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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]

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]

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]

Seismic activity in the Northern Canadian Cordillera is characterized by diffuse earthquakes that extend hundreds of km northwest from the Yakutat collision zone. We use 25 months of broadband seismic data from Mackenzie Mountain Earthscope Project (MMEP), USArray Transportable Array (TA), and permanent Canadian National Seismic Network stations to present a local earthquake catalog with high sensitivity to small regional events. Deep learning techniques are adopted for both seismic phase detection and association. Event relocations are performed to provide well constrained estimates of earthquake depth distributions. Clusters of seismicity spanning the upper crust are located in the central Richardson Mountains, along the Tintina fault, and in the northeast Selwyn Basin. These clusters suggest that the core of the Richardson Anticlinorium is tectonically active and that the Tintina fault is a locus for low levels of active deformation. We interpret seismicity in the northeast Selwyn Basin as primarily occurring in the hanging wall of the Plateau thrust fault and suggest that some combination of localized duplex structures and lithological strength contrasts both within the Selwyn Basin and between abutting Paleozoic shelf sequences may be responsible for seismicity in the Mackenzie Mountain foreland. Plain Language Summary: The Northern Canadian Cordillera is situated inboard of a transition from a right‐lateral tectonic regime in coastal southeast Alaska to a subduction setting beneath southern Alaska. It presents a unique case study to examine the length scales over which tectonic deformation is able to permeate into a continental interior. In this study we use data from a relatively dense regional network of seismometers in order to present the most spatially complete catalog of earthquakes in the Northern Canadian Cordillera to date. We use machine learning techniques for the detection and classification of small magnitude earthquakes and use our results to analyze regional tectonics. Active faults that host earthquakes are identified and we use the earthquake depth distributions to further distinguish between mechanisms for regional stress transfer. We find evidence that the Tintina fault is presently active along a confined area at a low rate. We also find that low magnitude earthquakes do not generally occur within the Mackenzie Mountain Fold‐Thrust belt but are prevalent within the foreland Selwyn Basin. We also detect a pattern of relatively deep (∼30 km) earthquakes in the northern Richardson Mountains. Key Points: Small magnitude earthquakes extending to 25 km depth are detected throughout the Selwyn Basin in the Mackenzie Mountain forelandThe Tintina fault hosts a range of low magnitude earthquakes in the uppermost 15 km of crust within a 400 km zone along strikeDeformation in the Richardson Mountains is localized onto a narrow band of faults centered on the Richardson Anticlinorium [ABSTRACT FROM AUTHOR]

This paper calculated the Duration Magnitude (MD) equation using data from the Iraqi Meteorological Organization and seismology (IMOS). It is an empirically determined equation and expressed as: MD = 0.8582 × log(τcoda) - 0.0000262 × R + 2.1203 ± c The epicentral distance and local geological conditions affect the duration of the coda. The data is obtained from 7 seismic stations in the network. The new proposed duration magnitude equation results from applying linear regression analysis to the data of a seismic signal duration with correlation coefficient R²=0.76; and a standard deviation value of 0.049. Station corrections indicated by Sc are also specified for the seismic stations that range from -0.024 to + 0.02. [ABSTRACT FROM AUTHOR]

To preserve the analog records of historical earthquakes which occurred in and around Iran in digital form and to make them available for modern analysis techniques that require waveforms, we decided to digitize seismograms of destructive earthquakes that occurred between 1960 and 1990. The seismograms have been recorded at five seismic stations deployed in Tehran, Mashhad, Kermanshah, Tabriz, and Shiraz by the Institute of Geophysics of the University of Tehran (IGUT). The restoration project of the old analog seismograms archived in the IGUT started in 2018. Seismograms were sorted by station and date to create a hardcopy databank, and then those from large earthquakes and their larger aftershocks were chosen for the scanning process. Seismograms of 52 significant earthquakes and their aftershocks with magnitudes greater than 5.0 that occurred in and around Iran were scanned. Besides that, seismograms of several significant and huge teleseismic earthquakes that occurred between 1960 and 1990 were scanned. At present, almost 600 scanned seismograms are available. The current digitization process in IGUT is done manually and is based on waveform vectorization. The provided databank is one of the most important sources of information for research in seismology and active tectonics in and around Iran. [ABSTRACT FROM AUTHOR]

Robust estimates of the Earth's temperature, composition and partial melt are difficult to obtain from seismic tomography models based only on wave velocities. Seismic attenuation is able to add crucial additional information and allows us to constrain 3-D variations in temperature as well as answer fundamental questions regarding the presence of water and partial melt within the mantle. A major problem with measuring attenuation is that scattering and focusing effects need to be included in order to distinguish between intrinsic attenuation (transformation of energy to heat) and scattering (redistribution of energy). Here, we will use whole Earth oscillations or normal modes, exploiting the fact that small scale scattering becomes less important at longer periods. In addition, focussing is implicitly included by simultaneously measuring the elastic and anelastic splitting function of a given normal mode. Normal mode data has rarely been used before to constrain 3-D variations in attenuation, hence we first need to assess the feasibility of using whole Earth oscillations to image 3-D variations in attenuation in Earth's upper and lower mantle. Here, we use simple models in forward calculations to evaluate the sensitivity of normal modes to 3-D attenuation variations, and find observable differences in normal mode spectra when 3-D variations in mantle attenuation are included. We also test if 3-D attenuation variations can be recovered using splitting function measurements for synthetic normal mode spectra and investigate potential problems with unaccounted earthquake magnitude variations, noise and wide-band cross-coupling between modes. We find that we are able to recover input synthetic anelastic splitting functions, even when they are an order of magnitude smaller than the elastic splitting functions. Biases in the earthquake seismic moment solutions may lead to 'ghost' anelastic splitting functions being recovered, especially for high degree structure. However, this 'ghost' signal is only relevant if the size of the input anelastic splitting function is 50 per cent smaller than the scaled amplitude signal of expected upper mantle attenuation. Random noise and normal mode coupling also lead to spurious effects in the anelastic splitting functions. However, they manly affect high degree anelastic structure in a non-branch consistent manner. Thus, we are able to robustly recover low anelastic degree structure (s max ≤ 4) in all tested cases. [ABSTRACT FROM AUTHOR]

An innovative fiber-optic hydrophone (FOH) was developed and investigated via an experiment at sea; it is capable of operating at a very low frequency of the seismic spectrum and detecting small magnitude earthquakes. The FOH exploits an optical fiber coil wrapped around a sensitive mandrel in a Michelson interferometric configuration. The FOH operated for about seven days at a water depth of 40 m, in the Campi Flegrei volcanic area (Southern Italy), and a few meters from a well-calibrated PZT hydrophone used as a reference. Thirty-three local earthquakes occurred during the simultaneous operation of the two hydrophones, allowing a straightforward comparison of the recordings. The local earthquakes occurred at an epicentral distance less than 2.5 km from the site of recording, and were estimated to be in the range of magnitude from −0.8 to 2.7. The analysis of the recorded earthquake waveforms in the frequency and time domains allowed retrieving the response function of the FOH in the frequency range from 5 to 70 Hz. The FOH responsivity in terms of acoustic pressure reached about 230 nm/Pa and was flat in the studied frequency range. Due to the high quality of the FOH recordings, this equipment is suitable for applications addressing submarine volcanic activity and the background seismicity of active faults in the ocean. [ABSTRACT FROM AUTHOR]

