Inversion Of Focal Mechanisms Research Articles

Stress field and tectonic regime of the Eastern Mediterranean from inversion of earthquake focal mechanisms

The formal stress inversions of 1258 earthquake focal mechanism solutions reveal the presence of six distinct stress regimes. However, the predominant tectonic regimes comprise normal faulting (39%), strike-slip faulting (29%), thrust faulting (25%), and unspecified oblique faulting (7%). The division of the study area into zones based on structural and deformation patterns has provided valuable insights into the variations in the stress field and tectonic processes within this seismically active region. For each designated zone, reduced stress tensors were derived using the improved right dihedron method and the rotation optimization technique implemented by the Win-Tensor software.The stress field derived from the entire dataset is characterized by R′ = 1.0 and an NW-SE SHmax orientation, strongly influenced by the tectonic stress regime of the Hellenic Arc. In specific regions such as CYS, CEM, and WEM, the prevailing tectonic regimes is characterized by a predominant compressive setting with a stress index (R' = 2.0 to 2.5). This pattern aligns with neotectonic deformation associated with the subduction of the African Plate beneath the Eurasian Plate. The EMZ Zone exhibits a strike-slip tectonic regime with an R' of 1.50, primarily influenced by the Dead Sea Transform Fault. This fault accommodates differential motion between the African and Arabian plates in the NNE direction, resulting in left-lateral strike-slip motion along the plate boundary. The mean slip tendency values, varying between 0.62 and 0.66, suggests that the faults in the study area are appropriately oriented for slip or shear displacement, indicating an elevated probability of reactivation.

Determination of earthquake focal mechanism via multi-task learning

A multi-task learning-based focal mechanism network (MTFMN) is proposed for calculating parameters of the focal mechanism of earthquakes by regression with incorporating expert prior knowledge. The model automatically learns feature representations of seismic waveforms and transforms the inversion task of the focal mechanism into multi-task learning. Experimental results suggest that MTFMN outperforms traditional methods in the task of earthquake focal mechanism and improves the accuracy of parameter estimation of focal mechanism. In addition, comparative experiments demonstrate MTFMN’s enhanced robustness and generalization capabilities compared to other methods. Our proposed methodology presents a more precise regression approach for focal mechanism inversion, with the potential to provide a better understanding of seismic events.

A New Method to Invert for Interseismic Deep Slip Along Closely Spaced Faults Using Surface Velocities and Subsurface Stressing‐Rate Tensors

AbstractInversions of interseismic geodetic surface velocities often cannot uniquely resolve the three‐dimensional slip‐rate distribution along closely spaced faults. Microseismic focal mechanisms reveal stress information at depth and may provide additional constraints for inversions that estimate slip rates. Here, we present a new inverse approach that utilizes both surface velocities and subsurface stressing‐rate tensors to constrain interseismic slip rates and activity of closely spaced faults. We assess the ability of the inverse approach to recover slip rate distributions from stressing‐rate tensors and surface velocities generated by two forward models: (a) a single strike‐slip fault model and (b) a complex southern San Andreas fault system (SAFS) model. The single fault model inversions reveal that a sparse array of regularly spaced stressing‐rate tensors can recover the forward model slip distribution better than surface velocity inversions alone. Because focal mechanism inversions currently provide normalized deviatoric stress tensors, we perform inversions for slip rate using full, deviatoric or normalized deviatoric forward‐model‐generated stressing‐rate tensors to assess the impact of removing stress magnitude from the constraining data. All the inversions, except for those that use normalized deviatoric stressing‐rate tensors, recover the forward model slip‐rate distribution well, even for the SAFS model. Jointly inverting stressing rate and velocity data best recovers the forward model slip‐rate distribution and may improve estimates of interseismic deep slip rates in regions of complex faulting, such as the southern SAFS; however, successful inversions of crustal data will require methods to estimate stressing‐rate magnitudes.

