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The structure of fault zones and the ruptures they host are inextricably linked. Fault zones are narrow, which has made imaging their structure at seismogenic depths a persistent problem. Fiber‐optic seismology allows for low‐maintenance, long‐term deployments of dense seismic arrays, which present new opportunities to address this problem. We use a fiber array that crosses the Garlock Fault to explore its structure. With a multifaceted imaging approach, we peel back the shallow structure around the fault to see how the fault changes with depth in the crust. We first generate a shallow velocity model across the fault with a joint inversion of active source and ambient noise data. Subsequently, we investigate the fault at deeper depths using travel‐time observations from local earthquakes. By comparing the shallow velocity model and the earthquake travel‐time observations, we find that the fault's low‐velocity zone below the top few hundred meters is at most unexpectedly narrow, potentially indicating fault zone healing. Using differential travel‐time measurements from earthquake pairs, we resolve a sharp bimaterial contrast at depth that suggests preferred westward rupture directivity. Plain Language Summary: Fault zone structure is important because it influences the physics of earthquake ruptures. Imaging fault zones at depth, where large earthquakes typically happen, is challenging because fault zones are narrow and seismic imaging resolution degrades with depth. Dense seismic arrays deployed across faults can help resolve important properties of fault zones at depth. Fiber optic seismology allows for the deployment of dense arrays across faults for long periods of time with low logistical burden. We use a fiber optic array that crosses the Garlock Fault to explore important characteristics of the fault zone at different depths. We find that there is no extensive low velocity feature at depth, potentially suggesting healing of the fault damage zone. Additionally, when we remove the contribution of the complicated velocity structure of the shallow crust, we recover a sharp velocity contrast across the fault which may have implications for the propagation behavior of future ruptures. Key Points: We use a distributed acoustic sensing array that crosses the Garlock Fault to investigate its structureWe find that the low velocity zone around the fault is mostly shallow, suggesting the damage zone at depth is at most narrowWe find a clear bimaterial contrast at depth, which was hidden by the shallow crust, that suggests a preferred westward directivity [ABSTRACT FROM AUTHOR]

As seismology has evolved, scientists have developed various earthquake magnitude scales tailored for specific scenarios, including Richter magnitude (ML) for local, moderate quakes, moment magnitude (Mw) for large-scale seismic events, energy magnitude (Me), which correlates directly with the total energy released, and duration magnitude (MD) for quickly assessing shallow or nearby seismic activities. Despite these advances, these scales were not specifically designed to address the unique challenges posed by mine earthquakes, particularly in coal mine environments where seismic events are typically of lower energy and lower perceptibility but have potentially significant impacts. This study applies the K-class magnitude system for the first time to address significant discrepancies in magnitude values for the same seismic events in mine earthquakes, particularly in coal mines when different formulas are used. However, because current seismic monitoring systems in domestic coal mines continue to employ a variety of magnitude scales, we have developed a corrected Richter scale (MLc) specifically tailored to the geological conditions of coal mines. This adaptation facilitates the conversion to and from the K class, enhancing the accuracy and relevance of seismic assessments in these environments. Therefore, we conducted continuous blasting experiments at the Dongtan Coal Mine and derived the corrected Richter magnitude MLc. Based on data analysis, we fitted the energy attenuation coefficient and compared the K values with four types of traditional magnitude results. Finally, we derived the conversion relationship between the K class and the corrected Richter magnitude MLc, establishing a mine earthquake measurement system. The successful application of the K class in coal mines suggests its potential to be extended to the entire field of induced microseismicity, including mininginduced microseismicity, shale gas extraction-induced microseismicity, and oil extraction-induced microseismicity, providing valuable insights and tools for a broader range of geophysical research and industry practices. [ABSTRACT FROM AUTHOR]

The dynamic geological features of the Pieniny Klippen Belt (PKB) in southern Poland are nowadays a focal point of researchers as it is recognised as an active zone of crustal discontinuity. In the present study, we employed long-term analysis of ground-based global navigation satellite system (GNSS) measurements (from 2004 to 2020) to probe the crustal deformation, strain rates, and rotational rates in the PKB unit and the surrounding region, i.e., Magura Nappe (MN) and Podhale Flysch (PF) units. Measured velocities from GNSS observables are modelled by the auto-regressive integrated moving average (ARIMA) method to comprehend the long-term tectonic deformation. Our results showed that the ARIMA-modelled velocity varied from ~ 0.15 to ~ 8.86 mm/yr, indicating about 8.71 mm/yr difference along all units. Such differences suggest that crustal slip along the active thrusts and folds is the major factor causing regional deformation. The strain rates in PKB are also varying from the western to the eastern part. The rotational rates in PKB show a counterclockwise (CCW) pattern similar to the strain rates. These patterns suggesting that the PKB was rotated in the CCW direction with a large angle during the Miocene period. Finally, we analysed the seismicity for a period from 2004 to 2020 by using Bayesian moment tensor inversion and multivariate Bayesian inversion. The Bayesian inversion was applied based on bootstrapping chain analysis to figure out the earthquake mechanism using moment tensor inversion for the mainshock that occurred in Poland on 20 July 2018. The inversion results for the 2018 earthquake resolved a thrusting mechanism with nodal plane-1 having a strike of 346°, dip of 32°, and rake of 92° and a nodal plane-2 with a strike of 163°, dip of 58°, and rake of 89°. Since the seismicity in the Poland region has experienced less significant earthquakes in the last century, it is reasonable to attribute this lower seismic activity to the correspondingly low slip rates discerned through geodetic monitoring efforts. [ABSTRACT FROM AUTHOR]

Geometric complexities play an important role in the nucleation, propagation, and termination of strike-slip earthquake ruptures. The 2021 Mw7.4 Maduo earthquake rupture initiated at a large releasing stepover with a complex fault intersection. In the epicentral region, we conducted detailed mapping and classification of the surface ruptures and slip measurements associated with the earthquake, combining high-resolution uncrewed aerial vehicle (UAV) images and optical image correlation with field investigations. Our findings indicate that the coseismic ruptures present discontinuous patterns mixed with numerous lateral spreadings due to strong ground shaking. The discontinuous surface ruptures are uncharacteristic in slip to account for the large and clear displacements of offset landforms in the epicentral region. Within the releasing stepovers, the deformation zone revealed from the optical image correlation map indicates that a fault may cut diagonally across the pull-apart basin at depth. The left-lateral horizontal coseismic displacements from field measurements are typically ≤0.6 m, significantly lower than the 1–2.7 m measured from the optical image correlation map. Such a discrepancy indicates a significant proportion of off-fault deformation or the possibility that the rupture stopped at a shallow depth during its initiation phase instead of extending to the surface. The fault network and multi-fault junctions west and south of the epicenter suggest a possible complex path, which retarded the westward propagation at the initial phase of rupture growth. A hampered initiation might enhance the seismic ground motion and the complex ground deformation features at the surface, including widespread shaking-related fissures. [ABSTRACT FROM AUTHOR]

