Search

The seismic attenuation relationship between ground motion parameters (such as peak acceleration and response spectra value) and seismic parameters (such as magnitude and epicentral distance) is an important foundation for seismic hazard analysis and the core of determining seismic input parameters for seismic resistance engineering. The acceleration envelope parameters, which describe the relationship between ground motion intensity and time variation, are primarily used for artificially synthesizing seismic motion. At present, little research has been performed on the attenuation relationship of acceleration envelope parameters in the Longmenshan fault zone on the eastern side of the Tibetan Plateau in China. Therefore, this study selected the Mw4-6 aftershock records of the 2008 Wenchuan earthquake that occurred on the Longmenshan fault zone and established a commonly used three segment envelope model parameter attenuation relationship. We classified aftershock records based on their source mechanisms and obtained attenuation relationship models for thrust slip aftershocks, thrust and strike slip aftershocks, and strike slip aftershocks. The results are as follows: (1) The thrust slip aftershock records had the longest rising stage which is the time difference from the arrival of P-waves to the beginning of the stable sustained stage of seismic motion. Thrust and strike slip aftershocks records had the longest stable sustained stage period and the slowest attenuation at the tail of the record. (2) The attenuation relationship of the acceleration envelope parameters commonly used in Chinese engineering for artificially synthesizing seismic motion is based on the strong earthquake records in the western United States. But, compared to the attenuation characteristics of the acceleration envelope function in the western United States, the Mw4-6 earthquake records on the Longmenshan fault zone had a slower attenuation rate at the tail of the record. So, accurately artificially synthesizing seismic motion through envelope parameter attenuation model requires the use of attenuation model established by earthquake records in this region. [ABSTRACT FROM AUTHOR]

In this paper, we study the tectonic stress around the South Caspian Basin (SCB), which includes the Kopeh Dagh, Alborz, Talesh, eastern Greater Caucasus mountain belts, the Apsheron sill, and the Balkhan-Ashkabad fault zone. We apply the stress inversion to focal mechanisms of 410 mainshocks that have occurred over the last 69 years. These mechanisms indicate that the surrounding fault zones of the SCB exhibit diverse types of faulting, ranging from thrust to strike-slip, normal, and their combinations. The results of the stress inversion align with the kinematics of the major fault zones bounding the SCB and emphasize the spatial heterogeneity of the stress field in this region. The region is predominantly under compression, but transpressive and strike-slip regimes are also present. This highlights the role of obliquely oriented basement faults with respect to the maximum horizontal compressive stress (SHmax) in accommodating deformation through convergent zones. The orientation of the SHmax is in the NE to NNE direction in the Kopeh Dagh, Alborz, Talesh, and Ashkabad-Balkhan fault zones, being rotated to NNE-N in the Greater Caucasus. The orientation of the SHmax relative to the convergence direction of the Arabian and Eurasian plates indicates that the Arabian-Eurasian oblique convergence-derived tectonic stress is the primary contributor to the total stress and deformation in this region. [ABSTRACT FROM AUTHOR]

The P-wave peak distribution in azimuth and takeoff angles, corrected for the distance through empirical attenuation laws, can reveal the radiation pattern amplitude of the source (Tarantino et al., 2019). This piece of information, jointly combined with the available polarities and S-wave peak amplitudes, can provide the full focal mechanism, constraining the solution also when only a few seismic station records are available, that is, in the case of microseismicity. We proposed a new technique, named P-, S-wave amplitude, and polarities (P-SAP), designed to compute the focal mechanism by jointly inverting the P-, S-wave amplitude ratios and P-wave polarities in a Bayesian framework and employing the octree strategy (Fang et al., 1996) to explore the space of possible solutions. The outputs are strike, dip, and rake angles of the most likely triplet (principal and auxiliary planes) with the related uncertainties, as well as other multiple solutions, if present. We tested the methodology to synthetic data, and we applied it to a microseismic sequence that occurred in Irpinia region, southern Italy. A background microseismicity occurs in a volume delimited by the faults activated during the 1980 M 6.9 Irpinia earthquake. This faults system is complex and composed of northwest-southeast-striking normal faults along the Apennines chain and an approximately east-west-oriented strike-slip fault, deep-seated in the Potenza area. A network of three-component accelerometers and velocimeters are currently deployed to monitor the area. The major topic of this work is validating the proposed P-SAP methodology for microseismicity studies. We also inferred the optimum stress tensor of the sequence, confirming that the microseismicity is controlled by the regional stress field and can reveal characteristics useful to highlight behaviors of larger-scale seismicity. [ABSTRACT FROM AUTHOR]

We numerically investigate the mechanisms that resulted in induced seismicity occurrence associated withCO2 injection at the Illinois Basin-Decatur Project (IBDP). We build a geologically consistent model that honors key stratigraphic horizons and 3D fault surfaces interpreted using surface seismic data and microseismicity locations. We populate our model with reservoir and geomechanical properties estimated using well-log and core data. We then performed coupled multiphase flow and geomechanics modeling to investigate the impact of CO2 injection on fault stability using the Coulomb failure criteria. We calibrate our flow model using measured reservoir pressure during the CO2 injection phase. Our model results show that pore-pressure diffusion along faults connecting the injection interval to the basement is essential to explain the destabilization of the regions where microseismicity occurred, and that poroelastic stresses alone would result in stabilization of those regions. Slip tendency analysis indicates that, due to their orientations with respect to the maximum horizontal stress direction, the faults where the microseismicity occurred were very close to failure prior to injection. These model results highlight the importance of accurate subsurface fault characterization for CO2 sequestration operations. [ABSTRACT FROM AUTHOR]