Volcanic inflation and deflation often precede eruptions and can lead to seismic velocity changes (dv/v $dv/v$) in the subsurface. Recently, interferometry on the coda of ambient noise‐cross‐correlation functions yielded encouraging results in detecting these changes at active volcanoes. Here, we analyze seismic data recorded at the Klyuchevskoy Volcanic Group in Kamchatka, Russia, between summer of 2015 and summer of 2016 to study signals related to volcanic activity. However, ubiquitous volcanic tremors introduce distortions in the noise wavefield that cause artifacts in the dv/v $dv/v$ estimates masking the impact of physical mechanisms. To avoid such instabilities, we propose a new technique called time‐segmented passive image interferometry. In this technique, we employ a hierarchical clustering algorithm to find periods in which the wavefield can be considered stationary. For these periods, we perform separate noise interferometry studies. To further increase the temporal resolution of our results, we use an AI‐driven approach to find stations with similar dv/v $dv/v$ responses and apply a spatial stack. The impacts of snow load and precipitation dominate the resulting dv/v $dv/v$ time series, as we demonstrate with the help of a simple model. In February 2016, we observe an abrupt velocity drop due to the M7.2 Zhupanov earthquake. Shortly after, we register a gradual velocity increase of about 0.3% at Bezymianny Volcano coinciding with surface deformation observed using remote sensing techniques. We suggest that the inflation of a shallow reservoir related to the beginning of Bezymianny's 2016/2017 eruptive cycle could have caused this local velocity increase and a decorrelation of the correlation function coda. Plain Language Summary: Before eruptions, volcanoes inflate due to the rising magma from below. Previous studies have found that these deformations can lead to small changes in the properties of the surrounding rock. We use passive image interferometry, a method that relies on the omnipresent background vibration of the Earth—mostly induced by the oceans, to measure these changes at the Klyuchevskoy Volcanic Group in Kamchatka, Russia. However, in Kamchatka, this background noise is masked and distorted by small earthquakes and tremors originating from the volcanoes themselves. We combine machine learning techniques with established monitoring methods to find times when these tremors remain similar. Afterward, we use data from these time periods in the conventional way to observe changes in the soil and the rock. Our results show that rain‐ and snowfall and the thickness of the snow cover exert the strongest influence on the properties of the rocks. Additionally, we found that a large magnitude 7.2 earthquake, which struck Kamchatka during our study, caused a slight weakening of the rocks due to microstructural damage. We register changes shortly before an eruption and suggest a connection to the beginning of an eruptive cycle in 2016. Key Points: Fluctuating noise conditions lead to distortions in noise interferometry studies, which we avoid with the help of machine learningThe seismic velocity on Kamchatka is affected by numerous mechanisms, amongst them environmental, tectonic, and volcanic eventsWe observe a velocity increase at Bezymianny during February 2016 and link it to the beginning of the eruptive cycle [ABSTRACT FROM AUTHOR]

Earthquakes are one of nature's most devastating disasters. Earthquake prediction is critical in seismology since its success can save lives, property, and infrastructure. Numerous technologies have been proposed to address this issue, including mathematical analysis, artificial intelligence, and machine learning algorithms. Unfortunately, due to earthquakes' dynamic and spontaneous nature, they frequently fail to provide positive results. The study uses deep learning techniques to predict the magnitude of an impending earthquake using eight mathematically calculated seismic indicators derived from Japan, Indonesia, and the Hindu-Kush Karakoram Himalayan (HKKH) region's earthquake catalogs. Three deep learning techniques, including Long Short-Term Memory (LSTM), Bi-directional Long Short-Term Memory (Bi-LSTM), and self-attention-based transformer, have been implemented to model the associations between calculated seismic indicators and potential earthquake incidents. These models have been evaluated with well-known matrices such as the Mean Absolute Error (MAE), Mean Squared Error (MSE), log-cosh loss, and Mean Squared Logarithmic Error (MSLE). The value of these cost functions converges to a small number for all models, indicating that these models effectively predict earthquake magnitudes. When these models were fed with an unknown test dataset from Japan, the LSTM model performed best with the least deviation metrics (MAE = 0.060, MSE = 0.006, log cosh = 0.042 and MSLE = 0.003). Similarly, the Bi-LSTM model delivered the ideal result for the Indonesia earthquake catalog (MAE = 0.073, MSE = 0.009, log cosh = 0.016, and MSLE = 0.009), while the transformer model produced the optimal result for the HKKH region (MAE = 0.062, MSE = 0.006, log cosh = 0.043, and MSLE = 0.003). Predicting earthquake magnitude at various locations using these methodologies produces significant and positive results for magnitudes ranging from 3.5 M to 6.0 M, paving the way for the ultimate robust prediction mechanism, that has not yet been developed. [ABSTRACT FROM AUTHOR]

Among the most important questions that await an answer in seismology, perhaps one is whether there is a correlation between the magnitudes of two successive seismic events. The answer to this question is considered of fundamental importance given the potential effect in forecasting models, such as Epidemic Type Aftershock Sequence models. After a meta-analysis of 29 papers, we speculate that given the lack of studies carried out with realistic physical models and given the possible bias due to the lack of events recorded in the experimental seismic catalogs, important improvements are necessary on both fronts to be sure to provide a statistically relevant answer. [ABSTRACT FROM AUTHOR]

The massive worldwide deglaciation leads to more frequent slope instabilities in mountainous terrains. The physical processes leading to such destabilizations are poorly constrained due to little monitoring of dynamic parameters at the local scale. Here we study a very large slow‐moving landslide (∼0.8 km2), on the flank of Tungnakvíslarjökull glacier in Iceland. Based on a combination of remote sensing images, we monitor the landslide and glacier kinematics over 75 years, with a focus over the period 1999–2019 when rapid glacier wastage has been observed. The landslide accelerates from 2 to 45 m/yr in the 6 years following a sudden increase in glacier mass loss. This acceleration coincides with intense quake activity (Mℓ < 2.8), recorded by a regional seismic network. We show that this seismicity is caused by the landslide sliding on a rough surface. The evolution of the quake magnitudes suggests a progressive segmentation of the landslide mass during its acceleration. Plain Language Summary: Glacier‐retreat may lead to a destabilization of adjacent slopes that can evolve into large landslides. The time‐scales, as well as the controlling factors of the landslide response to this glacier retreat are poorly known, due to few observations of landslide dynamics in the vicinity of glaciers over the long term. To fill this gap, we document the dynamics of a large landslide in Iceland, developing on the flank of Tungnakvíslarjökull glacier, that underwent strong retreat after 1994. Based on aerial images since 1945 and satellite images since 1987, we calculate the motion of the landslide through time. We show that the landslide accelerated from 2 to 45 m/yr in the 6 years following the onset of glacier retreat. This acceleration coincides with earthquakes of small magnitudes (Mℓ < 2.8), produced by the landslide motion. Their analysis suggests that the landslide mass is progressively fragmented during the acceleration. This study opens interesting perspectives for landslide studies by combining seismic records from regional networks and 3D motions from satellites. Key Points: We monitor the dynamics of a very large slow‐moving landslide over 75 years, developing on the flank of a glaciated valleyThe landslide kinematics is strongly controlled by the glacier retreat, and produce seismicity correlated with the landslide velocityThe acceleration is linked with progressive segmentation of the landslide mass [ABSTRACT FROM AUTHOR]

Rupture directivity and its potential frequency dependence is an open issue within the seismological community, especially for small‐to‐moderate events. Here, we provide a statistical overview based on empirical evidence of seismological observations, thanks to the large amount of high‐quality seismic recordings (more than 30,000 waveforms) from Central Italy, which represents an excellent and almost unique natural laboratory of normal faulting earthquakes in the magnitude range between 3.4 and 6.5 within the time frame 2008–2018. In order to detect an anisotropic distribution of ground motion amplitudes due to the rupture directivity, we fit the smoothed Fourier Amplitude Spectra (FAS) cleared of source‐, site‐ and path‐effects. According to our criteria, about 36% of the analyzed events (162 out of 456) are directive and the distribution of rupture direction is aligned with the strikes of the major faults of the Central Apennines. We find that the directivity is a band‐limited phenomenon whose width may extend up to five times the corner frequency. The results of this research provide useful insights to parameterize directivity, to be explicitly implemented in future ground motion modeling and scenario predictions. Plain Language Summary: In seismology, directivity is one of the source phenomena that causes large spatial variability of earthquake ground motions and is related to the features of the rupture propagation along the fault. The importance of this effect is very well known for large‐magnitude events (M > 6), while it is still an open issue for small‐to‐moderate events. The aim of this paper is to recognize directive events and quantify their strength in Central Italy, using a large data set of earthquakes in the magnitude range 3.4–6.5 that occurred between 2008 and 2018. We find that about 36% of the analyzed events are directive, caused by rupture propagations oriented along the NW‐SE alignment of the Central Apennines fault systems. Furthermore, we find that directivity is a band‐limited phenomenon and that as the directivity gets stronger, the frequency band becomes wider. Our contribution provides a useful insight with the possibility to improve the parametrization of directivity within the empirical seismic hazard assessment. Key Points: Statistical procedure to detect directivity effects in small‐to‐moderate earthquakes analyzing the aleatory residuals of the ground motionThe directive events are the 36% of the data set and the distribution of rupture direction is aligned with the strikes of the faults systemThe observed directivity is a frequency‐dependent band‐limited phenomenon, which can occur up to five times the value of corner frequency [ABSTRACT FROM AUTHOR]