The orientation of S Hmax is commonly estimated from in situ borehole breakouts and earthquake focal mechanisms. Borehole measurements are expensive, and therefore sparse, and earthquake measurements can only be made in regions with many well-characterized earthquakes. Here, we derive the stress-field orientation using stress-induced anisotropy in nonlinear elasticity. In this method, we measure the strain derivative of velocity as a function of azimuth. We use a natural pump-probe (NPP) approach which consists of measuring elastic wave speed using empirical Green's functions (probe) at different points of the earth tidal strain cycle (pump). The approach is validated using a larger data set in the Northern Alpine Foreland region where the orientation of maximum horizontal compressive stress is known from borehole breakouts and drilling-induced fractures. The technique resolves NNW-SSW to N-S directed S Hmax which is in good agreement with conventional methods and the recent crustal stress model. We confirm that the NPP method can be applied to dense large-scale seismic arrays. The technique is then applied to the Southern Alps to understand the contemporary stress pattern associated with the ongoing deformation due to counterclockwise rotation of the Adriatic plate with respect to the European plate. Our results explain why the two major faults in Northeastern Italy, the Giudicarie Fault and the Periadriatic Line (Pustertal–Gailtal Fault) are currently inactive, while the currently acting stress field allows faults in Slovenia to deform actively. We have demonstrated that the pump-probe method has the potential to fill in the measurement gap left by conventional approaches, both in terms of regional coverage and in depth. [ABSTRACT FROM AUTHOR]

There have been reports about anomalies in mobile geomagnetic data before earthquakes; however, whether it can be used as an indicator for identifying potential earthquake areas was not be explored. In this study, we propose two parameters for earthquake forecasting based on annual mobile geomagnetic observation data. The spatial horizontal and three components' changes are calculated in each year and then used to forecast moderate–large earthquakes (M ≥ 5.0) in southwest China in the subsequent period. It is found that earthquakes are more likely to occur in low H- or F-value regions. We statistically assess their forecasting performance by using Molchan's error diagram, and the results indicate that there is considerable precursory information in the spatial H and F values. It is concluded that mobile geomagnetic observations might be useful in middle-term earthquake forecasts in the study area. We discuss the physical mechanisms of H and F values to explain their reasonability. The methodology proposed in this study could be helpful in finding out the optimal solution for annual mobile geomagnetic measurements for middle-term earthquake forecasting. [ABSTRACT FROM AUTHOR]

Pixel Offset Tracking (POT) for optical imagery is a widely used method for extracting large-scale ground deformation. However, the influence of imaging parameters on the measurement accuracy of POT is still unclear. In this study, based on 16 pairs of Sentinel-2 images covering the period before and after the Ms6.9 Menyuan earthquake in 2022, we quantitatively assessed the effects of imaging bands, time intervals between image pairs, and differences in solar zenith angles on the measurement accuracy of optical POT. The results showed that the quality of ground deformation extracted using the near-infrared band was superior to other bands. The accuracy of optical POT measurements exhibited a negative correlation with both the time interval between image pairs and the differences in solar zenith angles. The maximum difference in optical POT measurement accuracy for the near-infrared band between image pairs with different time intervals (5/10/15 days) reached 30.3%, while the maximum difference in deformation measurement accuracy for pairs with different solar zenith angle differences was 30.56%. Utilizing the optimal POT image pair, the accuracy of co-seismic deformation measurement for the Menyuan earthquake improved by 48.3% compared to the worst image pair. The maximum co-seismic horizontal displacement caused by the earthquake was estimated to be 3.00 ± 0.51 m. [ABSTRACT FROM AUTHOR]

With the gradual development of and improvement in earthquake early warning systems (EEWS), more accurate real-time seismic intensity measurements (IMs) methods are needed to assess the impact range of earthquake intensities. Although traditional point source warning systems have made some progress in terms of predicting earthquake source parameters, they are still inadequate at assessing the accuracy of IMs predictions. In this paper, we aim to explore the current state of the field by reviewing real-time seismic IMs methods. First, we analyze different views on the ultimate earthquake magnitude and rupture initiation behavior. Then, we summarize the progress of IMs predictions as they relate to regional and field warnings. The applications of finite faults and simulated seismic wave fields in IMs predictions are analyzed. Finally, the methods used to evaluate IMs are discussed in terms of the accuracy of the IMs measured by different algorithms and the cost of alerts. The trend of IMs prediction methods in real time is diversified, and the integration of various types of warning algorithms and of various configurations of seismic station equipment in an integrated earthquake warning network is an important development trend for future EEWS construction. [ABSTRACT FROM AUTHOR]

Electromagnetic emissions from earthquakes are known as precursors and are of considerable importance for the purpose of early alarms. The propagation of low-frequency waves is favored, and the range between tens of mHz to tens of Hz has been heavily investigated in the last thirty years. This work describes the self-financed Opera 2015 project that initially consisted of six monitoring stations over Italy, equipped with electric and magnetic field sensors, among others. Insight of the designed antennas and low-noise electronic amplifiers provides both characterization of performance (similar to the best commercial products) and the elements to replicate the design for our own independent studies. Measured signals through data acquisition systems were then processed for spectral analysis and are available on the Opera 2015 website. Data provided by other world-known research institutes have also been considered for comparison. The work provides examples of processing methods and results representation, identifying many exogenous noise contributions of natural or human-made origin. The study of the results occurred for some years and led us to think that reliable precursors are confined to a short area around the earthquake due to the significant attenuation and the effect of overlapping noise sources. To this aim, a magnitude-distance indicator was developed to classify the detectability of the EQ events observed during 2015 and compared this with some other known earthquake events documented in the scientific literature. [ABSTRACT FROM AUTHOR]

We utilized relative polarity measurements and machine learning techniques to better resolve focal mechanisms and stress orientations considering a catalog of ∼29,000 relocated earthquakes that occurred during 1984–2021 in the southeastern San Francisco Bay Area. Earthquake focal mechanisms are commonly produced using P wave first motion polarities, which traditionally requires events to be well‐recorded across a seismic network with good focal sphere coverage. We adapted recently developed approaches that are less dependent on high signal‐to‐noise records and exploit similar waveforms to produce relative polarity and amplitude measurements between earthquake pairs. These techniques were previously only applied on localized earthquake sequences, and we further developed these approaches so that they can be utilized for regional catalogs. We validated or corrected manually identified polarities by performing polarity consensuses using earthquake pairs. Missing and unreliable polarity measurements were imputed using iterative random forests, an unsupervised ensemble machine learning method. Relative P and S wave amplitude measurements were made between earthquakes, constraining S/P ratios for low signal‐to‐noise waveforms. Using these techniques, we were able to reduce focal mechanism uncertainties by an average of ∼13° and produced well‐constrained focal mechanisms for ∼6 times as many earthquakes than those produced using only the traditionally derived polarities. We performed stress inversions using the focal mechanisms by grouping the focal mechanism results into a quadtree structure. Our stress results are consistent with previous work, albeit at a higher spatial resolution, and demonstrate these techniques can aid our understanding of fault structures and kinematics in more detail than was previously possible. Plain Language Summary: We determined whether earthquake compressional waves recorded on seismic stations caused either an initial upwards or downwards motion by comparing hundreds of thousands of waveforms against one another in a portion of the southeastern San Francisco Bay Area. If the waveforms were similar, the polarities for two events were likely the same. If the waveforms were anti‐similar (one waveform was "flipped"), then the polarities for the two events were likely different. Using these comparisons, we were able to correct polarities that had been manually selected in addition to increasing the number of polarity measurements. Some polarities for earthquakes recorded at a seismic station could still not be determined, for example, a station was installed after an earthquake occurred. To further increase the number of polarity measurements, we used a machine learning technique to estimate these missing polarities. All of these polarity measurements were used to determine how the faults hosting the cataloged earthquakes slipped, which allowed us to better understand stresses in the area. Key Points: We developed a new approach using relative polarity measurements between earthquakes to constrain focal mechanisms in regional catalogsConsidering a catalog of 29,179 earthquakes, we determined quality mechanisms for 18,226 events and used them to constrain stress fieldFocal mechanism stress inversion results are consistent with previous work, albeit at a higher resolution [ABSTRACT FROM AUTHOR]