The correlation between seismic and acoustic signals can be used to quantify the influence of earthquake ground motion on the output signals of infrasound sensors and thus evaluate the performance of infrasound sensors under field conditions. However, there have been relatively few studies, and earthquakes considered in those studies are large magnitude events. There remains a need to quantify correlation mechanisms at high frequencies using small earthquakes. In this study, we utilize a collocated seismoacoustic station BJT and a six-element infrasound array DQS to study different coupling mechanisms for infrasound sensors. From 2019 to 2021, 11 and 37 small magnitude earthquakes were detected by BJT and DQS, respectively. Seismoacoustic signals from these small magnitude earthquakes are used to compute the seismoacoustic spectral ratio and the observed spectral ratios are then compared with theoretical estimates. For local seismic-to-acoustic coupling (local infrasound), the highly local pressure field induced by high-frequency ground motion and the separation between seismoacoustic sensors reduce seismic and acoustic signal correlation, which prevents the use of small earthquakes in current studies. The seismic sensitivity of the infrasound sensor, MB3a is shown to be for ground acceleration or for ground velocity and may be an important source of high-frequency noise for an infrasound station using this sensor. Furthermore, an empirical relation is developed to constrain the characteristics of earthquakes that can generate coupled signals on the infrasound sensor. Our study complements previous work and provides insight into the improved interpretation of infrasound signals and methods of seismoacoustic station calibration. [ABSTRACT FROM AUTHOR]

Determining the depths of small crustal earthquakes is challenging in many regions of the world, because most seismic networks are too sparse to resolve trade-offs between depth and origin time with conventional arrival-time methods. Precise and accurate depth estimation is important, because it can help seismologists discriminate between earthquakes and explosions, which is relevant to monitoring nuclear test ban treaties and producing earthquake catalogs that are uncontaminated by mining blasts. Here, we examine the depth sensitivity of several physics-based waveform features for ~8000 earthquakes in southern California that have well-resolved depths from arrival-time inversion. We focus on small earthquakes (2 < ML < 4) recorded at local distances (<150 km), for which depth estimation is especially challenging. We find that differential magnitudes (MW/ML-MC) are positively correlated with focal depth, implying that coda wave excitation decreases with focal depth. We analyze a simple proxy for relative frequency content, ɸ ≊ log10(M0) + 3log10(fc)ɸ, and find that source spectra are preferentially enriched in high frequencies, or "blue-shifted," as focal depth increases. We also find that two spectral amplitude ratios Rg 0.5-2 Hz/Sg 0.5-8 Hz and Pg/Sg at 3-8 Hz decrease as focal depth increases. Using multilinear regression with these features as predictor variables, we develop models that can explain 11%-59% of the variance in depths within 10 subregions and 25% of the depth variance across southern California as a whole. We suggest that incorporating these features into a machine learning workflow could help resolve focal depths in regions that are poorly instrumented and lack large databases of well-located events. Some of the waveform features we evaluate in this study have previously been used as source discriminants, and our results imply that their effectiveness in discrimination is partially because explosions generally occur at shallower depths than earthquakes. [ABSTRACT FROM AUTHOR]

The article investigates the reliability of moment tensor (MT) inversion in time domain with use of first P-wave amplitude, a method used to determine the source mechanisms of earthquakes, across four different seismic networks. The study compares the synthetic tests results of MT inversion for two underground mining and two artificial reservoir monitoring seismic networks. The analysis was performed to assesses how consistency and accuracy of the results depend on different factors like: network configuration, events depth, velocity model, focal mechanism of event and applied noise. The findings highlight the impact of network configuration compared to other variables and data quality on the reliability of moment tensor inversion and provide insights into different factors which have to be considered to enhance MT accuracy. The significance of events depth in P-wave amplitude MT inversion and the necessity to consider velocity model influence, especially presence of high velocity gradient, is highlighted by the presented results. [ABSTRACT FROM AUTHOR]

Based on the seismic moment tensor (SMT) data of 270 earthquakes in the Central Tien Shan, which occurred from 1978 to 2021 and include 63 events from the Global Centroid Moment Tensor catalog and 207 events from the works of Aleksander D. Kostyuk and Naylya A. Sycheva, the focal parameters (seismotectonic deformations (STD), kinematic and dynamic parameters, source radius, and stress drop) were calculated. The STD calculation was based on the approaches proposed in the studies of Yuriy V. Riznichenko and Sergei L. Yunga. The area under consideration is characterized by such deformation modes as thrust, transpression, underpression, and oblique. The distribution of the Lode–Nadai coefficient was calculated and plotted. A significant part of the study area is characterized by simple compression deformation, dominated by simple compression and simple shear. To calculate the stress drop deltasigma, the values of the scalar seismic moment M0, which are determined in the SMT calculation, and the source radii r, which are calculated on the basis of theoretical and experimental models of the dependence of the source radius on the moment magnitude, were used. The radii r and stress drops deltasigma were calculated for two models of earthquake sources: the Brune model and the Madariaga– Kaneko–Shearer model. A catalog of dynamic parameters was compiled. The comparison of the kinematic and dynamic parameters of earthquakes was carried out, and the relationship of the dropped stresses with the type of movement in the source, as well as with the Lode–Nadai coefficient, was established. The results obtained in the study can be useful for specialists in other fields of knowledge – geodesy, geology, and geophysics.

The 6 August 2023 MW5.5 Pingyuan earthquake is the largest earthquake in the central North China Plain (NCP) over the past two decades. Due to the thick sedimentary cover, no corresponding active faults have been reported yet in the epicenter area. Thus, this earthquake presents a unique opportunity to delve into the buried active faults beneath the NCP. By integrating strong ground motion records, high-precision aftershock sequence relocation, and focal mechanism solutions, we gain insights into the seismotectonics of the Pingyuan earthquake. The aftershocks are clustered at depths ranging from 15 to 20 km and delineate a NE-SW trend, consistent with the distribution of ground motion records. A NE-SW nodal plane (226°) of the focal mechanism solutions is also derived from regional waveform inversion, suggesting that the mainshock was dominated by strike-slip motion with minor normal faulting component. Integrating regional geological data, we propose that an unrecognized fault between the NE-SW trending Gaotang and Lingxian-Yangxin faults is the seismogenic fault of this event. Based on the S-wave velocity structure beneath the NCP, this fault probably extends into the lower crust with a high angle. Considering the tectonic regime and stress state, we speculate that the interplay of shear strain between the Amurian and South China blocks and the hot upwelling magma from the subducted paleo Pacific flat slab significantly contributed to the generation of the Pingyuan earthquake.