Identifying subsurface fault planes via a stress inversion of earthquake focal mechanisms

SUMMARY We test three different methods for selecting the fault plane from the two nodal planes in an individual focal mechanism, which are implemented in a linear stress inversion technique, to constrain the geometry of subsurface seismogenic faults. The three fault-selection methods use the misfit angle between the slip orientation and maximum shear stress direction on the nodal planes, fault instability and slip tendency, respectively, to select the fault planes. These three fault-selection methods are applied to various types (synthetic, simulated and real) of focal mechanism data sets where the fault planes are already known. In particular, synthetic focal mechanism data sets are generated with the assumption of different levels of pore pressure variation (up to 60 per cent of the minimum principal stress) that may be involved in activating faults with diverse orientations. The instability method performs the best among the three fault-selection methods, with 60–100 per cent of the faults correctly selected in each of the tested data sets. The slip-tendency method is slightly less accurate than the instability method, especially when the earthquakes occur in a highly variable pore pressure environment, and the misfit-angle method is relatively ineffective in selecting the faults, especially in noisy data sets. The test results show that the instability method is the most effective in correctly selecting the faults when the instability of the selected fault plane is significantly higher than that of the auxiliary plane, whereby the instability ratio of the selected (fault) plane to the auxiliary plane is above ∼1.4; this constraint can improve our ability to identify subsurface seismic faults. We apply this stress inversion technique, which implements the instability method, to the 2016–2017 induced earthquakes in Pohang, South Korea. We invert 53 well-constrained, well-located focal mechanism solutions to derive the stress condition, with the nodal planes possessing a higher instability selected as faults. The earthquakes, which occurred in spatially distinct areas of the region, have been associated with water injection into two boreholes (PX-1 and PX-2). Approximately 70 per cent of the identified faults for the PX-2-related earthquakes are well aligned in terms of both their locations and orientations, thereby indicating that these earthquakes occurred along a single, large-scale fault. The fault planes with instability ratios above ∼1.3 are all correctly selected for the PX-2 fault. Although there are more variations in the identified fault orientations of the PX-1-related earthquakes, some of the fault planes with high instability ratios (>1.4) are generally subparallel to one another. Both the locations and orientations of these high-instability-ratio planes are well aligned, suggesting the presence of a large-scale fault that is subparallel to the PX-2 fault. This study demonstrates the potential of effectively identifying and imaging subsurface seismic faults using only information on fault mechanics (i.e. stress and focal mechanisms).

Towards fast focal mechanism inversion of shallow crustal earthquakes in the Chinese mainland

We have developed an automatic regional focal mechanism inversion system based on the Earthquake Rapid Report (ERR) system and the real-time three-component seismic waveform stream of 1 000 broadband seismic stations provided by the China Earthquake Networks Center (CENC). The system can rapidly provide a double couple solution and centroid depth within 5–15 ​min after receiving earthquake information from the ERR system. The data processing is triggered by earthquake information obtained from the ERR system. The system is capable of determining the focal mechanism of all shallow-depth earthquakes in the Chinese mainland with a magnitude of 5.5–6.5. It utilizes waveform data recorded by seismic stations located within 500 ​km from the epicenter, enabling the reporting of a focal mechanism solution within 5–15 ​min of an earthquake occurrence. Additionally, the system can assign a corresponding grade (A B C) to the focal mechanism solution. We processed a total of 301 earthquakes that occurred from 2021 to June 2022, and after the quality control, 166 of them were selected. These selected solutions were manually checked, and 160 of them were compiled in a focal mechanism catalog. This catalog can be conveniently downloaded online via the Internet. The automatic focal mechanism solution of earthquakes in eastern China exhibits a good agreement with that provided by the Global Centroid Moment Tensor (GCMT), when available. The average Kagan angle between this catalog and GCMT is 22°, and the average difference in MW is 0.17. Furthermore, compared with GCMT, the minimum magnitude of our catalog has been reduced from approximately 5.0 to 4.0. The correlation between the centroid depth and crustal thickness in the Chinese mainland confirms the distribution of the centroid depth.