A precise finite‐fault model including the fault geometry and slip distribution is essential to understand the physics of an earthquake. However, the conventional linear inversion of geodetic data for a finite‐fault model cannot fully resolve the fault geometry. In this study, we developed a Bayesian inversion framework that can comprehensively solve a non‐planar fault geometry, the corresponding fault slip distribution with spatially variable directions, and objective weighting for multiple data types. In the proposed framework, the probability distributions of all the model parameters are sampled using the Monte Carlo method. The developed methodology removes the requirement for manual intervention for the fault geometry and data weighting and can provide an ensemble of plausible model parameters. The performance of the developed method is tested and demonstrated through inversions for synthetic oblique‐slip faulting models. The results show that the constant rake assumption can significantly bias the estimates of fault geometry and data weighting, whereas additional consideration of the variability of slip orientations can allow plausible estimates of a non‐planar fault geometry with objective data weighting. We applied the method to the 2013 Mw 6.5 Lushan earthquake in Sichuan province, China. The result reveals dominant thrust slips with left‐lateral components and a curved fault geometry, with the confidence interval of the dip angles being between 20°–25° and 56°–58°. The proposed method provides useful insights into the scope of imaging a non‐planar fault geometry, and could help to interpret more complex earthquake sources in the future. Plain Language Summary: An earthquake originates from a rapid relative movement between two blocks on both sides of a fault. The earthquake can produce ground deformation, which can be measured by geodetic instruments deployed on the ground. Geophysicists can thus utilize the geodetic measurements to invert the earthquake attributes, including the fault geometry and the associated slips (i.e., the zone and amount of the relative movement between two blocks). Simultaneous estimation of the fault geometry parameters and the slip parameters is a nonlinear problem. The conventional inversion of geodetic data uses a linear adjustment method that cannot fully resolve the fault geometry. In addition, geodetic measurements have different precisions in terms of the types of instruments. We developed a nonlinear inversion based Bayesian inference framework to solve both these problems. The proposed method can correctly interpret an earthquake source with a non‐planar fault geometry. We applied the method to the 2013 Mw 6.5 Lushan earthquake in southwest China and determined a statistical estimate of the fault geometry and slips. The results indicate that the proposed method could help to improve our knowledge of complicated earthquake sources in the future. Key Points: We develop a Bayesian finite‐fault inversion for non‐planar fault geometry and spatially variable slip amplitude and rakeAccounting for variable directions of fault slips allows to better constrain non‐planar fault geometry and tune objective data weightingAccounting for the non‐planar fault geometry, the 2013 Lushan earthquake modeling reveals a dominant thrust slip with sinistral component [ABSTRACT FROM AUTHOR]

Evaluating various investigations for north-German gas fields, we discuss past and actual evolutions of the rock fabric in the light of dilatant driven and spontaneous contractant critical phenomena. Features of the latter were discovered by multi-stage triaxial tests with water-saturated sandstone samples and were similarly observed around the gas fields. A Mohr–Coulomb condition with quasi-local stress components ( σ ^ 1 ′ and σ ^ 3 ′ ), and variable parameters ϕ ' and c ^ ′ , can capture successive critical states of the solid fabric. The implied driven dilatation up to a collapse with contraction is captured by a stress-dilatancy relation. Fractal patterns of shear bands (faults) dominate if the smallest principal stress σ ^ 3 ′ exceeds c ^ ′ , otherwise cracks dominate and can lead to a rockburst. Triaxial tests with X-ray attenuation, seismometry including the splitting of shear waves and/or neutron beam diffraction contribute to clarification and validation. Seismic early warning and calculation models for various geotechnical operations with dominating faults can thus be improved, but the task is more difficult for rockbursts. [ABSTRACT FROM AUTHOR]

Earthquake early warning (EEW) systems provide seconds to tens of seconds of warning time before potentially-damaging ground motions are felt. For optimal warning times, seismic sensors should be installed as close as possible to expected earthquake sources. However, while the most hazardous earthquakes on Earth occur underwater, most seismological stations are located on-land; precious seconds may go by before these earthquakes are detected. In this work, we harness available optical fiber infrastructure for EEW using the novel approach of distributed acoustic sensing (DAS). DAS strain measurements of earthquakes from different regions are converted to ground motions using a real-time slant-stack approach, magnitudes are estimated using a theoretical earthquake source model, and ground shaking intensities are predicted via ground motion prediction equations. The results demonstrate the potential of DAS-based EEW and the significant time-gains that can be achieved compared to the use of standard sensors, in particular for offshore earthquakes. [ABSTRACT FROM AUTHOR]

We report on the effort of the International Seismological Centre (ISC, www.isc.ac.uk) to digitise station measurements for earthquakes as well as other types of seismic events that are currently listed in the ISC database without station data. The association of instrumental measurements to a seismic event is fundamental to verify its occurrence and it facilitates, when possible, its hypocentre relocation. We search for relevant station data in the Bureau Central de l'Association Internationale de Sismologie (BCIS). The BCIS bulletins were produced at the same time of the International Seismological Summary (ISS) but they appear to list more events with data than the ISS from the 1950s. Unfortunately, BCIS bulletins have not been digitised in a systematic way. We show that during the period going from the mid-1950s up to 1963 the BCIS is a unique source of station data that allowed us to verify thousands of earthquakes worldwide and significantly expand the global record of seismicity in those years. The data digitised in this work will play a key role in future works regarding earthquakes in the pre-digital era. [ABSTRACT FROM AUTHOR]

Several studies focusing on the anomalies of one specific parameter (such as magnetic, ionospheric, radon release, temperature, geodetic, etc.) before impending earthquakes are constantly challenged because their results can be regarded as noise, false positives or are not related to earthquakes at all. This rise concerns the viability of studying isolated physical phenomena before earthquakes. Nevertheless, it has recently been shown that all of the complexity of these pre-earthquake anomalies rises because they could share the same origin. Particularly, the evolution and concentration of uniaxial stresses within rock samples have shown the generation of fractal crack clustering before the macroscopic failure. As there are studies which considered that the magnetic anomalies are created by lithospheric cracks in the seismo-electromagnetic theory, it is expected that the crack clustering is a spatial feature of magnetic and non-magnetic anomalies measurements in ground, atmospheric and ionospheric environments. This could imply that the rise of multiparametric anomalies at specific locations and times, increases the reliability of impending earthquake detections. That is why this work develops a general theory of fractal-localization of different anomalies within the lithosphere in the framework of the seismo-electromagnetic theory. In addition, a general description of the fractal dimension in terms of scaling entropy change is obtained. This model could be regarded as the basis of future early warning systems for catastrophic earthquakes. [ABSTRACT FROM AUTHOR]