The 6 August 2023 MW5.5 Pingyuan earthquake is the largest earthquake in the central North China Plain (NCP) over the past two decades. Due to the thick sedimentary cover, no corresponding active faults have been reported yet in the epicenter area. Thus, this earthquake presents a unique opportunity to delve into the buried active faults beneath the NCP. By integrating strong ground motion records, high-precision aftershock sequence relocation, and focal mechanism solutions, we gain insights into the seismotectonics of the Pingyuan earthquake. The aftershocks are clustered at depths ranging from 15 to 20 km and delineate a NE-SW trend, consistent with the distribution of ground motion records. A NE-SW nodal plane (226°) of the focal mechanism solutions is also derived from regional waveform inversion, suggesting that the mainshock was dominated by strike-slip motion with minor normal faulting component. Integrating regional geological data, we propose that an unrecognized fault between the NE-SW trending Gaotang and Lingxian-Yangxin faults is the seismogenic fault of this event. Based on the S-wave velocity structure beneath the NCP, this fault probably extends into the lower crust with a high angle. Considering the tectonic regime and stress state, we speculate that the interplay of shear strain between the Amurian and South China blocks and the hot upwelling magma from the subducted paleo Pacific flat slab significantly contributed to the generation of the Pingyuan earthquake. [ABSTRACT FROM AUTHOR]

Abstract Two shallow earthquakes of moment magnitude 6.2 and 5.8 occurred in the central arm of Sulawesi on 26 July 2021 and 26 August 2021, respectively. The fault responsible for the earthquake had previously only been partially mapped, thus making further analysis of its characteristics crucial for the assessment of seismic hazard. In this study, we exploit data from a regional seismic network, relocate the associated seismicity using the double difference method with an updated velocity model, determine focal mechanisms from full-waveform inversion, and analyze the static stress changes caused by the mainshock. Our relocated hypocenters and focal mechanism solutions reveal two earthquake clusters, one at the Central Balantak Fault that exhibits normal slip on a SE–NW trending rupture, and the other at the West Balantak Fault, which exhibits dextral strike-slip motion on a SE–NW trending rupture. The additional static stress increase transferred by the Mw 6.2 mainshock may have triggered the subsequent Mw 5.8 event. A detailed assessment of previously unmapped faults in Central Sulawesi is essential for a more comprehensive understanding of seismic hazard in the region.

Abstract Understanding flow behavior in a geothermal reservoir is important for managing sustainable geothermal energy extraction. Fluid flow in geothermal reservoirs generally occurs in complex existing fracture systems in which the reservoirs are situated in highly fractured rocks. To simulate a discrete fracture model, the location and orientation of the fracture were computed using statistical processes and observational data. In many cases, estimating the location and orientation of fractures from 1D borehole logging data is challenging. In this study, we used microseismic data to build the fracture network systems and extract the detailed positions and dimensions of the fractures. We used the microseismic data recorded at the Okuaizu Geothermal Field, Fukushima Prefecture, Japan, from 2019 to 2021. First, we located the hypocenters, removing the effect of uncertainty in the velocity structure of the geothermal fluids. We relocated and clustered the seismic events based on waveform similarity. We analyzed each cluster to define the fracture orientation using principal component analysis (PCA) and focal mechanism (FM) analysis. We used the P polarity with the S/P ratio as a constraint for a better fault-plane solution. With PCA, we can extract the fracture dimension of each cluster. Our cluster analysis showed that the clusters were not always planar fractures, and we interpreted them as fracture zones. Based on the consistency between PCA and FM, each cluster/fracture zone was classified into three conceptual models to characterize the fracture network system in this field. This model showed variations in the orientation of small fractures within the fracture zone. We characterized the spatial variation in fracture distribution and orientations in the reservoir and demonstrated the fracture network system of this field. The fracture zone near the injection well has a N–S strike, and the dip is above 80°; however, the fracture zone in the northeastern part of the injection well has a NW–SE strike with a dip between 60° and 80°. The fracture network system estimated in this study is crucial for robust reservoir modeling because our model is more realistic, observation-orientated, and includes local anomalies of reservoir properties. Graphical Abstract

In this paper，the regional full waveform inversion using the broadband waveforms recorded by China Seismic Network were conducted，and the focal mechanism solutions of the 59 earthquakes with M≥4.0 occurred in the mainland of China during March to June 2024 were obtained. The types of these focal mechanism solutions show 18 reverse faulting，33 strike-slip faulting，7 normal faulting and 1 odd earthquakes.

Do large caves exist deep underground? Through seismological investigations, we speculate the existence of a large geological cave about 7 km beneath the surface. The complete moment tensor inversion method (gCAP ) was used to analyze the ML4.7 earthquake that occurred in Rongchang District, Chongqing, on December 27, 2016. The complete moment tensor solution of its focal mechanism was obtained, and the waveform fitting was good. The inversion results showed that this relatively special earthquake had a thrust and collapse type source and that the earthquake centroid depth was 7 km. Based on the focal mechanism results and the fact that the earthquake was located near an anticlinal fold, we infer that this earthquake was attributable to thrusting of a fault in the anticlinal fold, which simultaneously led to destruction of the stability of the geological cavity in the fold, leading to collapse. The focal mechanism results showed that the seismic moment tensor had mainly a horizontal component with less vertical component, which is consistent with the local tectonic stress. The results indicate that the complete moment tensor of the focal mechanism inversion method is an effective means for studying complex earthquakes. In addition, shale gas drilling in this area is very deep and there are hydraulic fracturing operations, suggesting that earthquakes may be induced by the water injection pressure.

