean Subduction Zone is computedSystematic stress drop variations between upper plate, interface and intermediate depth seismicity are revealedReliability and comparability of results are increased by using two methods, the spectral ratio and the spectral decomposition approach [ABSTRACT FROM AUTHOR]

The evolution of fluid injection‐induced seismicity, generally characterized through the number of events or their seismic moment, depends on, among other factors, the injected fluid volume. Migration of seismicity is observed during those sequences and might be caused by a range of mechanisms: fluid pressure diffusion, fluid‐induced aseismic slip propagating along a stimulated fault, interactions between earthquakes. Recent theoretical and observational developments underline the important effect on seismicity migration of structural parameters, like fault criticality, or injection parameters, like flow rate or pressurization rate. Here, we analyze two well‐studied injection‐induced seismic sequences at the Soultz‐sous‐Fôret and Basel geothermal sites, and find that the evolution of the seismicity front distance primarily depends on the injected fluid volume. Based on a fracture mechanics model, we develop new equations relating seismicity migration to injected fluid volume and frictional and structural properties of the fault. We find that the propagation of a fluid‐induced aseismic slip front along the stimulated fault, triggering seismicity, explains well the observations made on the two sequences. This model allows us to constrain parameters describing the seismicity front evolution and explains the diversity of migration patterns observed in injection‐induced and natural earthquake swarms. Plain Language Summary: Injection of fluids at depth may induce earthquakes, which can represent a hazard both for reservoir activities and populations. The magnitude and number of earthquakes depend, at first order, on the injected fluid volume. Induced seismicity has also been observed to migrate away from the injection area, with a diversity of velocities and patterns. Migration has been attributed to several mechanisms like the diffusion of a pore pressure perturbation along the fault, earthquakes triggering each other, or the propagation of a silent aseismic slip. Here, we combine recent theoretical and observational developments to propose a new model to explain the migration of seismicity in two well‐studied induced sequences in France and Switzerland. Our model directly relates migration distance to injected fluid volume and other physical parameters describing the frictional or stress properties of the stimulated faults. This generic model explains seismicity propagation and provides constraints on the relevant physical parameters controlling it. We apply our approach to earthquake swarms occurring in natural context, and reconstruct fluid circulation dynamics during those sequences. Key Points: Induced seismicity front position correlates with cumulative injected fluid volumeAn aseismic crack model driven by fluid injection explains the observed slip distribution and seismicity migrationsOur results enable the reconstruction of a wide range of migration patterns within earthquake swarms and their injection history [ABSTRACT FROM AUTHOR]

Geodynamic modeling and seismic studies have highlighted the possibility that a thin layer of low seismic velocities, potentially molten, may sit atop the core‐mantle boundary but has thus far eluded detection. In this study we employ normal modes, an independent data type to body waves, to assess the visibility of a seismically slow layer atop the core‐mantle boundary to normal mode center frequencies. Using forward modeling and a data set of 353 normal mode observations we find that some center frequencies are sensitive to one‐dimensional kilometer‐scale structure at the core‐mantle boundary. Furthermore, a global slow and dense layer 1–3 km thick is better‐fitting than no layer. The well‐fitting parameter space is broad with a wide range of possible seismic parameters, which precludes inferring a possible composition or phase. Our methodology cannot uniquely detect a layer in the Earth but one should be considered possible and accounted for in future studies. Plain Language Summary: Normal modes are long‐period oscillations of the whole Earth as it vibrates after large earthquakes. The frequency that a mode oscillates at depends on the interior structure of the Earth. Research suggests a global and thin layer of anomalous composition and low seismic wave speeds may have formed at the base of Earth's mantle, but would be difficult to observe seismically. We test and quantify the effect of this layer on the frequencies at which normal modes vibrate. We then compare these predictions to a large data set of normal mode frequency measurements to examine whether such a layer is consistent with observed data. We find that not only is a layer of 1–3 km thickness permitted by the modes, but that a layer being present improves the fit to the data. There are a wide range of parameters that adequately fit the data set so we cannot be specific about its properties. Furthermore a layer is likely not a unique way to improve the model. A seismically slow layer at the core‐mantle boundary has implications for processes in the mantle and outer core and the interaction between them. Key Points: Normal mode center frequencies are a sensitive data type to detect seismically anomalous thin layers at the core‐mantle boundaryA slow and dense layer on the order of 1 km thick atop the core‐mantle boundary can improve the fit to normal mode dataThe inclusion of a layer is likely not a unique way to improve the 1D model and layer properties remain uncertain [ABSTRACT FROM AUTHOR]