Earthquake prediction is a common scientific challenge for academics worldwide. This dilemma originates from the lack of precursory indicators that meet the sufficient and necessary conditions of earthquake occurrence, which may be the root cause of the failure of earthquake prediction. In light of this, a double-block catastrophic mechanics theory for earthquakes based on cross-fault Newton force measurement is proposed herein. Based on this theory and laboratory physical model tests of seismic Newton force monitoring, a new academic thought is envisioned "the sufficient and necessary condition for earthquake occurrence is the change of Newton force, and the sudden drop of Newton force on the fault surface can be used as a predictor of earthquake disaster." Several equipment systems have been independently developed, and the technology has been successfully applied to engineering practice. This concept has currently been proven in small-scale double-block catastrophic events such as landslides. Based on the double-block catastrophic mechanics theory, landslides and earthquakes have the similar nature but different scales. According to the on-site monitoring of landslides, it is verified that the sudden drop of Newton force can be used as a predictor of landslide disaster which successfully solves the problem of short-term landslide prediction. The introduction of cross-fault Newton force measurement technology and idea has laid a foundation for improving the method and level of international earthquake monitoring and solving the world-class scientific problem of short-term earthquake prediction. [ABSTRACT FROM AUTHOR]

As one of the most prevailing geodetic tools, the interferometric synthetic aperture radar (InSAR) technique can accurately obtain co-seismic displacements, but is limited to the one-dimensional line-of-sight (LOS) measurement. It is therefore difficult to completely reveal the real three-dimensional (3D) surface displacements with InSAR. By employing azimuth displacement observations from pixel offset tracking (POT) and multiple aperture InSAR (MAI) techniques, 3D displacements of large-magnitude earthquakes can be obtained by integrating the ascending and descending data. However, this method cannot be used to accurately realize the 3D surface displacement measurements of small-magnitude earthquakes due to the low accuracies of the POT/MAI-derived azimuth displacement measurements. In this paper, an alternative method is proposed to calculate co-seismic 3D displacements from ascending and descending InSAR-LOS observations with the dislocation model-based displacement direction constraint. The main contribution lies in the two virtual observation equations that are obtained from the dislocation model-based forward-modeling 3D displacements, which are then combined with the ascending/descending InSAR observations to calculate the 3D displacements. The basis of the two virtual observation equations is that the directions of the 3D displacement vectors are very similar for real and model-based 3D displacements. In addition, the weighted least squares (WLS) method is employed to solve the final 3D displacements, which aims to consider and balance the possible errors in the InSAR observations as well as the dislocation model-based displacement direction constraint. A simulation experiment demonstrates that the proposed method can achieve more accurate 3D displacements compared with the existing methods. The co-seismic 3D displacements of the 2020 Nima earthquake are then accurately obtained by the proposed method. The results show that co-seismic displacements are dominated by the vertical displacement, the magnitude of the horizontal displacement is relatively small, and the overall displacement pattern fits well with the tensile rupture. [ABSTRACT FROM AUTHOR]

Currently, it is still challenging to detect earthquakes by using the measurements of Global Navigation Satellite System (GNSS), especially while only adopting single-frequency GNSS. To increase the accuracy of earthquake detection and warning, extra information and techniques are required that lead to high costs. Therefore, this work tries to find a low-cost method with high-accuracy performance. The contributions of our research are twofold: (1) an improved earthquake-displacement estimation approach by considering the relation between earthquake and ionospheric disturbance is presented. For this purpose, we propose an undifferenced uncombined Single-Frequency Precise Point Positioning (SF-PPP) approach, in which both the ionospheric delay of each observed satellite and receiver Differential Code Bias (DCB) are parameterized. When processing the 1 Hz GPS data collected during the 2013 Mw7.0 Lushan earthquake and the 2011 Mw9.0 Tohoku-Oki earthquake, the proposed SF-PPP method can provide coseismic deformation signals accurately. Compared to the results from GAMIT/TRACK, the accuracy of the proposed SF-PPP was not influenced by the common mode errors that exist in the GAMIT/TRACK solutions. (2) Vertical Total Electron Content (VTEC) anomalies before an earthquake are investigated by applying time-series analysis and spatial interpolation methods. Furthermore, on the long-term scale, it is revealed that significant positive/negative VTEC anomalies appeared around the earthquake epicenter on the day the earthquake occurred compared to about 4–5 days before the earthquake, whereas, on the short-term scale, positive/negative VTEC anomalies emerged several-hours before or after an earthquake. [ABSTRACT FROM AUTHOR]

Objectives: Revealing the characteristics of deformation field related to earthquake preparation process is an important way to distinguish earthquake preparation stage and earthquake risk. In order to reveal the significance of crustal deformation observation, the deformation field related to the Wenchuan Ms 8.0 earthquake preparation and the relationship between the deformation field and the seismic activity are discussed from a large spatial scale. Methods: The original data of crustal tilt observed at Wenchuan station in the epicenter area of the Wenchuan earthquake since 2002 are collected, and the phased characteristics of crustal tilt change are extracted by using wavelet analysis method and piecewise fitting method. Based on the daily solution data of global navigation satellite system (GNSS) station coordinates since 2001, the baselines between Luzhou station and the surrounding stations are calculated. By comparing the baseline change with the Benioff creep, the synergistic characteristics between them are found. Results: The tilt rate in EW direction at Wenchuan station began to decrease significantly at the end of 2004, from 230 ms/a to 130 ms/a, and it decreased to 60 ms/a until the earthquake occurrence. There was no significant exponential acceleration change from the end of 2007 to the earthquake occurrence. The rapidly shortening anomaly of Guangzhou-Luzhou GNSS baseline in early 2005 has a high correlation with the earthquake creep loss anomaly, and the correlation coefficient is 0.89. The duration of rapidly shortening is about 2.5 years, and the magnitude of the corresponding earthquake is estimated to be 7.9, which is close to the magnitude of the Wenchuan earthquake. The Sumatra Ms 8.9 earthquake in 2004 has a significant impact on the stress-strain field in the eastern Tibet Plateau and accelerates the preparation process of the Wenchuan earthquake. Conclusions: The different influence of surrounding earthquakes should be considered in the analysis of crustal deformation and earthquake risk. The research results of this paper have reference value for understanding the significance of crustal deformation measurements and innovating earthquake prediction ideas. [ABSTRACT FROM AUTHOR]