On June 17, 2021, an ML3.6 earthquake occurred in Shuangqiao District, Chengde City, Hebei Province. Based on the data of Hebei regional digital seismic network, the double-difference location algorithm was used to reposition the earthquake sequence and a precise epicenter distribution was obtained. The fault plane parameters of this earthquake were computed using the small event fitting method as follows, strike 63.76° and dip 70.52°. The main shock’s optimal unit source time function, moment tensor solution, and double couple solution are obtained by CAP method. Among them, the best fault plane solution is strike 63.4° and dip 52.8°. Based on the fault center solution calculations, the final fault plane parameters were determined to be strike 63.59° and dip 61.66°, which aligns with the region’s Shuangtashan-Shuangfengsi fault. Considering regional tectonic characteristics, it is postulated that this earthquake may have been induced by the principal stress squeezing on the fault, representing a typical normal fault activity. This study provides crucial evidence for identifying the seismogenic structure of the Chengde earthquake and the stress field status in the area.

Stability problems in rock stress fields were highlighted. It is a direct source of force for crustal deformation, rock destruction, and earthquake occurrence. Through large-scale international cooperation, the global GPS velocity field and world stress map were updated. Plates with the soul of a power source are no longer a bunch of rigid tectonics, and for the first time the dynamism of their movements and internal stresses are revealed. This paper describes the methods of in-situ measurement of stress, the principles of division of stress tensor regimes, and the basic features of stress maps. In response to the earthquake focal mechanism, the author has drawn a variety of 3D perspective drawings, so that readers can easily grasp the geologic meaning of the earthquake focal mechanism in a way that is easy to read and understand by looking at the drawings. Finally, the background of the local dynamics is analyzed with two examples comparing the similarities and differences between the measured stress maps and the earthquake focal mechanism.

Two shallow earthquakes of moment magnitude 6.2 and 5.8 occurred in the central arm of Sulawesi on 26 July 2021 and 26 August 2021, respectively. The fault responsible for the earthquake had previously only been partially mapped, thus making further analysis of its characteristics crucial for the assessment of seismic hazard. In this study, we exploit data from a regional seismic network, relocate the associated seismicity using the double difference method with an updated velocity model, determine focal mechanisms from full-waveform inversion, and analyze the static stress changes caused by the mainshock. Our relocated hypocenters and focal mechanism solutions reveal two earthquake clusters, one at the Central Balantak Fault that exhibits normal slip on a SE–NW trending rupture, and the other at the West Balantak Fault, which exhibits dextral strike-slip motion on a SE–NW trending rupture. The additional static stress increase transferred by the Mw 6.2 mainshock may have triggered the subsequent Mw 5.8 event. A detailed assessment of previously unmapped faults in Central Sulawesi is essential for a more comprehensive understanding of seismic hazard in the region. [ABSTRACT FROM AUTHOR]

Understanding flow behavior in a geothermal reservoir is important for managing sustainable geothermal energy extraction. Fluid flow in geothermal reservoirs generally occurs in complex existing fracture systems in which the reservoirs are situated in highly fractured rocks. To simulate a discrete fracture model, the location and orientation of the fracture were computed using statistical processes and observational data. In many cases, estimating the location and orientation of fractures from 1D borehole logging data is challenging. In this study, we used microseismic data to build the fracture network systems and extract the detailed positions and dimensions of the fractures. We used the microseismic data recorded at the Okuaizu Geothermal Field, Fukushima Prefecture, Japan, from 2019 to 2021. First, we located the hypocenters, removing the effect of uncertainty in the velocity structure of the geothermal fluids. We relocated and clustered the seismic events based on waveform similarity. We analyzed each cluster to define the fracture orientation using principal component analysis (PCA) and focal mechanism (FM) analysis. We used the P polarity with the S/P ratio as a constraint for a better fault-plane solution. With PCA, we can extract the fracture dimension of each cluster. Our cluster analysis showed that the clusters were not always planar fractures, and we interpreted them as fracture zones. Based on the consistency between PCA and FM, each cluster/fracture zone was classified into three conceptual models to characterize the fracture network system in this field. This model showed variations in the orientation of small fractures within the fracture zone. We characterized the spatial variation in fracture distribution and orientations in the reservoir and demonstrated the fracture network system of this field. The fracture zone near the injection well has a N–S strike, and the dip is above 80°; however, the fracture zone in the northeastern part of the injection well has a NW–SE strike with a dip between 60° and 80°. The fracture network system estimated in this study is crucial for robust reservoir modeling because our model is more realistic, observation-orientated, and includes local anomalies of reservoir properties. [ABSTRACT FROM AUTHOR]

The complexity of the active tectonic system in East Indonesia has influenced various types of seismicity in the Seram Sea, resulting in several significant earthquakes. In the last decade, there were two devastating earthquakes, Mw 6.2 (October 31, 2017) occurred with the thrust mechanism as typical of active subduction while Mw 6.5 (September 26, 2019) occurred with a left-lateral mechanism due to the unknown active fault in the Seram Island and its surroundings. In the present study, we analyzed the current seismicity for 8-years (2015–2022) by using Bayesian moment tensor inversion for the two largest earthquakes and multivariate hypocenter clustering in the spatial-time-distance-magnitude (STDM) domain using an earthquake catalog. The Bayesian inversion was applied by using bootstrapping chain analysis with the regional seismic network with a distance of up to 4000 km to figure out the earthquake source mechanism. The inversion results show that both earthquakes have different mechanisms. The October 2017 event occurred in the actual plane with strike 262°, dip 21° and rake 68°, suggesting an active subduction with a steep dipping angle of 20°–25° towards the north direction. However, the September 2019 occurred with strike 346°, dip 85°, and rake −140°, indicating an offshore unidentified fault with rupture propagation towards the N–S direction and pure reverse slip. The grid search of the centroid location shows a converged solution with small shifting of 10–15 km to the east and 10 km to the south while the depth is consistent at 7–10 km. The obtained focal mechanism shows an uncertainty with 70–80% confidence intervals and a percentage of Double-Couple (DC) component > 90% that evidences the tectonic process. To support the earthquake mechanism results, earthquake clustering analysis was performed based on nearest-neighborhood distance with its components in STDM. The notable breaks in the STDM time series due to the Mw 6.2 (2017) and Mw 6.5 (2019) earthquakes are discussed, supported by a Multitaper spectral analysis for effective visualization of STDM spectral peaks. Finally, the relation between STDM time series is studied by magnitude-squared coherence with 95% confidence intervals. As a result, we found that although the magnitude of coherence was not high, they are weakly related to each other. This implies that the occurrence of future seismicity somehow depends upon these parameters. The comprehensive results of this study may support the existence of the South Seram subduction system that can be used to update the mitigation plan for different earthquake characteristics shortly. [ABSTRACT FROM AUTHOR]