Among the multitude of magnitude scales developed to measure the size of an earthquake, the surface wave magnitude Ms is the only magnitude type that can be computed since the dawn of modern observational seismology (beginning of the 20th century) for most shallow earthquakes worldwide. This is possible thanks to the work of station operators, analysts and researchers that performed measurements of surface wave amplitudes and periods on analogue instruments well before the development of recent digital seismological practice. As a result of a monumental undertaking to digitize such pre-1971 measurements from printed bulletins and integrate them in parametric data form into the database of the International Seismological Centre (ISC, http://www.isc.ac.uk , last access: August 2021), we are able to recompute Ms using a large set of stations and obtain it for the first time for several hundred earthquakes. We summarize the work started at the ISC in 2010 which aims to provide the seismological and broader geoscience community with a revised Ms dataset (i.e., catalogue as well as the underlying station data) starting from December 1904 up to the last complete year reviewed by the ISC (currently 2018). This Ms dataset is available at the ISC Dataset Repository at 10.31905/0N4HOS2D. [ABSTRACT FROM AUTHOR]

Part of North-Western Himalaya particularly Uttarakhand has been identified by several eminent seismologists as gap area where large size earthquake (greater than magnitude 7) is expected. The region has witnessed two earthquakes having a magnitude greater than 6 in last 25 years (March 28, 1999 Chamoli earthquake and October 20, 1991 Uttarkashi earthquake) both having an epicenter in Gharwal region of Uttarakhand. Several studies have shown that a future large earthquake in central Himalayas can generate severe ground motion in the National capital region of Delhi, which is about 300 km from this expected source. Several densely populated cities and villages having total population of several millions are located between Delhi and Uttarakhand. These towns/cities will be severely affected by a large earthquake having an epicenter in this region. Using advancements in communication technology and real-time seismology, a project to have an earthquake early warning system for Northern India is under progress at Indian Institute of Technology, Roorkee. In this project, a dense network of 84 accelerometers has been installed covering an area of about 100 × 40 km in Garhwal region. This network has an average station to station distance of less than 10 km and all the sensors are streaming data, which is being processed in real time at central server stationed at Roorkee. This paper describes details of the network, sensors, present status of development, performance of instrumentation during recent events, and processing details. [ABSTRACT FROM AUTHOR]

Improving the capability of seismic network to detect weak seismic events is one of the timeless challenges in seismology: the greater is the number of detected and locatable seismic events, the greater insights on the mechanisms responsible for seismic activation may be gained. Here we implement and apply a single-station template matching algorithm to detect events belonging to the fluid-injection induced seismicity cluster located in the High Agri Valley, Southern Italy, using the continuous seismic data stream of the closest station of the INSIEME network. To take into account the diversity of waveforms, albeit belonging to the same seismic cluster, eight different master templates were adopted. Afterwards, using all the stations of the network, we provide a seismic catalogue consisting of 196 located earthquakes, in the magnitude range − 1.2 ≤ Ml ≤ 1.2, with a completeness magnitude Mc = − 0.5 ± 0.1. This rich seismic catalogue allows us to describe the damage zone of a SW dipping fault, characterized by a variety of fractures critically stressed in the dip range between ~ 45° and ~ 75°. The time-evolution of seismicity clearly shows seismic swarm distribution characteristics with many events of similar magnitude, and the seismicity well correlates with injection operational parameters (i.e. injected volumes and injection pressures). [ABSTRACT FROM AUTHOR]

How earthquakes initiate is still a largely debated question in earthquake science. On the lab scale, rupture initiation is well studied, and detailed models of how rupture nucleation evolves have been developed. Contrarily, for real earthquakes, only a few high‐resolution observations of this process are available today, mostly limited to mainshock magnitudes > M5. Consequently, there is still no consensus on whether and how laboratory results can be transferred to real earthquakes. Here we show that rupture nucleation phenomena observed on the lab scale can also be imaged on the microearthquake‐scale with little instrumental effort. Our results highlight the potential of the applied analysis workflow to significantly improve the observation quality of seismicity patterns and immediate foreshock phenomena in microearthquake sequences. Our approach can help to narrow the existing observational gap to the lab scale and may contribute to a better understanding of the mechanisms of earthquake initiation in the future. Plain Language Summary: One of the key questions in seismology is how earthquakes start. Until now, many models of earthquake initiation have been established through laboratory experiments. But for real earthquakes, only a few detailed observations of rupture initiation exist because they are mainly restricted to large earthquakes (M > 5). Consequently, it is not known today if and how the results obtained in the laboratory can be transferred to real earthquakes. In this study, we show that rupture initiation can be studied with close‐to‐lab‐scale detail in foreshock sequences to small earthquakes (M > 2.5). Small earthquakes occur much more frequently than large ones ‐ generally 10 times more per unit decrease in earthquake size (i.e., magnitude). Therefore, our results indicate that the database of detailed rupture initiations in real earthquakes can be extended substantially. Our analysis method that generates decade‐long, high‐resolution, and consistent catalogs of sequences of very small earthquakes, helps to narrow the existing observational gap to the lab scale. In this way, it can contribute to a better understanding of earthquake initiation mechanisms in the future. Key Points: Observe precursory phenomena to ML ≤ 3.2 mainevents in a microseismic sequenceImage high‐resolution spatiotemporal evolution of the immediate foreshock zone of small earthquakesObserve lab‐scale‐like rupture initiation phenomena on the field scale with little instrumental effort similar to lab scale experiments [ABSTRACT FROM AUTHOR]

Datong Window is at the junction of Datong Basin and Liulengshan Range in the northern part of Shanxi Graben, China. It is important to analyze the relationship between the frequency of small earthquakes and the deep velocity structure in this region since the earthquake swarm in 1989 with the maximum surface-wave magnitude Ms of 5.9. A high-resolution 3D P-wave velocity model of the crust in this region was obtained with double-difference tomography using seismic data from July 2009 to June 2019. The results show that the Datong-Yanggao seismic sequence occurred, in general, within the NE rupture zone of the earthquake swarm of 1989. The sequences from 1991, 1999, and 2010 were in the rupture zone of 1989. A series of earthquakes developed in this region, which is at the intersection of multiple faults and a low-velocity anomaly of the intermediate-lower crust. A gap of length of about 3 km was found at the southwest end of the NE Dawangcun Fault (near Lujialing). Several moderate earthquake ruptures failed to break through it. It was speculated that it may be a potential asperity. There is a possibility of an earthquake with magnitude about 5 from the gap. The relocation results for the earthquakes show that intensive small-earthquake events in Datong Window corresponding to moderate-to-strong earthquakes in Shanxi Graben were highly clustered horizontally and distributed vertically in a column. In contrast, when there was no moderate earthquake in Shanxi Graben after intensive small-earthquake events, the spatial distribution of these events in Datong Window was scattered and dispersive. [ABSTRACT FROM AUTHOR]

How earthquakes initiate is still a largely debated question in earthquake science. On the lab scale, rupture initiation is well studied, and detailed models of how rupture nucleation evolves have been developed. Contrarily, for real earthquakes, only a few high‐resolution observations of this process are available today, mostly limited to mainshock magnitudes > M5. Consequently, there is still no consensus on whether and how laboratory results can be transferred to real earthquakes. Here we show that rupture nucleation phenomena observed on the lab scale can also be imaged on the microearthquake‐scale with little instrumental effort. Our results highlight the potential of the applied analysis workflow to significantly improve the observation quality of seismicity patterns and immediate foreshock phenomena in microearthquake sequences. Our approach can help to narrow the existing observational gap to the lab scale and may contribute to a better understanding of the mechanisms of earthquake initiation in the future. Plain Language Summary: One of the key questions in seismology is how earthquakes start. Until now, many models of earthquake initiation have been established through laboratory experiments. But for real earthquakes, only a few detailed observations of rupture initiation exist because they are mainly restricted to large earthquakes (M > 5). Consequently, it is not known today if and how the results obtained in the laboratory can be transferred to real earthquakes. In this study, we show that rupture initiation can be studied with close‐to‐lab‐scale detail in foreshock sequences to small earthquakes (M > 2.5). Small earthquakes occur much more frequently than large ones ‐ generally 10 times more per unit decrease in earthquake size (i.e., magnitude). Therefore, our results indicate that the database of detailed rupture initiations in real earthquakes can be extended substantially. Our analysis method that generates decade‐long, high‐resolution, and consistent catalogs of sequences of very small earthquakes, helps to narrow the existing observational gap to the lab scale. In this way, it can contribute to a better understanding of earthquake initiation mechanisms in the future. Key Points: Observe precursory phenomena to ML ≤ 3.2 mainevents in a microseismic sequenceImage high‐resolution spatiotemporal evolution of the immediate foreshock zone of small earthquakesObserve lab‐scale‐like rupture initiation phenomena on the field scale with little instrumental effort similar to lab scale experiments [ABSTRACT FROM AUTHOR]