Earthquake stress drop is an important source parameter that directly links to strong ground motion and fundamental questions in earthquake physics. Stress drop estimations may contain significant uncertainties due to such factors as variations in material properties and data limitations, which limit the applications of stress drop interpretations. Using a high‐resolution borehole network, we estimate stress drop for 4551 (M0‐4) earthquakes on the San Andreas Fault at Parkfield, California, between 2001 and 2016 using spectral decomposition and an improved stacking method. To evaluate the influence of spatiotemporal variations of material properties on stress drop estimations, we apply different strategies to account for spatial variations of velocity and attenuation changes, and divide earthquakes into three separate time periods to correct temporal variations of attenuation. These results show that appropriate corrections can significantly reduce the scatter in stress drop estimates, and decrease apparent depth and magnitude dependence. We find that insufficient bandwidth can cause systematic underestimation of stress drop estimates and increased scatter. The stress drop measurements from the high‐frequency borehole recordings exhibit complex stable spatial patterns with no clear correlation with the nature of fault slip, or the slip distribution of the 2004 M6 earthquake. Temporal variations are significantly smaller, less well resolved and varying spatially. They do not affect the long‐term stress drop spatial variations, suggesting local material properties may control the spatial heterogeneity of stress drop. Plain Language Summary: Earthquake stress drop (the change in shear stress before and after the earthquake) reflects the properties of the fault where earthquakes occur. Determining the scaling relationships between earthquake stress drop and magnitude enables us to predict the behavior of infrequent large earthquakes from the measurements of the more abundant small earthquakes. Measurement of stress drop is challenging, especially for small earthquakes. Different studies can obtain different stress drop values for the same earthquakes, leading to different interpretations. We use recordings from a high‐resolution borehole network in Parkfield, California to measure source parameters for small earthquakes. We examine the influence of data limitation (e.g., frequency bandwidth) and corrections for material properties on stress drop resolution. We find that if the bandwidth is too narrow, the resulting source parameters can be systematically underestimated with large scatter. Insufficient corrections of material properties can lead to biased interpretation of stress drop patterns and scaling relationships. Our final results show that the stress drops of earthquakes in Parkfield do not depend on magnitude or depth, but exhibit strong spatial variations that are stable with time. The 2004 M6 earthquake caused temporal variations of stress drop, and the temporal changes are different at different fault patches. Key Points: Borehole data and an improved spectral decomposition approach provide better constraints on estimates of corner frequency and stress dropFrequency bandwidth, and corrections for variable attenuation and velocity with depth most influence stress drop estimates and interpretationsSpatial pattern of stress drop is heterogeneous on a ∼1 km scale, and stable with time; temporal changes are smaller and spatially variable [ABSTRACT FROM AUTHOR]

On Jul. 1873，French missionaries built an observatory in Xujiahui Shanghai. On Jan. 22nd 1904，the observatory began to observe the earthquakes and published many reports，annuals and records. Chinese priest Pierre Hoang （Huang Bolu） was the bishop’s secretary in Jiangnan parish. Invited by the director of Xujiahui observatory，he edited 8 volumes of Chinese version “the Earthquakes” from 1906 to 1909. Later he translated it into French version with 2 columns and named “the Catalog of Earthquakes Reported in China” . The first column was published on Nov. 1909 by Xujiahui Tushanwan Press and the second column on Oct. 1913. This is the first Chinese geography and earthquake monograph edited by a Chinese scholar. In the first column，there are chronological earthquake records of 19 province territories including Fu，Zhou，Ting and County. In the second column，there are national chronological earthquake records in gregorian order，attached by a national earthquake city distribution map，distribution map of 139 earthquake locations and an academic report of Chinese earthquake activities cycle.“The Catalog of Earthquakes Reported in China” gave Dr. Weng Wenhao much inspiration，and later he sent Li Shanbang to Xujiahui observatory to learn seismic technology. After that，Li Shanbang took charge of Peking Jiufeng Seismic Station. Xujiahui Shanghai deserved the title of birthplace of Chinese modern earthquake science.

Hydraulic fracturing is performed to enhance rock permeability, for example, in the frame of geothermal energy production or shale gas exploitation, and can potentially trigger induced seismicity. The tracking of increased permeabilities and the fracturing extent is often based on the microseismic event distribution within the stimulated rock volume, but it is debated whether the microseismic activity adequately depicts the fracture formation. We are able to record tilt signals that appear as long‐period transients (< $< $180 s) on two broadband seismometers installed close (17–72 m) to newly formed, meter‐scale hydraulic fractures. With this observation, we can overcome the limitations of the microseismic monitoring alone and verify the fracture mapping. Our analysis for the first time combines a catalog of previously analyzed acoustic emissions ([AEs] durations of 20 ms), indirectly mapping the fractures, with unique tilt signals, that provide independent, direct insights into the deformation of the rock. The analysis allows to identify different phases of the fracturing process including the (re)opening, growth, and aftergrowth of fractures. Further, it helps to differentiate between the formation of complex fracture networks and single macrofractures, and it validates the AE fracture mapping. Our findings contribute to a better understanding of the fracturing processes, which may help to reduce fluid‐injection‐induced seismicity and validate efficient fracture formation. Plain Language Summary: Hydraulic fracturing (HF) describes the opening of fractures in rocks by injecting fluids under high pressure. The new fractures not only can facilitate the extraction of shale gas but can also be used to heat up water in the subsurface in enhanced geothermal systems, a corner stone of renewable energy production. The fracture formation is inherently accompanied by small, nonfelt earthquakes (microseismic events). Occasionally, larger events felt by the population can be induced by the subsurface operations. Avoiding such events is important for the acceptance of HF operations and requires a detailed knowledge about the fracture formation. We jointly analyze two very different data sets recorded during mine‐scale HF experiments: (a) the tilting of the ground caused by the opening of the fractures, as recorded by broadband seismometers—usually deployed for earthquake monitoring—installed close to the experiments and (b) a catalog of acoustic emissions, seismic signals of few milliseconds emitted by tiny cracks around the forming hydraulic fracture. The novel joint analysis allows to characterize the fracturing processes in greater detail, contributing to the understanding of the physical processes, which may help to understand fluid‐injection‐induced seismicity and validate the formation of hydraulic fractures. Key Points: We present and model unique observations of tilt signals induced by hydraulic fractures on broadband seismometersThe tilt magnitude is correlated with the injected volume and provides independent insight into fracture volume and fracturing efficiencyA joint analysis of tilt and acoustic emissions helps characterizing the fracture growth and provides evidence for aftergrowth [ABSTRACT FROM AUTHOR]

An explosive sound was heard over a wide area of Sapporo city, Hokkaido, Japan, on 26 April 2021. It was accompanied by minor shaking and was considered likely to be a sonic boom produced by a fireball. However, no luminous phenomenon was observed. We analyzed seismic data to identify the source of the sonic boom, and estimated the trajectory of the object from the associated signal arrival times. The source was located ∼100 km off the west coast of Hokkaido at an altitude of 50 km. The elevation angle was greater than 66° and the estimated velocity was higher than that of a fighter plane. Although the luminous phenomenon was not observed, the trajectory model suggested that the source of the sonic boom was not an aircraft and most likely a fireball. We compared trajectory models estimated from optical and seismic observations for past sonic boom events, and they were found to be in good agreement. Although our trajectory model is simplified and assumes that sound and the fireball have a constant speed, the model is highly accurate. The results show that seismic data can be used to estimate the trajectory of objects producing sonic booms, even when visual observations are unavailable. Plain Language Summary: An unexplained explosive sound was heard around Sapporo city, Hokkaido, Japan at 20:00 JST on 26 April 2021. The sound and the resulting minor tremors were widely felt throughout the densely populated city. To determine the source of the explosive sound, we analyzed the data obtained by seismometers and found that the signals of the tremors had been recorded. The explosive sound represented a sonic boom, which is a loud sound generated by an object traveling faster than the speed of sound. Based on the arrival pattern of the signal, we estimated the location and moving direction associated with the source of this sound. The source was located about 100 km off the west coast of Hokkaido and the estimated velocity was higher than that of a fighter plane. Although no luminous phenomenon was observed, we believe the sonic boom was generated by a fireball. Key Points: We analyzed seismic data to identify the source of an explosive sound heard in Sapporo city, Hokkaido, Japan, on 26 April 2021A trajectory model was established, and the source was identified as an object producing a sonic boom that was most likely a fireballSeismic data can be used to estimate the trajectory of objects producing sonic booms, even those not visually observed [ABSTRACT FROM AUTHOR]