Microseismic monitoring is pivotal for mine safety, offering early warnings for coal mining operations. By introducing the MTfit and MSATSI inversion methods to study the focal mechanisms of microseismic events and the stress field in coal mines, we applied these techniques for the first time to the 63upper06 working face in the Dongtan Coal Mine. We selected 107 high-quality microseismic events during the mining process and used MTfit to invert their focal mechanisms. Moreover, we used MSATSI to determine the direction of the maximum horizontal principal compressive stress in the region. This allowed us to establish a connection between mining activities, focal mechanisms, and rock strata fractures. Results show the predominance of normal and reverse faulting with significant CLVD components, underlining tensile and compressive failures as dominant in the mine's seismicity. Further, the study subdivides the research area into three subregions for detailed stress analysis: Subregion 1 is located below the working face with a maximum principal stress of 20.3 MPa, oriented between N60°E and N75°E; Subregion 2 is above the working face and exhibits more complex stress conditions with a maximum principal stress of 30.1 MPa; and Subregion 3 is situated at the roof's right side of the working face, with a maximum principal stress of 20.3 MPa, oriented at 30°northwest. It provides an important reference for seismic risk assessment and disaster prevention and control during mining. [ABSTRACT FROM AUTHOR]

Copyright of Progress in Earthquake Sciences is the property of China Earthquake Administration, Institute of Geophysics and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Reviews of Geophysics & Planetary Physics is the property of Editorial Office of Reviews of Geophysics & Planetary Physics and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Anomalously overturned thrust faults, lineaments and segmentation causing cross‐cutting basement structures characterize the tectonic setting of Sikkim Himalaya. However, its seismotectonics is poorly constrained along with the speculated northward extension of the Dhubri‐Chungthang Fault Zone (DCFZ) causing segmentation. Here, we utilize the precise location of newly acquired local earthquake data and fault plane solutions using full‐waveform moment tensor inversion to better constrain seismically active zones. Transtensional shearing along the Main Himalayan Thrust in central Sikkim is possibly incited by fluid‐rich upper‐crust. Cessation of the mapped 20 km wide mid‐crustal seismogenic zone of DCFZ at Chungthang and its northward discontinuation into the Higher Himalayas is confirmed by the striking variation in focal mechanisms. Earthquakes along imbricated segments in the lower‐crust originate possibly in response to crustal shortening. Extensional shearing along the Moho triggers seismicity to the northwest of Sikkim. Such complex tectonic dynamics instigating persistent seismicity indicates high potential for future great earthquakes in Sikkim Himalaya. Plain Language Summary: A dense seismic network operational from April 2019 to May 2023 has been continuously recording earthquakes occurring in and around Sikkim Himalaya. In this study, we have located the earthquakes occurring within 200 km radius of Sikkim Himalaya to determine which regions are seismically active. We further analyzed these earthquakes to deduce deformation in the source region, which comprises of how the accumulated stress in the Earth gets released. This study aided in mapping the fault zones and its sense of motion, which are responsible for the persistent occurrence of earthquakes. We provide the possible extent of the debated mid‐crustal Dhubri‐Chungthang Fault Zone, which traverses across the study region and is stated to orthogonally divide the arcuate Himalayas. Our study also provides evidence for earthquakes originating in the lower‐crust due to overlapping of parallelly dipping layers of the Moho. Earthquakes occurring in upper‐crust and along the Moho boundary attest to previously obtained results. Such complex multi‐fault dynamics increases the seismic hazard potential in Sikkim Himalaya. Key Points: High resolution local earthquake catalog for Sikkim Himalaya recorded at the longest running seismic networkDelineation of possible seismogenic extent of the highly debated Dhubri‐Chungthang Fault ZoneEvidence of lower crustal earthquakes along imbricated Moho [ABSTRACT FROM AUTHOR]

Copyright of Progress in Earthquake Sciences is the property of China Earthquake Administration, Institute of Geophysics and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Progress in Earthquake Sciences is the property of China Earthquake Administration, Institute of Geophysics and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The seismic attenuation relationship between ground motion parameters (such as peak acceleration and response spectra value) and seismic parameters (such as magnitude and epicentral distance) is an important foundation for seismic hazard analysis and the core of determining seismic input parameters for seismic resistance engineering. The acceleration envelope parameters, which describe the relationship between ground motion intensity and time variation, are primarily used for artificially synthesizing seismic motion. At present, little research has been performed on the attenuation relationship of acceleration envelope parameters in the Longmenshan fault zone on the eastern side of the Tibetan Plateau in China. Therefore, this study selected the Mw4-6 aftershock records of the 2008 Wenchuan earthquake that occurred on the Longmenshan fault zone and established a commonly used three segment envelope model parameter attenuation relationship. We classified aftershock records based on their source mechanisms and obtained attenuation relationship models for thrust slip aftershocks, thrust and strike slip aftershocks, and strike slip aftershocks. The results are as follows: (1) The thrust slip aftershock records had the longest rising stage which is the time difference from the arrival of P-waves to the beginning of the stable sustained stage of seismic motion. Thrust and strike slip aftershocks records had the longest stable sustained stage period and the slowest attenuation at the tail of the record. (2) The attenuation relationship of the acceleration envelope parameters commonly used in Chinese engineering for artificially synthesizing seismic motion is based on the strong earthquake records in the western United States. But, compared to the attenuation characteristics of the acceleration envelope function in the western United States, the Mw4-6 earthquake records on the Longmenshan fault zone had a slower attenuation rate at the tail of the record. So, accurately artificially synthesizing seismic motion through envelope parameter attenuation model requires the use of attenuation model established by earthquake records in this region.