On‐fault earthquake magnitude distributions are calculated for northern Caribbean faults using estimates of fault slip and regional seismicity parameters. Integer programming, a combinatorial optimization method, is used to determine the optimal spatial arrangement of earthquakes sampled from a truncated Gutenberg‐Richter distribution that minimizes the global misfit in slip rates on a complex fault system. Slip rates and their uncertainty on major faults are derived from a previously published GPS block model for the region, with fault traces determined from offshore geophysical mapping and previously published onshore studies. The optimal spatial arrangement of the sampled earthquakes is compared with the 500‐year history of earthquake observations. Rupture segmentation of the subduction interface along the Hispaniola‐Puerto Rico Trench (PRT) fault and seismic coupling on the PRT fault appear to exert the primary control over this spatial arrangement. Introducing a rupture barrier for the Hispaniola‐PRT fault northwest of Mona Passage, based on geophysical and seismicity observations, and assigning a low slip rate of 2 mm/yr on the PRT fault are most consistent with historical earthquakes in the region. The addition of low slip‐rate secondary faults as well as segmentation of the Hispaniola and Septentrional strike‐slip fault improves the consistency with historical seismicity. An important observation from the modeling is that varying the slip rate on the PRT fault and different segmentation scenarios result in significant changes to the optimal magnitude distribution on faults farther away. In general, optimal on‐fault magnitude distributions are more complex and inter‐dependent than is typically assumed in probabilistic seismic hazard analysis and probabilistic tsunami hazard analysis. Plain Language Summary: High population density and unreinforced structures make the northern Caribbean region especially susceptible to earthquakes. The region's complex fault system includes a major offshore fault north of Hispaniola and Puerto Rico separating two tectonic plates, as well as several major crustal faults crossing the islands. During the region's 500‐year recorded history, major earthquakes and tsunamis, including the devastating 2010 and 2021 earthquakes in Haiti, resulted from movement along these faults. The range of magnitudes for each fault is critical in assessing earthquake hazard throughout the region. In this study, we employ a novel method, integer programming, from the disparate field of operations research to determine the optimal distribution of large earthquakes on the northern Caribbean fault system. Rather than assessing each fault independently, the distribution of earthquakes under various scenarios is determined for the system as a whole, with specific assumptions regarding fault geometry and slip rate on one fault affecting the forecasted magnitudes on other faults. We find that segmentation of major faults increases the frequency earthquakes in the range of magnitude 6.0–7.0. In addition, the magnitude of the largest earthquakes occurring on the offshore plate boundary faults may be significantly less than previously assumed for tsunami hazard assessments. Key Points: On‐fault earthquake magnitude distributions for northern Caribbean faults are calculated using the integer programming methodHistorical earthquakes, including the 2010 and 2021 Haiti earthquakes, are used to evaluate scenarios of fault segmentation and couplingFault characterization assumptions on one fault influence earthquake magnitude distributions on other faults farther away [ABSTRACT FROM AUTHOR]

Among the multitude of magnitude scales developed to measure the size of an earthquake, the surface wave magnitude MS is the only magnitude type that can be computed since the dawn of modern observational seismology (beginning of the 20th century) for most shallow earthquakes worldwide. This is possible thanks to the work of station operators, analysts and researchers that performed measurements of surface wave amplitudes and periods on analogue instruments well before the development of recent digital seismological practice. As a result of a monumental undertaking to digitize such pre-1971 measurements from printed bulletins and integrate them in parametric data form into the database of the International Seismo- logical Centre (ISC, www.isc.ac.uk, last access: August 2021), we are able to recompute MS using a large set of stations and obtain it for the first time for several hundred earthquakes. We summarize the work started at the ISC in 2010 which aims to provide the seismological and broader geoscience community with a revised MS dataset (i.e., catalogue as well as the underlying station data) starting from December 1904 up to the last complete year reviewed by the ISC (currently 2018). This MS dataset is available at the ISC Dataset Repository at https://doi.org/10.31905/0N4HOS2D. [ABSTRACT FROM AUTHOR]

Performance and possible causes of seismic activity in the southern of Santiago de Cuba province were evaluated, based on the geological and tectonic information available and records of the earthquakes occurred in this region in January 17, 2016 at 08:17 UTC. The analysis of the space-time-energy parameters of the earthquakes showed the coherence between the results of three of the networks monitoring the region, namely, the Cuban National Seismological Service (SSNC), the United States Geological Service (USGS) and the German Center for Geo-scientific Research (GEOFON)). For three of the earthquakes, the seismic moment tensors were calculated by reversing the corresponding waveforms. Comparing the results of two of them with those obtained by other agencies that monitor the area was totally consistent with fault planes oriented N-S with compressive focal mechanisms. The third earthquake, which occurred a week later in the east of the province, also had a compressive mechanism, although with a NW SE orientation on the fault plane. The seismic activity that initially performed like a swarm of earthquakes, performed differently later; similar to aftershocks registered after a higher-energy earthquake, following Omori's Law, that is, the type of premonitorymain earthquake event-aftershocks. [ABSTRACT FROM AUTHOR]

From California to British Columbia, the Pacific Northwest coast bears an omnipresent earthquake and tsunami hazard from the Cascadia subduction zone. Multiple lines of evidence suggests that magnitude eight and greater megathrust earthquakes have occurred ‐ the most recent being 321 years ago (i.e., 1700 A.D.). Outstanding questions for the next great megathrust event include where it will initiate, what conditions are favorable for rupture to span the convergent margin, and how much slip may be expected. We develop the first 3‐D fully dynamic rupture simulations for the Cascadia subduction zone that are driven by fault stress, strength and friction to address these questions. The initial dynamic stress drop distribution in our simulations is constrained by geodetic coupling models, with segment locations taken from geologic analyses. We document the sensitivity of nucleation location and stress drop to the final seismic moment and coseismic subsidence amplitudes. We find that the final earthquake size strongly depends on the amount of slip deficit in the central Cascadia region, which is inferred to be creeping interseismically, for a given initiation location in southern or northern Cascadia. Several simulations are also presented here that can closely approximate recorded coastal subsidence from the 1700 A.D. event without invoking localized high‐stress asperities along the down‐dip locked region of the megathrust. These results can be used to inform earthquake and tsunami hazards for not only Cascadia, but other subduction zones that have limited seismic observations but a wealth of geodetic inference. Plain Language Summary: The largest earthquakes on Earth occur along faults that develop between two tectonic plates that come into contact. Termed megathrust earthquakes, these catastrophic events are responsible for generating both strong ground‐shaking and tsunamis. The Cascadia megathrust fault straddles the Pacific coastline of North America and from evidence in both the United States and Japan, we know this fault last slipped in 1700 A.D. We have combined models of strain buildup (geodetic coupling models) with state‐of‐the‐art 3‐D computer simulations to understand the potential hazard of a future earthquake in Cascadia and show what factors might lead to the fault slipping its entire length. We compare our simulations to geologic measurements of permanent ground movement from 1700 A.D. Our results demonstrate that no matter where the earthquake is allowed to start, coupling models showing strain accumulation to the top of the fault easily leads to big earthquakes. We also look into what 1700 A.D. event may have looked like and show several scenarios that fit the geologic data very closely. This work represents the first set of 3‐D simulations that use the laws of physics to see what may control the size of future earthquakes in Cascadia. Key Points: We design the first fully dynamic 3‐D earthquake simulations based on geodetic coupling models for the Cascadia megathrustSegmentation in the stress drop is needed to produce subsidence amplitudes consistent with observed megathrust earthquakesDynamic rupture simulations demonstrate how fault friction and stress levels may control margin‐wide rupture [ABSTRACT FROM AUTHOR]