Despite the importance of viscoelasticity in the evolution of crustal stress/strain being widely recognized, the interpretation of interseismic geodetic measurements for assessing earthquake potential is still based overwhelmingly on elastic models. The reasons for this disparity include conflating deformation rates with deformation itself and the lack of a succinct representation of the seismic readiness of a locked fault in a viscoelastic Earth. Using a classical viscoelastic model for strike-slip faults, we reiterate the commonly overlooked message that, if the recurrence interval is long, most of the strain energy for the next earthquake accrues early in the cycle, and low strain rates later in the cycle by no means indicate diminished rupture potential. Fault stress stays near failure for much of the late interseismic period which may explain why slow slip-rate faults have more variable recurrence intervals than fast slip-rate faults. We propose to use displacement deficit instead of slip deficit to represent seismic readiness. Plain Language Summary Modern satellite measurements can reveal how quickly faults are being loaded by tectonic plate motions, and seismic hazard models use these loading rates as proxy for the likelihood of a pending earthquake. However, because of the partially fluid-like behavior of Earth's interior, these loading rates have actually evolved with time since the last rupture. For faults with long intervals between successive earthquakes, these rates slow down substantially as the next event draws near. We, therefore, caution that slow rates of loading should not be assumed to reflect limited earthquake potential. [ABSTRACT FROM AUTHOR]

In the field of seismic interferometry, cross-correlations are used to extract Green's function from ambient noise data. By applying a single station variation of the method, using autocorrelations, we are in principle able to retrieve zero-offset reflections in a stratified Earth. These reflections are valuable as they do not require an active seismic source and, being zero-offset, are better constrained in space than passive earthquake based measurements. However, studies that target Moho signals with ambient noise autocorrelations often give ambiguous results with unclear Moho reflections. Using a modified processing scheme and phase-weighted stacking, we determine the Moho P -wave reflection time from vertical autocorrelation traces for a test station with a known simple crustal structure (HYB in Hyderabad, India). However, in spite of the simplicity of the structure, the autocorrelation traces show several phases not related to direct reflections. Although we are able to match some of these additional phases in a qualitative way with synthetic modelling, their presence makes it hard to identify the reflection phases without prior knowledge. This prior knowledge can be provided by receiver functions. Receiver functions (arising from mode conversions) are sensitive to the same boundaries as autocorrelations, so should have a high degree of comparability and opportunity for combined analysis but in themselves are not able to independently resolve V P , V S and Moho depth. Using the timing suggested by the receiver functions as a guide, we observe the Moho S -wave reflection on the horizontal autocorrelation of the north component but not on the east component. The timing of the S reflection is consistent with the timing of the PpSs–PsPs receiver function multiple, which also depends only on the S velocity and Moho depth. Finally, we combine P receiver functions and autocorrelations from HYB in a depth–velocity stacking scheme that gives us independent estimates for V P , V S and Moho depth. These are found to be in good agreement with several studies that also supplement receiver functions to obtain unique crustal parameters. By applying the autocorrelation method to a portion of the EASI transect crossing the Bohemian Massif in central Europe, we find approximate consistency with Moho depths determined from receiver functions and spatial coherence between stations, thereby demonstrating that the method is also applicable for temporary deployments. Although application of the autocorrelation method requires great care in phase identification, it has the potential to resolve both average crustal P and S velocities alongside Moho depth in conjunction with receiver functions. [ABSTRACT FROM AUTHOR]

Our models and understanding of the dynamics of earthquake rupture are based largely on estimates of earthquake source parameters, such as stress drop and radiated seismic energy. Unfortunately, the measurements, especially those of small and moderate-sized earthquakes (magnitude less than about 5 or 6), are not well resolved, containing significant random and potentially systematic uncertainties. The aim of this review is to provide a context in which to understand the challenges involved in estimating these measurements, and to assess the quality and reliability of reported measurements of earthquake source parameters. I also discuss some of the ways progress is being made towards more reliable parameter measurements. At present, whether the earthquake source is entirely self-similar, or not, and which factors and processes control the physics of the rupture remains, at least in the author's opinion, largely unconstrained. Detailed analysis of the best recorded earthquakes, using the increasing quantity and quality of data available, and methods less dependent on simplistic source models is one approach that may help provide better constraints. This article is part of the theme issue 'Fracture dynamics of solid materials: from particles to the globe'. [ABSTRACT FROM AUTHOR]

Earthquake disasters can cause enormous social and economic damage, and therefore the sustainability of infrastructure requires the mitigation of earthquake consequences. In seismic design of infrastructures, it is essential to estimate the response of the site during earthquake. Geotechnical engineers have developed quantitative methods for analyzing the seismic ground response. This study proposes a multilayer perceptron (MLP) model to evaluate the seismic response of the surface based on the seismic motion at the bedrock (or 100 m level), and compares its performance with that of a conventional model. A total of 6 sites, with 100 earthquake events at each site, were selected from the Kiban Kyoshin Network (KiK-net) and used as datasets. The acceleration response spectra were calculated from the predicted and measured (baseline) acceleration histories and compared. The proposed MLP model predicted the magnitudes of response and the natural periods where the response amplifies closely with the measured ground motions (baseline). The MLP model outperformed the conventional model for seismic ground response analysis. However, the proposed model did not perform as well for earthquakes whose response spectra exceed 2 g due to a deficiency in large earthquake measurements in the training datasets. [ABSTRACT FROM AUTHOR]

This contribution is addressed to an introductive university course on the correlation existing between radon emission and earthquakes processes held following a flipped-class approach where students receive didactic materials prior to face-to-face lessons. This research was initially started to investigate the real correlation between Radon emission from the Earth and the occurrence of strong earthquakes by using measurements of hourly Radon flow variation. During quiet seismogenic conditions, we observe an unvarying level of Radon emission in the air. Before a strong earthquake, substantial variations of Radon (222Rn) concentration have been observed in the air, probably because of the increase of thermodynamic energy inside the Earth. The physical processes affecting earthquakes are still not fully understood; therefore, we are interested in investigating a wide variety of signals observed before an earthquake, ranging from chemical, electric, and magnetic variations. The goal is to be able to estimate the earthquake magnitude, timing also location in advance with a good approximation. The experimental observation and research studies were carried out by G. Giuliani Permanent Foundation since 2002 in Abruzzo. The innovative methodology of observations with Gamma detectors allowed us to reveal a close correlation between the different physical phenomena during the preparation phase of strong earthquakes. We master the methodology of measuring the hourly flow of 222Rn gas decay, which provides a good correlation with the occurrence of strong earthquakes. To advance the reliability of our assessment, we added more parameters to our observations, such as magnetic and RF. The joint analysis advances our understanding of the processes underlying the earthquake occurrence. The experimental observation of Radon has been tested for more than a decade in the Abruzzo region. The initial results provided the baseline of reliable correlation between radon variations and earthquakes that could be used as an alert mechanism for the forthcoming seismic events. The multiparametric approach of detecting pre-earthquakes signals provided the robustness in detecting the earthquake preparation phase. There are no doubts that by expanding the network of gamma sensors, we achieve much better signal detection, which is critical for the better spatial correlation of Radon variations with the earthquake processes. [ABSTRACT FROM AUTHOR]