Abstract Mining-induced earthquakes are unnatural seismic events that frequently occur in high-position hard and thick rock strata during coal mining. Considering the frequent occurrence of strong mining-induced earthquakes in the Dongtan mining area, this study analysed the fracture migration characteristics of hard and thick rock strata and the focal mechanism of mining-induced earthquakes based on Volasov’s thick-plate and moment tensor inversion theories. The results showed that the main key strata were difficult to break under single-panel mining conditions because of the thick and high-strength rock strata and breakage of the main key strata is caused by multiple-panel mining. Volasov’s thick-plate theoretical calculation indicated an initial fracture span of the main key strata was 314 m, which is consistent with the actual mining distance of the working face. This verified that strong mining-induced earthquakes were induced by the initial fracture of the main key strata. In coal mining, the pure shear failure type of mining-induced earthquakes indicated the highest percentage, and the shear fracture of rock strata was the primary cause of strong mining-induced earthquakes. The dip angle of the focal fracture surface in mining-induced earthquakes was generally within 15°. Through an analysis of the focal mechanism of mining-induced earthquakes, it has a certain guiding role in explaining the mechanism of mining-induced earthquakes.

By collecting focal mechanism solutions of earthquakes at different depths from Northeast Japan to Northeast China during 1957 to 2022, the joint iterative stress inversion method is used to calculate the tectonic stress field in the subduction zone and the tectonic stress field state in Northeast China. The results show that the shallow area of the Japan trench is not only affected by the subduction and extrusion of the Pacific plate, but also related to the extrusion of the North American plate. The deep source earthquakes in Northeast China are closely related to the long-term action of the trench subduction zone. The formation of the Changbaishan volcanic area has a great relationship with the thrust of the subduction zone. The seismicity of the subduction zone indirectly controls the formation and activity of the volcanic area in Northeast Asia. The principal compressive stress axis distributes in the NEE-SWW direction in the Yingkou area of Liaoning, while the principal tensile stress axis distributes in the NNW-SSE direction. Based on the results of the focal mechanism of shallow earthquakes in the volcanic area and its surrounding areas, it is believed that the current tectonic stress field in the northeastern volcanic area continues the overall structure of stress in the northeastern region, with the principal compressive stress axis in the NEE-SWW direction and the principal tensile stress axis in the NNW-SSE direction.

In this paper, the regional full waveform inversion using the broadband waveforms recorded by China Seismic Network were conducted, and the focal mechanism solutions of the 62 earthquakes with M≥4.0 occurred in the mainland of China during November 2023 to February 2024 were obtained. The types of these focal mechanism solutions show 45 reverse faulting, 13 strike-slip faulting, 3 normal faulting and 1 odd earthquakes.

The M6.2 earthquake in Jishishan, Gansu Province, on December 18, 2023, caused extraordinary earthquake disasters. It was located in the northern part of the north−south seismic zone, which is a key area for earthquake monitoring in China. The newly built dense strong motion stations in this area provide unprecedented conditions for high-precision earthquake relocation, especially the earthquake focal depth. This paper uses the newly built strong motion and traditional broadband seismic networks to relocate the source locations of the M3.0 and above aftershocks and to invert their focal mechanisms. The horizontal error of earthquake location is estimated to be 0.5−1 km, and the vertical error is 1−2 km. The focal depth range of aftershocks is 9.6−14.6 km, distributed in a 12-km-long strip with SSE direction. Aftershocks in the south are more concentrated horizontally and vertically, while aftershocks in the north are more scattered. The focal mechanisms of the main shock and aftershocks are relatively consistent, and the P-axis orientation is consistent with the regional strain direction. There is a seismic blank area of M3.0 and above, about 3−5 km between the main shock and aftershocks. It is suggested that the energy released by the main shock rupture is concentrated in this area. Based on the earthquake location and focal mechanism of the main shock, it is inferred that the Northern Lajishan fault zone is the seismogenic structure of the main shock, and the main shock did not occur on the main fault, but on a secondary fault. The initial rupture depth and centroid depth of the main shock were 12.8 and 14.0 km, respectively. The source rupture depth may not be the main reason for the severe earthquake disaster.

The 2021 Yangbi earthquake sequence occurred on the southwestern margin of the Chuandian block in China, which is a typical foreshock-mainshock-aftershock sequence. We investigate source properties of this earthquake sequence with seismic and geodetic data. Focal mechanisms of M ≥ 2.0 events in this earthquake sequence are determined using P wave first-motion polarity plus S/P amplitude ratio data from local short-period and broadband seismic stations. The determined focal mechanisms together with spatial distribution of the earthquake sequence indicate the overall northwest-southeast-trending seismogenic faults with dominant strike-slip motions. We conduct kinematic finite-fault inversion with strong-motion, local broadband seismic, and coseismic InSAR plus GNSS displacement data, finding that the mainshock ruptured a major asperity unilaterally with a rupture length and duration being approximately 15 km and 6.5 s, respectively. The released scalar seismic moment is 1.7 × 1018 Nm, corresponding to a moment magnitude MW 6.1. Besides, stress field inversion results manifest that source area is characterized by a strike-slip stress regime and the local stress field has changed before and after the mainshock. We suggest that rupture initiation, propagation, and arrest of the 2021 Yangbi earthquake sequence may be regulated by the immanent strength and stress heterogeneities of the seismogenic fault.