Extreme temperature and pressure conditions on the surface of Venus present formidable technological challenges against performing ground‐based seismology. Efficient coupling between the Venusian atmosphere and the solid planet theoretically allows the study of seismically generated acoustic waves using balloons in the upper atmosphere, where conditions are far more clement. However, earthquake detection from a balloon has never been demonstrated. We present the first detection of an earthquake from a balloon‐borne microbarometer near Ridgecrest, CA in July 2019 and include a detailed analysis of the dependence of seismic infrasound, as measured from a balloon on earthquake source parameters, topography, and crustal and atmospheric structure. Our comprehensive analysis of seismo‐acoustic phenomenology demonstrates that seismic activity is detectable from a high‐altitude platform on Earth, and that Rayleigh wave‐induced infrasound can be used to constrain subsurface velocities, paving the way for the detection and characterization of such signals on Venus. Plain Language Summary: The interior structure of Venus remains unknown due to lack of in situ seismic observations. Adverse temperature and pressure conditions on the Venusian surface limit the lifetimes of landers to a few hours, which poses a technological challenge against performing ground‐based seismology to detect venusquakes. Seismic energy on Venus, as on Earth, can be transmitted into the atmosphere through mechanical coupling and propagate as low‐frequency sound (infrasound). Infrasound from earthquakes travels long distances and has been detected from ground‐based stations. This mechanism may allow the detection of seismically generated pressure disturbances on Venus using balloons, enabling remote seismology from its upper atmosphere, where temperature and pressure conditions are far more clement and longer mission lifetimes are likely. However, the feasibility of such a technique has yet to be established through the detection of ground motion following an earthquake using a freely floating balloon. We demonstrate the first detection of an earthquake from a high‐altitude balloon. We explore the dependence of the pressure signal seen by the balloon on parameters such as the magnitude, focal mechanism, location of the earthquake, surface topography, and atmospheric structure. We also show how the signal recorded at the balloon can be used to study the subsurface. Key Points: First detection of a natural earthquake using balloon‐borne infrasound dataRayleigh wave‐induced infrasound dispersion characteristics provide constraints on subsurface velocitiesShallow waveguides, focal mechanism, and subwavelength topographic changes control infrasound amplitude and dispersion by weak earthquakes [ABSTRACT FROM AUTHOR]

At seismic slip rates, flash‐weakening can significantly reduce the coefficient of friction, and the magnitude of weakening increases with surface temperature. To quantify the distribution of flash temperature, high‐speed double‐direct shear experiments were conducted on Westerly granite blocks using velocity steps from 1 mm/s to 900 mm/s at 9 MPa normal stress. We employed a high‐speed infrared camera to measure surface temperatures on the moving block during sliding, and utilized a novel sliding‐surface geometry to control the mm‐scale contact history. Following the initial weakening upon the velocity step, the blocks slide at a constant coefficient of friction. Surface temperatures are inhomogeneously distributed across the sliding surface, and increase with displacement. To determine the local normal stress distribution at the mm‐scale, we combine a one‐dimensional thermal model with conventional flash‐weakening models that incorporate a surface temperature‐dependence informed by the controlled, mm‐scale contact history. Early contacts experience local normal stress exceeding 40 times the applied normal stress. As sliding progresses, the local normal stress at the hottest contacts decreases as contact area increases, leading to local normal stresses ranging from 2 to 6 times the applied normal stress on most contacts by 30 mm of slip. Increases in surface temperature, which would decrease the coefficient of friction, are buffered by wear processes that increase contact area and decrease the local normal stress. Treatments of flash heating are advanced by incorporating improved characterization of the state of the sliding surface at the mm and larger scales during sliding. Key Points: Flash surface‐temperature and local normal stress at millimeter and larger scale are inhomogeneous on rock interfaces during seismic slipMm‐scale contacts comprise ∼10% of the apparent area, flash to 200˚C–400˚C, and support local normal stress 6–20 times the macroscopic stressDynamic friction remains constant with increasing flash temperature as wear reduces local normal stress and increases contact area with slip [ABSTRACT FROM AUTHOR]

Earthquakes of 1 to 3 magnitudes eventually reaching up to 8, take place in Cuba and adjacent areas. Although those with a magnitude greater than 3 are generally perceptible, the ones usually causing damage are those with a magnitude greater than 5; however, its negative impact largely depends on the state of the built infrastructure and the hypocenter. This essay is a contribution to risk perception in Cuban population and it introduces the concept of cities in seismic alert, which is based on the principle that where destructive seismic events occurred in the past it is very likely they will be repeated in the future, unless the infrastructure be substantially improved. A set of recommendations is also proposed for Cuban cities defined in seismic alert. [ABSTRACT FROM AUTHOR]

Earthquakes have been studied by means of seismometers recording the elastic waves travelling through the interior of our planet. Global Navigation Satellite System and Synthetic Aperture Radar surveys, measuring surface displacements, have provided additional information on earthquakes, as well as on those solid Earth processes responsible for them, such as subduction, collision and extension and the inter-seismic strain accumulation. This instrumentation is deployed over land and thus misses the seas, often surrounding regions where large earthquakes occur. This limitation is nowadays overcome by space gravity missions, thanks to their uniform coverage of the Earth, both inland and offshore. In this perspective, Gravitational Seismology has been identified as a new application of the Next-Generation Gravity Mission (NGGM), with the aim of evaluating its overall performance and of assessing the detectability of earthquake gravity signatures, as well as of those from active tectonics and inter-seismic deformation. Within the framework of self-gravitating viscoelastic Earth models, we have simulated the co- and post-seismic gravity signatures of 291 scenario earthquakes, with different occurrence times and geographical locations, focal mechanisms, depths and lines of strike, and included into the background gravity feeding the NGGM closed-loop simulation which provides observables of multiple pairs of GRACE-like satellites, given the instrument noise. NGGM earthquake detectability is herein defined on the possibility of estimating the amplitude of the original gravity signature of each earthquake by inversion of synthetic NGGM gravity data, consisting of 156 28-day gravity field solutions (about 11 years). For about two thirds of earthquakes of magnitude as low as 7, comparable with the 1980 Irpinia intraplate earthquake, the amplitudes have been estimated with a relative error less than 10% (and less than 50% for almost all the earthquakes), assuming as known the time variable contributions from atmosphere, oceans, hydrology, continental ice and glacial isostatic adjustment. When these contributions are inverted simultaneously with the earthquake ones, instead, we have had to increase the earthquake magnitude to 7.8 in order to estimate more than half of their amplitudes with a relative error less than 10%. We thus have shown that the NGGM will be able to detect, in most cases, the co- and post-seismic signatures of earthquakes of at least magnitude 7.8 and that this lower magnitude threshold can decrease down to magnitude 7 by improving the modelling of the background gravity field. [ABSTRACT FROM AUTHOR]

The largest earthquakes recorded in northern Alaska (Mw 6.4 and Mw 6.0) occurred ∼6 hr apart on 12 August 2018, in the northeastern Brooks Range. The earthquakes were captured by Sentinel‐1 interferometric synthetic aperture radar (InSAR) satellites and Earthscope Transportable Array seismic data, giving insight into the little‐known active tectonic processes of Arctic Alaska, obscured until recently by sparse data availability. In this study, InSAR modeling, teleseismic back projections, calibrated hypocentral relocations, and regional moment tensor solutions resolve two previously unknown, SSW dipping right‐lateral fault segments. These are the first active faults identified as conjugate to the NE trending sinistral Canning displacement zone directly to the west, which is therefore a more complex zone of diffuse faulting than previously thought. The northeastern Brooks Range has been characterized as an area of low to moderate seismic hazard, but these earthquakes illustrate the potential for larger, possibly destructive events in a region earmarked for rapid resource development. Plain Language Summary: The largest earthquakes recorded in northern Alaska (magnitude 6.4 and magnitude 6.0) occurred ∼6 hr apart on 12 August 2018. Few active faults are mapped in this region despite widespread seismicity, and the current tectonic setting remains unclear due to limited available data and the remote location. We use satellite radar images and seismic data to resolve two previously unknown fault segments, along which the magnitude 6.4 earthquake ruptured unilaterally eastward. This fault geometry demonstrates that the Canning displacement zone, the main tectonic feature in the area, is a more complex zone of diffuse faulting than previously thought. These results are also important for reassessing seismic hazard by illustrating the potential for damaging earthquakes on seemingly aseismic faults. Key Points: The largest earthquakes recorded in northern Alaska (Mw 6.4 and Mw 6.0) occurred on previously unknown active faultsInSAR and calibrated hypocenter relocations indicate that the faults are conjugate to the Canning displacement zoneThe Canning displacement zone involves a complex fault network and vertical‐axis block rotations [ABSTRACT FROM AUTHOR]

We investigate the variability of Brune stress drop (∆σ) and apparent stress (τa) of 23 earthquakes occurred in a small crustal volume adjacent to the hypocenter of the destructive Mw 6.1 L'Aquila earthquake. Their magnitude range is 2.7 ≤ Mw ≤ 4.1. Interevent variability of stress drop and apparent stress results in a factor of 10, well beyond the individual‐event uncertainty. Radiation efficiency ηsw = τa/∆σ varies mostly between 0.1 and 0.2, but in the days immediately before and after the main shock, ηsw tends to be smaller decreasing to values as low as 0.06. This may be the consequence of ruptures migrating in those days into a focal volume with higher dynamic strength. The temporal change of ηsw is tentatively interpreted as a spatial variation due to the earthquake migration into the locked portion of the fault that originated the main shock. Consistently, no variation in stress drop and apparent stress is observed between foreshocks and aftershocks but the smallest and largest ∆σ result in a good correlation with the largest and smallest b‐values, respectively, imaged by other authors in the rupture nucleation volume. Key Points: Variations of stress parameters are significant beyond estimate uncertaintiesStress release of aftershocks does not differ from the one of foreshocksA finer spatial resolution is reached compared to previous studies [ABSTRACT FROM AUTHOR]