Seismological research in Yunnan Province, China, and its tectonic implication between the Xianshuihe-Xiaojiang fault system and the Red River fault zone

The tectonic crosscutting relationship between the two most tectonic important Xianshuihe-Xiaojiang fault system (XFS) and the Red River fault zone (RRF) is a key basic problem but it is controversial now. These obscure further leads to a hot argument about the geodynamic model of the southeast margin of the Tibetan Plateau. In order to answer whether the XFS has cut across the RRF and extended southwardly, multiple seismological methods, including seismic relocation, b-value analysis, seismic energy, density study, focal mechanism inversion, and regional stress field research, are applied in the Yunnan Province area, China, where the XFS and the RRF intersects with each other. The results comprehensively demonstrate that the southern segment of the XFS has not been affected by the RRF, and it has continued for a length after crossing the RRF, but the Dian Bien Phu fault zone should not be an extension fault of the XFS. Along the SW direction, starting from the middle segment of the XFS, and cutting across the Qujiang fault, Shiping-Jianshui fault zone, RRF, Ailaoshan fault zone, Wuliangshan fault zone, and the southern section of the Daluo fault, the belt should be treated as the eastern boundary of the clockwise rotational in geodynamics model of the Tibetan Plateau in this study area. Based on these conclusion above and previous recognitions, a new geodynamic evolution model is proposed.

Earthquake focal mechanisms with distributed acoustic sensing

Earthquake focal mechanisms provide critical in-situ insights about the subsurface faulting geometry and stress state. For frequent small earthquakes (magnitude< 3.5), their focal mechanisms are routinely determined using first-arrival polarities picked on the vertical component of seismometers. Nevertheless, their quality is usually limited by the azimuthal coverage of the local seismic network. The emerging distributed acoustic sensing (DAS) technology, which can convert pre-existing telecommunication cables into arrays of strain/strain-rate meters, can potentially fill the azimuthal gap and enhance constraints on the nodal plane orientation through its long sensing range and dense spatial sampling. However, determining first-arrival polarities on DAS is challenging due to its single-component sensing and low signal-to-noise ratio for direct body waves. Here, we present a data-driven method that measures P-wave polarities on a DAS array based on cross-correlations between earthquake pairs. We validate the inferred polarities using the regional network catalog on two DAS arrays, deployed in California and each comprising ~ 5000 channels. We demonstrate that a joint focal mechanism inversion combining conventional and DAS polarity picks improves the accuracy and reduces the uncertainty in the focal plane orientation. Our results highlight the significant potential of integrating DAS with conventional networks for investigating high-resolution earthquake source mechanisms.

Detailed View of the Seismogenic Structures and Processes of the 2022 Bayan Har Intraplate Earthquake Swarm on the East Margin of the Qinghai–Tibet Plateau

Abstract At 16:03 on 9 June 2022 (UTC), an Mw 5.5 earthquake followed by several Mw&amp;gt;4 events, including the largest event of Mw 5.8 within a few hours, occurred in the Maerkang area near the Caodeng Hot Spring Town, located in the south-central part of the Bayan Har plate on the eastern margin of the Qinghai–Tibet plateau. The earthquake swarm allows understanding the tectonic stress environment of the Bayan Har plate and is an example of a typical moderate-to-strong intraplate earthquake swarm. This article comprehensively analyzes the detailed seismogenic fault structure of the swarm by means of precise hypocenter relocation, focal mechanism inversion for Mw&amp;gt;4 earthquakes, inversion of the tectonic stress field in different regions of the Bayan Har plate, tidal strain calculation, and seismicity statistics. The results show that the swarm was not directly related to the nearby mapped Songgang fault, but rather resulted from the successive activation of a series of unknown faults. The precise hypocenter distribution, together with focal mechanism solutions of major earthquakes, illuminates five major seismogenic faults with conjugate relationships and stepover. Spatial and temporal migration of hypocenters, stress transfer, and tidal correlations demonstrate that cascade triggering, afterslip, and overpressured fluid might have jointly played a role in causing the earthquake swarm. As an output of this research, a set of verifiable datasets are provided as a basis for further in-depth research.