The damping estimation of an eight-story steel building equipped with oil dampers is examined, carried out by adopting a proposed framework, which consists of an enhanced strain-energy method and an improved direct method for model updating. The building is located at Tohoku Institute of Technology and is equipped with a structural monitoring system that measures its seismic response, including floor acceleration and displacement and force of oil dampers. The enhanced strain-energy method is first developed and employed to assess the supplemental damping and stiffness provided by oil dampers, herein quantified in the form of equivalent damping ratios and natural frequencies. Then, modal characteristics extracted from the earthquake measurements are modified accordingly and utilized for the building model updating, in which mass and stiffness matrices are corrected by the improved direct method. The updated model accurately reproduces the target modal data, especially measured mode participation factors, and is further used for the building response predictions. Through prediction validations, the precision of the modified modal parameters is verified. Finally, a large earthquake event is chosen to demonstrate the effectiveness of the proposed framework for the damping estimation of the investigated building. [ABSTRACT FROM AUTHOR]

In this paper, we illustrate some of the current methods for the exploitation of data from Earth Observing satellites to measure and understand earthquakes and shallow crustal tectonics. The aim of applying such methods to Earth Observation data is to improve our knowledge of the active fault sources that generate earthquake shaking hazards. We provide examples of the use of Earth Observation, including the measurement and modelling of earthquake deformation processes and the earthquake cycle using both radar and optical imagery. We also highlight the importance of combining these orbiting satellite datasets with airborne, in situ and ground-based geophysical measurements to fully characterise the spatial and timescale of temporal scales of the triggering of earthquakes from an example of surface water loading. Finally, we conclude with an outlook on the anticipated shift from the more established method of observing earthquakes to the systematic measurement of the longer-term accumulation of crustal strain. [ABSTRACT FROM AUTHOR]

The article discusses a new breakthrough methodology for forecasting events and phenomena that are diverse in their physical nature. The article uses the total electron content trends (TEC) of the ionosphere as initial data for earthquake forecasting. The total electron content of the Earth's ionosphere is a good indicator of its state and its dynamics. It has been found that the most effective for earthquake forecasting are TEC indicators, recorded regularly during the several years preceding the forecasted earthquake. [ABSTRACT FROM AUTHOR]

Based upon the synthetic Rayleigh wave at different epicentral distances and real earthquake Rayleigh wave, S transform is used to measure their group velocities, compared with the Multiple Filter Technique (MFT) which is the most commonly used method for group-velocity measurements. When the period is greater than 15 s, especially than 40 s, S transform has higher accuracy than MFT at all epicenter distances. When the period is less than or equal to 15 s, the accuracy of S transform is lower than that of MFT at epicentral distances of 1000 km and 8000 km (especially 8000 km), and the accuracy of such two methods is similar at the other epicentral distances. On the whole, S transform is more accurate than MFT. Furthermore, MFT is dominantly dependent on the value of the Gaussian filter parameter a, but S transform is self-adaptive. Therefore, S transform is a more stable and accurate method than MFT for group velocity measurement of earthquake Rayleigh waves. [ABSTRACT FROM AUTHOR]

The formation of fold–thrust dislocations of stratified rocks in the Earth's crust has been considered as a consequence of the lateral compression of such rocks by converging platforms or lithospheric plates over the last 150 years within the framework of a geosyncline concept and then according to the concept of plate tectonics. Such convergences are reliably confirmed by current geodetic measurements. At the same time there are less popular ideas that present the formation of these dislocations as a result of various local, autonomous processes. The essence of the problem is that the domination of directionally ordered horizontal compression over the value of lithostatic pressure in the Earth's crust, which is unambiguously determined according to the data of focal mechanisms of earthquakes and in the measurements on stresses in situ in mine workings, makes it necessary to prefer the global mechanism of compression in the form of convergence of horizontally moving lithospheric plates. This convergence is reliably established on a global scale and by modern high-precision geodetic GPS measurements. However, similar GPS measurements, but on regional or local networks rather than on global ones, have shown an opposite result in a few cases. They revealed an increase in the width of representative segments of the mountain belts, rather than a decrease. A detailed study of the geological structure of such areas has indicated that the main role in their formation is played by scaly thrusts, sometimes transforming into small covers, and linear folds and groups of such folds associated with these dislocations and subordinate to them. The formation of such a cover-thrust tectonic structure continues now. For several years, the authors have developed a hypothesis according to which fold–thrust dislocations in the Earth's crust are formed as a result of an increase in the volume of stratified rocks and as a consequence of an increase in the area of these rocks. They cause volume expansion stresses, similar to the stresses of external compression; crumple; and are dissected by thrusts and reverse faults, tending to cover adjacent territories. It is assumed that the increase in the volume (volume expansion) and the areas of stratified rocks is a result of the entry of additional mineral material with ascending flows of deep mineralized fluids. The increase in the volume and the width of the mountain structure during the formation of its tectonic structure is indirectly confirmed by the near-vertical formations detected in the Earth's crust in several regions by geophysical methods, which have different velocity characteristics compared to the enclosing environment. These formations can be interpreted as channels of permeability for the penetration of deep fluids—the cause of an increase in rock volume. This suggests that the process of autonomous folding and thrusting in the Earth's crust really exists in the regional plan, first and foremost, within the mobile mountain belts, along with a global plate-tectonic mechanism. [ABSTRACT FROM AUTHOR]

Virtual reality (VR) has made it possible for users to access novel digital experiences. An interesting question that arises in the context of VR is whether it appears or feels different to users when different virtual environments are used. This study investigates the effect of VR head-mounted display (HMD) and desktop computer-facilitated VR on users' sense of presence (spatial presence and immersion) and task-oriented self-efficacy when exposed to an earthquake education VR system. A quasi-experiment design was used with a sample of 96 university students. The results revealed that the VR system had positive impacts on the users' earthquake preparedness self-efficacy. Although the experiment group (n = 39) had repeated experiences, as they first used desktop VR followed by VR HMD for the same content, users indicated a higher sense of spatial presence and immersion while using VR HMD than when using desktop VR. In addition, a VR HMD single-group pre- and posttest experimental design was performed with 20 participants, and the differences between the pretest and posttest measurements of earthquake preparedness and self-efficacy were determined to be significant. The qualitative results reveal that the visual stimulus and motion are relevant in composing the VR experience. [ABSTRACT FROM AUTHOR]

Ambient noise is useful for characterizing frequency‐dependent noise levels and for assessing data quality for seismic stations. We use 4 years of ambient noise spectra from 16 stations in central Alaska to examine environmental and structural influences on seismic stations. The region contains a major river (Tanana River) that is ice covered for half the year and is underlain by a sedimentary basin (Nenana basin) that strongly influences the seismic wavefield. Nenana basin amplifies ambient seismic noise by 12–16 dB at 0.1–0.7 Hz and 17–30 dB at 0.7–3 Hz. A meteorological station and river gauge at Nenana provide environmental data for comparison with seismic stations. During the summer, the Tanana River produces noise levels elevated by 30–40 dB at frequencies near 10 Hz, as recorded by all five stations within 100 m of the main river channel. The Tanana River sediment is predominantly silt and sand in this region; therefore, we attribute the 10‐Hz river signal to turbulence within the water and to unsteady shearing on the river bottom. The influence of wind is apparent on seismic noise at low (<0.05 Hz) frequencies, due to atmospheric‐induced tilting, and at high (>2.0 Hz) frequencies, due to unsteady shearing and turbulence near the ground. Our empirical findings motivate future studies, such as how flow from air or water couples to the ground and how deep sedimentary basins influence the ambient noise wavefield. Our results have implications for seismic site selection, environmental monitoring, and detection and characterization of earthquakes. Key Points: Four years of ambient noise data from 16 new seismometers in central Alaska is analyzed and compared with wind speed and river gauge dataThe Tanana River produces elevated seismic noise levels near 10 Hz that we attribute to water turbulence and shear along the river bottomThe Nenana basin amplifies seismic noise within two frequency ranges: 0.1–0.7 Hz, attributed to the deep basin structure, and 0.7–3.0 Hz [ABSTRACT FROM AUTHOR]