This paper presents the solution of an inverse problem in Seismology, which aims at deriving the seismic source parameters from P and S seismic waves. In particular, the paper gives the deduction of the seismic-moment tensor. The problem is tackled in this paper under three particular circumstances. First, we use the amplitude of the far-field (P and S) seismic waves as input data. We use the analytical expression of the seismic waves in a homogeneous isotropic body with a seismic-moment source of tensorial forces, the source being localized both in space and time. We assume that the position of the seismic source is known. The far-field waves provide three equations for the six unknown parameters of the general tensor of the seismic moment, such that the system of equations is under-determined. Second, the Kostrov vectorial (dyadic) representation of the seismic moment for a shear faulting is used. This representation relates the seismic moment to the focal displacement in the fault and the orientation of the fault (moment-displacement relation); it reduces the seismic moment to four unknown parameters. Third, the fourth missing equation is derived from the energy conservation and the covariance condition. The four equations derived here are solved and the seismic moment is determined, as well as other parameters of the seismic source, like focal volume, focal slip, fault orientation, and duration of the seismic activity in the source. It turns out that the seismic moment is traceless, its magnitude is of the order of the elastic energy stored in the focal region (as expected), and the solution is governed by the unit quadratic form associated with the seismic-moment tensor (related to the magnitude of the longitudinal displacement in the P wave). A useful picture of the seismic moment is the conic represented by the associated quadratic form, which is a hyperbola (seismic hyperbola). This hyperbola provides an image for the focal region: its asymptotes are oriented along the focal displacement and the normal to the fault. Also, the special case of an isotropic seismic moment is presented. Numerical examples are provided for this procedure, and the limitations are discussed. [ABSTRACT FROM AUTHOR]

The objective of this research is to develop a new approach to estimate earthquake arrival azimuth using seismological records of the "El Rosal" station, near to the city of Bogota - Colombia, by applying support vector machines (SVMs). The algorithm was trained with time series descriptors of 863 events recorded from January 1998 to October 2008, considering only events with magnitude = 2 ML. The earthquake signals were filtered in order to remove diverse kind of low and high-frequency noise not related to typical seismic activity in the area. During training stages of SVMs, several combinations of kernel exponent and complexity factor were applied to time series of 5, 10 and 15 seconds along with earthquake magnitudes of 2.0, 2.5, 3.0 and 3.5 ML. The best classification of SVMs was obtained using time windows of 5 seconds and earthquake magnitudes greater than 3.0 ML with kernel exponent of 10 and complexity factor of 2, showing an accuracy of 45.4 degrees. This research is an improvement of previous works related to earthquake arrival azimuth determination from single station data employing machine learning techniques. [ABSTRACT FROM AUTHOR]

The magnitude of postseismic slip is useful for constraining physical models of fault slip. Here we examine the postseismic slip following intermediate‐magnitude (M 4 to 5) earthquakes by systematically analyzing data from borehole strainmeters in central and northern California. We assess the noise in the data and identify 11 earthquakes that generated interpretable strain records. We estimate the earthquakes' postseismic to coseismic moment ratios by comparing the coseismic strain changes with strain changes induced by afterslip in the following 1.5 days. The median estimated postseismic moment is 0.45 times the coseismic moment, with a 90% confidence interval between 0.25 and 0.60. This postseismic moment is slightly larger than typically observed following large (M > 6) earthquakes but smaller than observed following small (M2 to 4) earthquakes. The intermediate‐magnitude postseismic slip suggests a size dependence in the dynamics of earthquakes or in the properties of fault areas that surround earthquakes. Key Points: We examine postseismic slip following M4‐5 earthquakes using borehole strain dataThe median estimated postseismic moment is between 0.25 and 0.6 times the coseismic momentThe postseismic moment ratio is intermediate between estimates for small and large earthquakes [ABSTRACT FROM AUTHOR]

On 5 April 2017, an Mw6.1 earthquake occurred about 50 km NE of the city of Fariman, northeast Iran. Several hundreds of aftershocks including two M > 5 events followed the main shock. The quake struck numerous towns and villages across the region, killed one person, and injured tens of people. Many schools and universities were evacuated around the epicentral area, and a lot of people left their residences for a few days. The northeastward motion of the central Iran toward Eurasia influences the epicentral region. Regional movements occur by shortening on the northwest-trending reverse faults. We studied teleseismic source parameters of this earthquake by applying different moment tensor decomposition methods including grid search for the nodal planes of the best double couple; linear inversion for a deviatoric moment tensor; grid search for the best double-couple moment tensor; grid search for the best deviatoric moment tensor; and grid search for the best full moment. Based on the moment tensors, the event occurred on a reverse fault following the regional compressional motion. The results of this study will provide useful information for future regional seismotectonic investigations and are of significant use for applications such as regional seismic hazard evaluations. [ABSTRACT FROM AUTHOR]

We determine earthquake stress drops directly from the Arias intensity database of NGA‐West2. Arias intensity (Arias, 1970, https://www.osti.gov/biblio/4167721) is an engineering measure proportional to the integral of the absolute value of acceleration squared, over the significant duration of the signal. As such, it is closely related to root mean square acceleration and can readily be connected to earthquake stress drop (Hanks & McGuire, 1981, https://pubs.geoscienceworld.org/ssa/bssa/article/71/6/2071/102118/the-character-of-high-frequency-strong-ground). Arias intensity records out to 100‐km yield stable stress drops for moderate‐to‐large magnitude earthquakes, M6.5+; for smaller events ~M4.5–6.5, only closer‐in records yield stable results. For the 116 events considered, stress drops are about 35% larger for class 1 mainshocks than for traditional on‐fault class 2 aftershocks, and smaller for those aftershocks close to the main fault plane. Aftershock stress drops show large variability, however, implying that on average they rerupture weakened patches but can also rupture intact rock or high‐stress asperities. We observe an increase of stress drop with earthquake depth similar to that of other studies but do not find any significant faulting mechanism dependence. The variability of the Arias intensity‐based stress drop is lower than that of eGf‐based stress drops from Baltay et al. (2010, https://doi.org/10.1029/2009JB006736; 2011, https://doi.org/10.1029/2011GL046698), and nearly on par with variability seen in ground‐motion prediction equations. The Arias intensity stress drop is a novel and promising method to estimate stress drop without the need for path and site corrections and yields further insight into the connection between source physics and ground motion. Key Points: Earthquake stress drop can robustly be estimated from Arias intensity, the squared cumulative accelerationTo 100 km, attenuation does not affect stress drop for large M6.5+ earthquakes, but only closer‐in records can be used for smaller eventsDue to the low variability in stress drop, we can model trends with depth and differences between mainshock and aftershock classes [ABSTRACT FROM AUTHOR]

A seismic array observation across the central focal area of the 2015 Gorkha earthquake was conducted to investigate aftershock distribution and crustal structure. Most aftershocks near Kathmandu were located above the Main Himalayan Thrust (MHT). Earthquake reflection imaging revealed the geometry of the MHT, which lies at ~10‐km depth at 75 km away from the Main Frontal Thrust. We found three areas of seismic velocity variability around the MHT above 15 km depth: (1) a high‐Vp zone where the MHT changes its dip angle from 5° to 13°, coinciding with a local maximum in coseismic slip, (2) a low‐Vp zone showing less coseismic slip, and (3) a high‐Vp zone where coseismic slip decreased further but afterslip occurred. These differences in slip behavior may indicate changes in frictional properties on the fault, which are reflected in the heterogeneity of the velocity structure. Plain Language Summary: On 25 April 2015, the Gorkha earthquake (Mw 7.8) struck central Nepal and resulted in nearly 9,000 fatalities. Destructive earthquakes have occurred repeatedly in the Himalayan region. The Himalayan seismogenic zone is located in the India‐Eurasia plate collision zone where the Indian plate underthrusts the Eurasian plate along the Main Himalayan Thrust. Revealing the crustal structure of the India‐Eurasia plate collision zone is important to constraining the process of earthquake occurrence and the orogenic process. Seismic array observation is an effective tool to investigate the crustal structure. Here, we present the results of seismic array observation across the central focal area of the 2015 Gorkha earthquake, which shows the geometry of the Main Himalayan Thrust and seismic velocity variability along its length. We find a high‐Vp zone which coincides with a large coseismic slip zone (>6 m). A low‐Vp zone shows less coseismic slip. Another high‐Vp zone located in the southern side of the low‐Vp zone corresponds to an area of afterslip. These results connect heterogeneous crustal structure with fault slip behavior and suggest that variability in the seismic properties of the crustal structure around the plate boundary may affect fault slip behavior. Key Points: Source fault geometry of the 2015 Gorkha earthquake can be imaged by using an earthquake reflection imaging methodA high‐Vp zone where the dip angle of the Main Himalayan Thrust changes coincides with the area of maximum coseismic slipHeterogeneous crustal structure around the Main Himalayan Thrust reflects changes in the frictional properties of the fault [ABSTRACT FROM AUTHOR]