Anomalous Crustal Stress in the Eastern Tennessee Seismic Zone

AbstractThe eastern Tennessee seismic zone (ETSZ) experiences the second highest rates of natural seismicity in the central and eastern United States (CEUS), following the New Madrid area, yet the cause of elevated earthquake rates is unknown. We probe the origin of ETSZ seismicity using geomechanically constrained stress inversions of earthquake focal mechanisms from 57 earthquakes, including 24 newly derived here and five from the recent events not used in the previous stress studies. Highly oblique northwest–southeast (NW–SE) extension that is unique in the CEUS dominates the ETSZ—central Alabama to southeastern Kentucky—and preferentially reactivates normal to strike-slip faults in the northeast (NE) and southwest (SW) quadrants (strikes 018°–086° or 196°–272° and dips 55°–90°). This extension cannot be explained by the compressive tectonic plate-boundary tractions that cause oblique NE–SW contraction elsewhere in the CEUS. Although our analyses do not uniquely determine the origin of the anomalous stress, we favor isostatic disequilibrium, due to anything from surface processes to crust–mantle interactions, as the possible cause. Increased long-term seismic hazard in the ETSZ may be controlled by and confined to the spatial extent of this anomalous seismotectonic state.

DiTingMotion: A deep-learning first-motion-polarity classifier and its application to focal mechanism inversion

Accurate P-wave first-motion-polarity (FMP) information can contribute to solving earthquake focal mechanisms, especially for small earthquakes, to which waveform-based methods are generally inapplicable due to the computationally expensive high-frequency waveform simulations and inaccurate velocity models. In this paper, we propose a deep-learning-based method for the automatic determination of the FMPs, named “DiTingMotion”. DiTingMotion was trained with the P-wave FMP labels from the “DiTing” and SCSN-FMP datasets, and it achieved ∼97.8% accuracy on both datasets. The model maintains ∼83% accuracy on data labeled as “Emergent”, of which the FMP labels are challenging to identify for seismic analysts. Integrated with HASH, we developed a workflow for automated focal mechanism inversion using the FMPs identified by DiTingMotion and applied it to the 2019 M 6.4 Ridgecrest earthquake sequence for performance evaluation. In this case, DiTingMotion yields comparable focal mechanism results to that using manually determined FMPs by SCSN on the same data. The results proved that the DiTingMotion has a good generalization ability and broad application prospect in rapid earthquake focal mechanism inversion.

A New Focal Mechanism Calculation Algorithm (REFOC) Using Inter‐Event Relative Radiation Patterns: Application to the Earthquakes in the Parkfield Area

AbstractAccurate earthquake focal mechanisms are essential for solving fault zone structure, estimating stress variations, and assessing seismic hazards. Small earthquakes' focal mechanisms are usually solved using P‐wave first‐motion polarities and/or S‐/P‐wave amplitude ratios, which are limited due to the low signal‐to‐noise ratio of small‐earthquake waveforms and a limited number of three‐component seismograms. To increase the number of high‐quality focal mechanisms, we develop a method that utilizes the inter‐event relative radiation patterns to perform a joint focal mechanism inversion of numerous clustered events (called REFOC). The method first uses P‐wave polarities and S‐/P‐wave amplitude ratios to constrain the initial solutions and then combines these solutions and the inter‐event P‐/P‐wave and S‐/S‐wave amplitude ratios to refine solutions. For example, we apply the method to 38,413 earthquakes in the Parkfield region, California. The REFOC outperforms traditional methods with 57% more solutions with &lt;55° uncertainties (&gt;70% of the catalog events) and 126% more solutions with &lt;25° uncertainties (&gt;40% of the catalog events), illuminating unprecedented fine‐scale rupture processes. Instead of rupturing along the main fault, many M &lt; 2 focal mechanisms show &gt;45° angular differences from the 2004 Mw 6.0 Parkfield earthquake. The variation of focal mechanism properties is spatially related to the variation of fault strength and geometry, and temporally correlated with the shear stress variations before, during, and after the 2004 Mw 6.0 earthquake. The observations highlight the potential of applying REFOC to monitor unprecedented details of fault zone structure and stress field, providing new insights into fault rupture physics, seismotectonic processes, and seismic hazards.