We use seismic noise cross-correlations to obtain a 3-D tomography model of SV -wave velocities beneath the western Indian Ocean, in the depth range of the oceanic crust and uppermost mantle. The study area covers 2000 × 2000 km2 between Madagascar and the three spreading ridges of the Indian Ocean, centred on the volcanic hotspot of La Réunion. We use seismograms from 38 ocean bottom seismometers (OBSs) deployed by the RHUM-RUM project and 10 island stations on La Réunion, Madagascar, Mauritius, Rodrigues, and Tromelin. Phase cross-correlations are calculated for 1119 OBS-to-OBS, land-to-OBS, and land-to-land station pairs, and a phase-weighted stacking algorithm yields robust group velocity measurements in the period range of 3–50 s. We demonstrate that OBS correlations across large interstation distances of >2000 km are of sufficiently high quality for large-scale tomography of ocean basins. Many OBSs yielded similarly good group velocity measurements as land stations. Besides Rayleigh waves, the noise correlations contain a low-velocity wave type propagating at 0.8–1.5 km s−1 over distances exceeding 1000 km, presumably Scholte waves travelling through seafloor sediments. The 100 highest-quality group velocity curves are selected for tomographic inversion at crustal and lithospheric depths. The inversion is executed jointly with a data set of longer-period, Rayleigh-wave phase and group velocity measurements from earthquakes, which had previously yielded a 3-D model of Indian Ocean lithosphere and asthenosphere. Robust resolution tests and plausible structural findings in the upper 30 km validate the use of noise-derived OBS correlations for adding crustal structure to earthquake-derived tomography of the oceanic mantle. Relative to crustal reference model CRUST1.0, our new shear-velocity model tends to enhance both slow and fast anomalies. It reveals slow anomalies at 20 km depth beneath La Réunion, Mauritius, Rodrigues Ridge, Madagascar Rise, and beneath the Central Indian spreading ridge. These structures can clearly be associated with increased crustal thickness and/or volcanic activity. Locally thickened crust beneath La Réunion and Mauritius is probably related to magmatic underplating by the hotspot. In addition, these islands are characterized by a thickened lithosphere that may reflect the depleted, dehydrated mantle regions from which the crustal melts where sourced. Our tomography model is available as electronic supplement. [ABSTRACT FROM AUTHOR]

The average shear wave velocity of the top 30 m of soil profile (V S30) is considered as an indicator of local-site effects in seismic hazard assessment (SHA) studies. Previous research works have demonstrated that in addition to actual V S30 values, their spatial correlation characteristics have considerable effects on the estimated values of earthquake intensity measurements, particularly when the seismic risk assessment of spatially distributed assets and infrastructure networks is of interest. Tehran city, in addition to its proximity to several faults, is situated on a variety of sedimentary textures and encompasses a large number of residential buildings and infrastructure networks. The current study utilizes advanced spatial statistics methods and investigates the spatial correlation characteristics of the random field of V S30 values in Tehran. Moreover, by employing a more comprehensive database of V S30 values measured using geoseismic approaches and using eight spatial interpolation methods, a new estimated V S30 map of Tehran city is provided. The geological formations of the study area are also investigated considering the distribution of sample points with different V S30 values. The findings of the current research can be utilized in spatially correlated SHA studies in the Tehran region and can be viewed as a step towards filling the data gap required for seismic risk assessment studies in this region. Moreover, by identifying the local variations in site classes, the findings of the current study can be used for further detailing of geological formations. • Spatial correlation attributes of V S30 values are identified to be used in spatially correlated seismic hazard assessment. • Advanced spatial statistics methods are utilized to develop V S30 models. • A new V S30 map of the Tehran region has been derived through the implementation of advanced geostatistical techniques. • The relationship between geological and geoseismic characteristics of the Tehran region is investigated. [ABSTRACT FROM AUTHOR]

The horizontal-to-vertical spectral ratio (HVSR) has seen widespread use as a tool for measuring a site's resonant behavior. However, the processing of HVSR with traditional methods can be tedious and time-consuming when the HVSR is complex and exhibits multiple resonances. This work proposes the AutoHVSR algorithm that allows for fully-automated processing of HVSR measurements, including those with zero, one, or multiple clear resonances. The AutoHVSR algorithm accepts microtremor or earthquake recordings and user-defined HVSR processing parameters as input and returns the computed HVSR from each time window or earthquake record, the mean/median HVSR curve, and statistics on each automatically identified HVSR resonance in terms of frequency and amplitude. The AutoHVSR algorithm integrates robust signal processing and computational methods with state-of-the-art machine-learning models trained using a diverse dataset of 1108 HVSR measurements. The AutoHVSR algorithm demonstrates excellent performance by correctly determining the number of HVSR resonances for 1098 of the 1108 HVSR measurements (>99%) and predicting the mean resonant frequency of the correctly identified resonances with a root mean square error (RMSE) of 0.05 Hz. Furthermore, the AutoHVSR algorithm was able to produce these predictions in 13 min (including HVSR processing time) compared to the 30 h required for traditional processing (a speed up of 138). The AutoHVSR algorithm is further demonstrated on a challenging dataset from Canterbury, New Zealand that included many HVSR curves with multiple and/or ambiguous resonances. Despite the challenging nature of the Canterbury dataset, the AutoHVSR algorithm was capable of correctly determining the number of HVSR resonances for 113 of the 129 HVSR measurements (>87%) and predicting the mean resonant frequency of the correctly identified resonances with a RMSE of 0.10 Hz. The AutoHVSR algorithm produced these predictions under 2 min (including HVSR processing time) compared to the 4 h required for traditional processing (a speed up of 120). Finally, while the AutoHVSR algorithm was developed using microtremor measurements where the horizontal components were combined using the geometric mean, it is shown to extend without modification to microtremor HVSR measurements where the two horizontal components are rotated azimuthally and to HVSR measurements from earthquake recordings. The AutoHVSR algorithm has been made publicly available in v0.3.0 of HVSRweb, it can be accessed at https://hvsrweb.designsafe-ci.org/. • The AutoHVSR algorithm is proposed for fully-automating HVSR processing. • AutoHVSR can process HVSR with zero, one, or multiple clear resonances. • The predictions of AutoHVSR are fast, reliable, and reproducible. • AutoHVSR can be applied broadly including azimuthally-rotated and earthquake HVSR. • AutoHVSR has been made publicly accessible through HVSRweb. [ABSTRACT FROM AUTHOR]

The article discusses an optimization strategy designed to improve the International Seismological Centre (ISC) earthquake focal mechanism computations. The authors detail how this can minimize an objective misfit function before describing how the optimized automatic picker to increase the number of first motion auto-picked polarities.