The size of an earthquake can be defined either from the seismic moment (M 0) or in terms of radiated seismic energy (E r). These two parameters look at the source complexity from different perspectives: M 0 is a static measure of the earthquake size, whereas E r is related to the rupture kinematics and dynamics. For practical applications and for dissemination purposes, the logarithms of M 0 and E r are used to define the moment magnitude M w and the energy magnitude M E, respectively. The introduction of M w and M E partially obscure the complementarity of M 0 and E r. The reason is due to the assumptions needed to define any magnitude scale. For example, in defining M w, the apparent stress (i.e. the ratio between M 0 and E r multiplied by the rigidity) was assumed to be constant, and under this condition, M w and M E values would only differ by an off-set which, in turn, depends on the average apparent stress of the analysed data set. In any case, when the apparent stress is variable and, for example, scales with M 0, the value of M E derived from M w cannot be used to infer E r. In this study, we investigate the similarities and differences between M w and M E in connection with the scaling of the source parameters using a data set of around 4700 earthquakes recorded at both global and regional scales and belonging to four data sets. These cover different geographical areas and extensions and are composed by either natural or induced earthquakes in the magnitude range 1.5 ≤ M w ≤ 9.0. Our results show that M E is better than M w in capturing the high-frequency ground shaking variability whenever the stress drop differs from the reference value adopted to define M w. We show that M E accounts for variations in the rupture processes, introducing systematic event-dependent deviations from the mean regional peak ground motion velocity scaling. Therefore, M E might be a valid alternative to M w for deriving ground motion prediction equations for seismic hazard studies in areas where strong systematic stress drop scaling with M 0 are found, such as observed for induced earthquakes in geothermal regions. Furthermore, we analyse the different data sets in terms of their cumulative frequency–magnitude distribution, considering both M E and M w. We show that the b values from M w (b Mw) and M E (b ME) can be significantly different when the stress drop shows a systematic scaling relationship with M 0. We found that b ME is nearly constant for all data sets, while b M w shows an inverse linear scaling with apparent stress. [ABSTRACT FROM AUTHOR]

Local magnitudes calculated at stations less than 10 km from earthquakes in the British Isles are up to one unit of magnitude higher than local magnitudes calculated at more distant stations. This causes a considerable overestimate of the event magnitude, particularly for small events, which are only recorded at short distances. Data from Central Italy and Norway show that the same problem also occurs in other regions, suggesting that this is a more general issue for local magnitude scales. We investigate the addition of a new exponential term to the general form of the local magnitude scale. This corrects for the higher-than-expected amplitudes at short hypocentral distances. We find that the addition of this new term improves magnitude estimates in the three studied regions and magnitudes at short distances are no longer overestimated. This allows the use of a single scale that can be used at all distances, with a smooth transition between short and long distances. For the UK, the amended scale is |$M\mathrm{ _{ L}}\ = \log (\mathrm{ {amp}})\ + 1.11\log (r) + 0.00189r - 1.16{e^{ - 0.2r}} - 2.09$| and this is the scale now used by the British Geological Survey. [ABSTRACT FROM AUTHOR]

The most common earthquake forecasting models assume that the magnitude of the next earthquake is independent from the past. This feature severely limits the capability to forecast large earthquakes with high probabilities. Here we investigate empirically on the magnitude-independence assumption, exploring if: (i) background and triggered earthquakes have the same frequency–magnitude distribution, (ii) variations of seismicity in the space–time–magnitude domain encode some information on the future earthquakes size. For this purpose, and to verify the stability of the findings, we consider seismic catalogues covering different space–time–magnitude windows, such as the Alto Tiberina Near Fault Observatory (TABOO), the California and Japanese seismic catalogues. Our approach is inspired by the nearest-neighbour method proposed by Baiesi & Paczuski and elaborated by Zaliapin et al. to distinguish between triggered and background earthquakes. Here we implement the same metric-based correlation to identify the precursory seismicity of any triggered earthquake; this allows us to analyse, for each triggered earthquake, the space–time–magnitude distribution of the seismicity that likely contributed to its occurrence. Our results show that the magnitude-independence assumption holds reasonably well in all catalogues, with a remarkable exception that is consistent with a previous independent study; this departure from the magnitude-independence assumption shows that larger events tend to nucleate at a higher distance from the ongoing sequence. We also notice that the reliability of this assumption may depend on the spatial scale considered; it holds for seismic catalogues of large areas, but we identify possible departures in small areas, reflecting different ways to release locally seismic energy. Finally, we come across an important issue that may lead to misleading results in similar studies, that is, if a seismic catalogue appears overall complete above a fixed magnitude threshold, it may still yield spurious signals into the analysis. Specifically, we show that some significant departures from the magnitude-independence assumption do not survive when considering spatiotemporal variations of the magnitude of completeness. [ABSTRACT FROM AUTHOR]

The determination of accurate source depths for globally observed earthquakes has long been one of the most problematic issues in earthquake source seismology. The optimal method for constraining the depths of teleseismically observed earthquakes is through the identification of near‐source surface reflections—depth phases. However, observing such phases is complicated by the relatively small amplitude of these arrivals compared to the background noise, particularly for earthquakes with Mw < 5.5. In this study, I present a methodology for leveraging the recent expansion in global seismic network coverage to enhance the identification of coherent depth phases through an automated stacking routine using a globally distributed array. While depth solutions for each individual depth phase are often nonunique, the identification of potential depths across multiple different phases, where each shows the correct distance‐dependent delay, provides a robust way to semiautomatically determine the source depth of an earthquake. I present a range of examples for the processing routine developed, along with an example for its regional application, to the North Chilean subduction zone. The technique presented offers an opportunity to improve depth estimates for earthquakes down to Mw4.9 and, requiring significantly less analyst input than other techniques offering a similar resolution, has the potential to be applied to large earthquake data sets. Plain Language Summary: The determination of accurate depths for globally observed earthquakes is critical for our understanding of the structure and deformation of tectonic plates but has proven to be one of the most intractable problems in earthquake seismology. Here, I present a method aimed at using the recent vast expansion in global seismic data coverage to enhance the detection of a suite of seismic phases that reflect from the Earths surface directly above the earthquake source—these are often referred to as "depth phases." These phases provide the best known constraint on earthquake depths for remotely observed earthquakes, especially in cases where several of these phases can be observed for a given earthquake. However, they are often difficult to detect, due to their low amplitude, and the complexity of the associated waveform. While limitations remain on the magnitude and source depth ranges over which the technique presented here can be applied, this approach enhances the detection of these low‐amplitude phases, allowing them to be identified for smaller earthquakes than has routinely been possible in the past. As this approach uses data observed from around the world, it can potentially be applied to large earthquake data sets in remote areas, where near‐field data are unavailable. Key Points: I present a method for stacking global seismic data to extract coherent depth phasesThe observation of multiple depth phases allows for the near‐automatic determination of source depths for earthquakes down to Mw 4.9As a regional example, I show the results of this technique as applied to 324 earthquakes in the North Chile subduction zone [ABSTRACT FROM AUTHOR]