Copyright of Progress in Earthquake Sciences is the property of China Earthquake Administration, Institute of Geophysics and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Earthquake (1000-3274) is the property of Editorial Office of Earthquake Journal and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Earthquake (1000-3274) is the property of Editorial Office of Earthquake Journal and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

On December 18, 2023, the Jishishan area in Gansu Province was jolted by a MS 6.2 earthquake, which is the most powerful seismic event that occurred throughout the year in China. The earthquake occurred along the NWtrending Lajishan fault (LJSF), a large tectonic transformation zone. After this event, China Earthquake Networks Center (CENC) has timely published several reports about seismic sources for emergency responses. The earthquake early warning system issued the first alert 4.9 s after the earthquake occurrence, providing prompt notification that effectively mitigated panics, injuries, and deaths of residents. The near real-time focal mechanism solution indicates that this earthquake is associated with a thrust fault. The distribution of aftershocks, the rupture process, and the recorded amplitudes from seismic monitoring and GNSS stations, all suggest that the mainshock rupture predominately propagates to the northwest direction. The duration of the rupture process is ~12 s, and the largest slip is located at approximately 6.3 km to the NNW from the epicenter, with a peak slip of 0.12 m at ~8 km depth. Seismic station N0028 recorded the highest instrumental intensity, which is 9.4 on the Mercalli scale. The estimated intensity map shows a seismic intensity reaching up to IX near the rupture area, consistent with field survey results. The aftershocks (up to December 22, 2023) are mostly distributed in the northwest direction within ~20 km of the epicenter. This earthquake caused serious casualties and house collapses, which requires further investigations into the impact of this earthquake. [ABSTRACT FROM AUTHOR]

Coal mining in thick-hard roof areas leads to frequent high-energy seismicity and may even induce coal–rock dynamic disasters such as rockburst. It is important to explore the focal mechanism of mining-induced seismicity to understand the failure behaviour of the coal–rock mass and the characteristics of the stress field. It is helpful to propose a seismic absorption method. Taking the 6306 working face of the Dongtan coal mine as the study area, the rupture type, focal mechanism, characteristics of the local stress field, and main controlling factors of the mining-induced 22 high-energy seismic events were investigated using the probabilistic full-waveform optimization inversion and the stress field inversion. The main conclusions were as follows: (1) the rupture types of 22 seismic events were shear, tensile, compression, and mixed failure, and the corresponding number of events was 11, 4, 1, and 6, respectively; (2) the focal mechanism of the above events was eight strike-slips, twelve reverse fault slips, and two normal fault slips; and (3) the stress field using the inversion of focal mechanisms was discovered. There was no significant variation of the maximum compressive stress σ1 and minimum compressive stress σ3 directions in the seismicity concentration area, fold 1, and fold 2. The principal stresses σ1 and σ3 were the main controlling factors inducing seismicity. This study is helpful in revealing the mechanism of the mining-induced high-energy seismicity and proposing corresponding prevention measures in the future. Highlights: A possible full-waveform optimization inversion method is developed to analyse the focal mechanism of a thick-hard roof zone The main controlling factors of the induced seismic activity are investigated through the stress field inversion The stress field inversion results have excellent potential to guide the design of hydraulic fracturing in the coal mine [ABSTRACT FROM AUTHOR]

The seismometer onboard InSight NASA Mars mission discovered a seismically active planet. We focused on the strongest event named S1222a (4 May 2022, Mw ∼ 4.7), which was recorded by the Very Broad Band sensors and associated channel ELYSE and is located 37.2° away from InSight. We use two different methods based on a point source approach for an elastic, horizontally layered medium to retrieve source parameters of S1222a. In the first case, the seismic moment tensor inversion of high‐frequency seismogram data is calculated using a matrix method for the direct waves. The process includes the generation of records in displacement using the frequency‐wavenumber integration technique. A method of inversion of the moment tensor of direct P‐ and S‐waves, less sensitive to path effects than reflected and transformed waves, is presented, which significantly increases the accuracy and reliability of the method. In the second case, tensors were calculated using common low‐frequency full‐waveform inversion and the tests to verify the plausibility of this solution obtained from the single station calculation were performed and the uncertainty estimations for inversions can be useful in future research. Plain Language Summary: The paper presents the seismic moment tensor solution and the focal mechanism for the largest event S1222a (4 May 2022, magnitude Mw4.7) recorded at one seismic station on Mars. We consider two different basic approaches to address the problem of the unavoidable inaccuracy of seismic waves modeling: focusing only on direct waves and using low‐frequency full‐waveform inversion. The moment tensor and focal mechanism of the S1222a event were obtained, as well as the optimal depth of a source. Despite the range of possible, it is encouraging that independent studies based on different methodologies, and using different structural models, point to reasonable solutions. The estimates of the focal mechanism when the single‐station inversion is calculated are taken into account and the stability tests to verify our solutions were performed. The importance of this study lies in expanding the possibilities of how to calculate this kind of tasks. Key Points: We consider two different methods based on a point source approach to retrieve source parameters of S1222aA method of inversion of the moment tensor of direct waves, less sensitive to path effects than reflected and transformed waves, is presentedWe obtained the results of seismic tensor solution and time‐independent focal mechanism [ABSTRACT FROM AUTHOR]

Copyright of Progress in Earthquake Sciences is the property of China Earthquake Administration, Institute of Geophysics and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

In the Eastern Alps, the indentation of the Adriatic promontory since the Cenozoic affected the kinematics of separate crustal domains bounded by faults that accommodate lateral extrusion processes and differential shortening. Deciphering the pattern of crustal stresses in the orogen interior is challenging, due to the lack of in situ stress measurements at crustal depths. We define stress regimes and the orientations of the most‐compressive horizontal stress (SHmax) by integrating published results with new data, including stress analysis from fault plane solutions, estimation of crustal anisotropy through the shear wave splitting analysis, paleostress determination from fault slip data, and computation of cumulative seismic displacements. The retrieved regional SHmax are generally consistent with N‐S convergence. In the northern part of the study area, current stress orientations are almost parallel to paleo‐SHmax, suggesting a rather uniform compressional regime since the late Cenozoic. Conversely, sharp deflections and divergence with paleo‐SHmax appear at the western border of the Adriatic promontory across major transpressive and extensional shear zones and in the Southalpine domain, indicating a change in tectonically induced second‐order stresses. A current strike‐slip regime with subordinate orogen‐parallel seismic displacements affects a belt to north of the Periadriatic Lineament and NE‐extension characterizes the Ortles‐Engandine region. Seismic anisotropy locally exhibits fault‐parallel fast axes (Brenner‐Giudicarie fault‐systems, Dinaric and Southalpine thrusts), whereas stress‐induced anisotropy parallel to SHmax characterizes the southern part of the orogen. Cumulative seismic displacements are small compared to geodetic ones, and unravel partitioning of deformation into second‐order transpressive and extensional belts in response to indentation. Key Points: Stress patterns in the Eastern Alps from earthquakes, seismic anisotropy and fault slip dataPartitioning of second‐order stresses and seismic strain into crustal domains in response to collision and lateral extrusionCrustal seismic anisotropy primarily controlled by active stresses, major faults and pervasive fabric in the basement [ABSTRACT FROM AUTHOR]