A refined catalogue of focal mechanisms for the Intra‑Carpathian region of Romania: implications for the stress field and seismogenic features assessment

We present a refined and the most complete catalogue of the focal mechanisms for the Intra-Carpathian region of Romania. It contains the high-quality solutions computed for 1217 earthquakes recorded between 1909 and 2018. Primary data gathered from the original seismograms and seismic bulletins have been used to compute the source parameters and focal mechanisms solutions. The focal mechanisms have been obtained using the HASH method by the polarities and S/P amplitudes ratios inversion. Our catalogue provides data necessary for the investigation of the contemporary stress field at different scales with high spatial and temporal resolution. We determined the stress field characteristics through formal inversion of focal mechanisms and also computed the reactivation potential of the active fault systems using the Win‑Tensor program. The stress field is heterogeneous, with SHmax significantly deviating from the first-order stress field direction and also with strong local variations in the stress regime.

FocMech-Flow: Automatic Determination of P-Wave First-Motion Polarity and Focal Mechanism Inversion and Application to the 2021 Yangbi Earthquake Sequence

P-wave first-motion polarity is important for the inversion of earthquake focal mechanism solutions. The focal mechanism solution can further contribute to our understanding of the source rupture process, the fault structure, and the regional stress field characteristics. By using the abundant focal mechanism solutions of small and moderate earthquakes, we can deepen our understanding of fault geometry and the seismogenic environment. In this paper, we propose an automatic workflow, FocMech-Flow (Focal Mechanism-Flow), for identifying P-wave first-motion polarity and focal mechanism inversion with deep learning and applied it to the 2021 Yangbi earthquake sequence. We use a deep learning model named DiTingMotion to detect the P-wave first-motion polarity of 2389 waveforms, resulting in 98.49% accuracy of polarity discrimination compared with human experts. The focal mechanisms of 112 earthquakes are obtained by using the CHNYTX program, which is 3.7 times more than that of the waveform inversion method, and the results are highly consistent. The analysis shows that the focal mechanisms of the foreshock sequence of the Yangbi earthquake are highly consistent and are all of the strike-slip type; the focal mechanisms of the aftershock sequence are complex, mainly the strike-slip type, but there are also reverse and normal fault types. This study shows that the deep learning method has high reliability in determining the P-wave first-motion polarity, and FocMech-Flow can obtain a large number of focal mechanism solutions from small and moderate earthquakes, having promising application in fine-scale stress inversion.

GPU-acceleration 3D rotated-staggered-grid solutions to microseismic anisotropic wave equation with moment tensor implementation

To improve the accuracy of microseismic inversion, seismic anisotropy and moment tensor source should be carefully considered in the forward modelling stage. In this study, 3D microseismic anisotropy wave forward modelling with a moment tensor source was proposed. The modelling was carried out based on a rotated-staggered-grid (RSG) scheme. In contrast to staggered-grids, the RSG scheme defines the velocity components and densities at the same grid, as do the stress components and elastic parameters. Therefore, the elastic moduli do not need to be interpolated. In addition, the detailed formulation and implementation of moment-tensor source loaded on the RSG was presented by equating the source to the stress increments. Meanwhile, the RSG-based 3D wave equation forward modelling was performed in parallel using compute unified device architecture (CUDA) programming on a graphics processing unit (GPU) to improve its efficiency. Numerical simulations including homogeneous and anisotropic models were carried out using the method proposed in this paper, and compared with other methods to prove the reliability of this method. Furthermore, the high efficiency of the proposed approach was evaluated. The results show that the computational efficiency of proposed method can be improved by about two orders of magnitude compared with traditional central processing unit (CPU) computing methods. It could not only help the analysis of microseismic full wavefield records, but also provide support for passive source inversion, including location and focal mechanism inversion, and velocities inversion.