The 12 November 2017 Mw 7.3 Ezgeleh‐Sarpolzahab earthquake is the largest instrumentally recorded earthquake in the Zagros Simply Folded Belt by a factor of ∼10 in seismic moment. Exploiting local, regional, and teleseismic data and synthetic aperture radar interferometry imagery, we characterize the rupture, its aftershock sequence, background seismicity, and regional tectonics. The mainshock ruptured slowly (∼2 km/s), unilaterally southward, for ∼40 km along an oblique (dextral‐thrust) fault that dips ∼14°E beneath the northwestern Lurestan arc. Slip is confined to basement depths of ∼12–18 km, resolvably beneath the sedimentary cover which is ∼8 km thick in this area. The gentle dip angle and basement location allow for a broad slip area, explaining the large magnitude relative to earthquakes in the main Fars arc of the Zagros, where shallower, steeper faults are limited in rupture extent by weak sedimentary layers. Early aftershocks concentrate around the southern and western edges of the mainshock slip area and therefore cluster in the direction of rupture propagation, implying a contribution from dynamic triggering. A cluster of events ∼100 km to the south near Mandali (Iraq) reactivated the ∼50° dipping Zagros Foredeep Fault. The basement fault responsible for the Ezgeleh‐Sarpolzahab earthquake probably accounts for the ∼1 km elevation contrast between the Lurestan arc and the Kirkuk embayment but is distinct from sections of the Mountain Front Fault that define frontal escarpments elsewhere in the Zagros. It may be related to a seismic interface underlying the central and southern Lurestan arc, and a key concern is whether or not the more extensive regional structure is also seismogenic. Key Points: The Mw 7.3 Ezgeleh‐Sarpolzahab earthquake ruptured a gently dipping oblique fault in the Lurestan arc basementThis faulting may be pervasive across the Lurestan arc, though it is unlikely seismogenic everywhereRelocated aftershocks correlate strongly with directivity implying a component of dynamic triggering [ABSTRACT FROM AUTHOR]

Foreshocks provide valuable information on the nucleation process of an upcoming large earthquake. We applied high‐resolution similar‐waveform techniques for earthquake detection, location, and source parameter estimation to understand the space‐time evolution of a foreshock sequence and its relationship to the mainshock hypocenter. The 1999 Mw 7.1 Hector Mine, California, earthquake was preceded by 50 foreshocks (−0.4 ≤ M ≤ 3.7) during the 20 hr before the mainshock. Foreshock activity did not accelerate leading up to the mainshock. Their locations moved north with time, rupturing adjacent areas along the fault plane with little overlap, but remained within a compact <2 km3 volume. The mainshock initiated at a location where previous foreshocks had locally increased the shear stress. These observations are consistent with a triggered cascade of stress transfer, where previous foreshocks load adjacent fault patches to rupture as additional foreshocks, and eventually the mainshock. Plain Language Summary: Foreshocks are smaller earthquakes that happen near, but before, a large earthquake. We can use foreshocks as a tool to help understand how large earthquakes begin. One model of earthquake initiation holds that small foreshocks directly trigger other small foreshocks by changing the stress on the fault and, eventually, a large earthquake occurs if the conditions are favorable. Since earthquakes of all sizes would start out the same way, it is impossible to predict ahead of time which earthquakes grow large. An alternative explanation holds that smaller foreshocks do not directly trigger the large earthquake but instead are a symptom of something else happening underground, such as slow fault movement that does not release ground‐shaking waves. Several foreshocks occurred in the 20 hr before the 1999 magnitude 7.1 Hector Mine, California, earthquake. Analyzing records of the ground shaking from these foreshocks, we found that they are consistent with the former model, where each foreshock triggered the next one to happen like a line of falling dominoes. We discovered that the large earthquake started out the same way as the smaller foreshocks, so we could not tell ahead of time that it would eventually grow large. Key Points: Foreshocks moved north with time, toward eventual mainshock location; seismicity rate did not accelerate leading up to mainshockForeshocks ruptured adjacent areas along fault with little overlap; mainshock initiated where foreshocks locally increased stressHector Mine foreshocks and mainshock were triggered by a cascade of stress transfer [ABSTRACT FROM AUTHOR]

We combine multi‐array relative back‐projection of high‐frequency P waves and finite‐fault modeling of low‐frequency P waves from the 8 September 2017 Mw 8.2 Chiapas, Mexico earthquake to image unilateral rupture on a southeast‐northwest striking subvertical fault plane over depths of 10–70 km. Improved multi‐array relative back‐projection estimates of rupture speed and fault geometry provide key refinements to the slip model from finite‐fault modeling. Compilation of prior seismicity and aftershocks reveals the earthquake initiated in the lower plane of a double Benioff zone (DBZ) and ruptured with large slip across the entire DBZ. The previously aseismic interior does not appear to be due to lack of stress as both planes show downdip extension stresses. Instead, tomographic imaging of DBZs find high Poisson's ratio with fast velocities in the DBZ interior, so we propose that highly hydrated harzburgite generates velocity‐strengthening behavior that inhibits earthquake nucleation, yet it can slip seismically during dynamic weakening from a large throughgoing rupture. Plain Language Summary: We studied the detailed rupture properties of the damaging Mw 8.2 earthquake that occurred in Chiapas, Mexico on 8 September 2017, using a complimentary combination of techniques to refine our estimates of the rupture speed, depth, and geometry. We found that the rupture began near a depth of 60 km inside a subducting plate and rupture propagated upward across the entire plate, including a portion that is normally aseismic. Together, these observations suggest that the interior of the subducting plate cannot normally produce earthquakes, but the seismic threshold can be reached through processes such as dehydration of rocks being pushed to higher pressures and temperature or from the fault slip of nearby large earthquakes that dynamically weaken the rocks. Key Points: Combining multi‐array relative back‐projection and finite fault modeling improves rupture models of the earthquakeThe earthquake nucleated on the bottom plane of a double Benioff zone (DBZ) and ruptured across the normally aseismic zone between the two planesWe propose that the slab interior is sufficiently stressed but high hydration inhibits earthquake slip unless large ruptures create dynamic weakening [ABSTRACT FROM AUTHOR]

The Sentinel-1 mission comprises two synthetic aperture radar satellites, each with a 12-d orbital repeat, orbiting 6 d apart within a narrow tube. The mission design promises the ability to respond quickly to earthquakes with InSAR, and to facilitate production of interferograms with good interferometric correlation globally. We report on our efforts to study global seismicity using Sentinel-1 Interferometric Wide-Swath data between April 2015 and December 2016. We select 35 potentially detectable terrestrial earthquakes in the range 5.5 ≤ M w ≤ 7.8 on the basis of their locations, depths and magnitudes, and process the first post-event interferogram with the shortest possible time span for each using the ISCE software. We evaluate each interferogram for earthquake deformation signals by visual inspection. We can identify deformation signals attributable to earthquakes in 18 of these interferograms (51 per cent); a further six interferograms (17 per cent) have ambiguous interferometric phase affected by tropospheric noise. 11 events (31 per cent) could not be identified from their interferograms. The majority of these failed detections were due to interferogram decorrelation, particularly apparent for earthquakes that occurred between 15°N and 15°S, where climate conditions promote dense vegetation. The majority of the ambiguous interferograms are affected by tropospheric noise, suggesting that techniques to mitigate such noise could improve detection performance. The largest event we do not detect with Sentinel-1 data is a M w7.0 earthquake that occurred in Vanuatu in April 2016; we also fail to detect the 2016 M w6.2 Kurayoshi earthquake in one out of two possible 24-d interferograms. We propose these as upper and lower estimates on the magnitude of completeness for earthquakes studied with Sentinel-1 data; to lower the magnitude of completeness we suggest that more frequent (e.g. 6-d) recurrence may be necessary in low latitude areas. [ABSTRACT FROM AUTHOR]

Abstract We observe remarkably periodic patterns of seismicity rates and magnitudes at the Fimbul Ice Shelf, East Antarctica, correlating with the cycles of the ocean tide. Our analysis covers 19 years of continuous seismic recordings from Antarctic broadband stations. Seismicity commences abruptly during austral summer 2011 at a location near the ocean front in a shallow water region. Dozens of highly repetitive events occur in semi-diurnal cycles, with magnitudes and rates fluctuating steadily with the tide. In contrast to the common unpredictability of earthquake magnitudes, the event magnitudes show deterministic trends within single cycles and strong correlations with spring tides and tide height. The events occur quasi-periodically and the highly constrained event sources migrate landwards during rising tide. We show that a simple, mechanical model can explain most of the observations. Our model assumes stick-slip motion on a patch of grounded ice shelf, which is forced by the variations of the ocean-tide height and ice flow. The well fitted observations give new insights into the general process of frictional triggering of earthquakes, while providing independent evidence of variations in ice shelf thickness and grounding. Highlights • Cryoseismicity at the Fimbul Ice Shelf, Antarctica, from two decades of observation. • Complex tidal modulation pattern provides restrictions for modelling. • Events occur as stick-slip motion during tidal migration of local grounding line. • Both short- and long-term seismicity trends are predicted by the model. • Activity is independent evidence of localised ice shelf thickening and grounding. [ABSTRACT FROM AUTHOR]