Microseismic monitoring is pivotal for mine safety, offering early warnings for coal mining operations. By introducing the MTfit and MSATSI inversion methods to study the focal mechanisms of microseismic events and the stress field in coal mines, we applied these techniques for the first time to the 63upper06 working face in the Dongtan Coal Mine. We selected 107 high-quality microseismic events during the mining process and used MTfit to invert their focal mechanisms. Moreover, we used MSATSI to determine the direction of the maximum horizontal principal compressive stress in the region. This allowed us to establish a connection between mining activities, focal mechanisms, and rock strata fractures. Results show the predominance of normal and reverse faulting with significant CLVD components, underlining tensile and compressive failures as dominant in the mine’s seismicity. Further, the study subdivides the research area into three subregions for detailed stress analysis: Subregion 1 is located below the working face with a maximum principal stress of 20.3 MPa, oriented between N60°E and N75°E; Subregion 2 is above the working face and exhibits more complex stress conditions with a maximum principal stress of 30.1 MPa; and Subregion 3 is situated at the roof’s right side of the working face, with a maximum principal stress of 20.3 MPa, oriented at 30°northwest. It provides an important reference for seismic risk assessment and disaster prevention and control during mining.

Se recopila una base de datos de 2547 mecanismos focales de terremotos para modelar el campo de esfuerzo tectónico en Costa Rica, sur de Nicaragua y norte de Panamá. Se determinan 33 tensores regionales de esfuerzo. La orientación general del esfuerzo horizontal máximo (SH Max) es subparalela a la dirección de convergencia N 32° E de la placa de Cocos con la placa Caribe a lo largo de la fosa mesoamericana, con una rotación horaria de +- 20° por debajo de los 30 km de profundidad. Se identifican tres órdenes de esfuerzos: el primero debido al movimiento absoluto de la placa de Cocos, el segundo generado por la interacción triple de esta con las placas Caribe y Nazca, y el tercero debido a contrastes de densidad impuestos por las cordilleras volcánicas, ígneas y sedimentarias activas y paralelas a la fosa, las cuales en combinación con fallas regionales y sus interacciones, actúan en conjunto como deflectores de los esfuerzos tectónicos locales que rotan hacia el norte. Los datos GNSS observados y modelados del campo de velocidades horizontales, orientados al NNE, presentan una correlación notable con las direcciones del SH Max interpoladas en la capa superior. Estas características, junto con las permutaciones locales y regionales de los ejes del elipsoide de esfuerzos, explican las interacciones de los regímenes tectónicos y sus complicaciones neotectónicas. Se genera un escenario 2D-3D detallado con límites sismo tectónicos realistas y más objetivos, utilizando categorías de regímenes de esfuerzo junto con la relación tectónica R'. Los resultados tienen aplicación práctica en estudios de amenaza y riesgo sísmico, exploración de recursos naturales.

Abstract Anomalously overturned thrust faults, lineaments and segmentation causing cross‐cutting basement structures characterize the tectonic setting of Sikkim Himalaya. However, its seismotectonics is poorly constrained along with the speculated northward extension of the Dhubri‐Chungthang Fault Zone (DCFZ) causing segmentation. Here, we utilize the precise location of newly acquired local earthquake data and fault plane solutions using full‐waveform moment tensor inversion to better constrain seismically active zones. Transtensional shearing along the Main Himalayan Thrust in central Sikkim is possibly incited by fluid‐rich upper‐crust. Cessation of the mapped 20 km wide mid‐crustal seismogenic zone of DCFZ at Chungthang and its northward discontinuation into the Higher Himalayas is confirmed by the striking variation in focal mechanisms. Earthquakes along imbricated segments in the lower‐crust originate possibly in response to crustal shortening. Extensional shearing along the Moho triggers seismicity to the northwest of Sikkim. Such complex tectonic dynamics instigating persistent seismicity indicates high potential for future great earthquakes in Sikkim Himalaya.

Different from other normal modes of the Earth's free oscillation that depend on all the six components (Mrr, Mtt, Mpp, Mrt, Mrp, and Mtp) of the centroid moment tensor, the amplitudes of the radial modes depend on the Mrr component (e.g., scalar moment (M0), dip (δ), and slip (λ)) and hypocenter depth of the focal mechanism, and hence can be easily used to constrain these parameters of the focal mechanism. In this study, we use the superconducting gravimeter (SG) records after the 2011 Tohoku earthquake to analyze the radial modes 0S0 and 1S0. Based on the solutions of the focal mechanism provided by the GCMT and USGS, we can obtain the theoretical amplitudes of these two radial modes. Comparing the theoretical amplitudes with the observation amplitudes, it is found that there are obvious differences between the former and the latter, which means that the GCMT and USGS focal mechanisms cannot well represent the real focal mechanism of the 2011 event. Taking the GCMT solution as a reference and changing the depth and the three parameters of the Mrr moment, the scalar moment (M0) and the dip (δ) have significant influences, but the effects of the slip (λ) and the depth are minor. After comparisons, we provide a new constraint (M0 = 5.8 ± 0.09 × 1022 N·m, δ = 10.1 ± 0.08°, λ = 88°, and depth = 20 km) for the focal mechanism of the 2011 event. In addition, we further determine the center frequency (1.631567 ± 2.6e-6 mHz) and quality factor (2046.4± 50.1) of the 1S0 mode.