Current Stress Pattern and Geodynamics of the Baikal Rift System

Abstract—The crustal stress field of the Baikal Rift System has been reconstructed by tectonophysical inversion of focal mechanisms from the catalog of earthquakes recorded by the regional seismological network. Cataclastic analysis of fault slip data developed at the Shmidt Institute of the Physics of the Earth (Moscow) revealed previously unknown features in the behavior of principal stresses. Namely, the maximum deviatoric stresses diverge off the rift axis while the normalized spherical and deviatoric stress tensor components reach high magnitudes in the crust of the Baikal Basin. The obtained stress pattern of the Baikal Rift System is consistent with the rift origin by a joint action of a vertical mantle flow (upwelling branch of convection) and a horizontal flow in the asthenosphere which drives the NW–SE motion of the Amur plate off Eurasia.

Relative Focal Mechanism Inversion and Its Application to Ridgecrest Sequence

AbstractEarthquake focal mechanisms are important for characterizing the subsurface faulting geometry and evaluating stress distributions. Existing approaches usually strive to determine the absolute focal mechanisms and may be subject to large uncertainties due to incomprehensive knowledge of the velocity model, particularly for moderate-to-small earthquakes. Alternatively, difficulties that arise from the velocity model can be largely mitigated by inverting the relative data variations in a series of earthquakes, because effects from the velocity model are systematic among all events in the vicinity. In this study, we propose a novel relative focal mechanism inversion (RFMI) method to invert the second-order variations in a series of focal mechanisms utilizing a well-constrained primary event. We test the RFMI method on both synthetic data and 251 real earthquakes (M ≥3) in the 2019 Ridgecrest sequence. The synthetic test results show that the RFMI method is robust and insusceptible to location errors (&amp;lt;2 km) and systematic velocity errors (5%). The real data application results demonstrate improved consistency among the inverted focal mechanisms, resulting in better characterization of the fault orientations than the Southern California Seismic Network (SCSN) focal mechanism catalog. The retrieved earthquake depths are also well correlated with the depths of the Mw 6.4 and 7.1 mainshocks. Waveform cross-correlation analysis verifies the reliability of the results. Furthermore, dynamic stress monitoring is enabled with decent resolution. The proposed RFMI method paves a new path toward achieving a rich number of reliable earthquake focal mechanisms, which will benefit the investigation of the earthquake process.

MCMTpy: A Python Package for Source Parameters Inversion Based on Cut-and-Paste Algorithm and Markov Chain Monte Carlo

Abstract Accurate earthquake source parameters (e.g., magnitude, source location, and focal mechanism) are of key importance in seismic source studies and seismic hazard assessments. The routine workflow of source parameters estimation consists of two steps: source location inversion and focal mechanism inversion. Separate inversion of source parameters is subject to the cumulative uncertainties of both two steps inversion processes. Markov Chain Monte Carlo (MCMC), as global optimization, has been adopted in many nonlinear inversion problems to reduce cumulative errors and provide uncertainty assessment, but the application of MCMC is strongly subject to prior information. In this study, we present a new Python package MCMTpy. MCMTpy exploits the Cut-And-Paste (CAP) algorithm and Bayesian inference, using Markov Chain to implement the source location inversion and focal mechanism inversion in one inversion workflow. The new approach can effectively reduce the prior model dependence, and is closely integrated into the current seismological programming ecosystem. To demonstrate the effectiveness of the new package, we applied the MCMTpy to the 2021 Ms 6.4 Yangbi earthquake, Yunnan, China, and 2008 Mw 5.2 Mt. Carmel Earthquake, Illinois. A comparison between our results and other catalogs (e.g., Global Centroid Moment Tensor and U.S. Geological Survey W-phase) solutions illustrates that both double-couple and moment tensor solutions can be reliably recovered. The robustness and limitations of our approach are demonstrated by an experiment with 30 different initial models and an experiment with the grid-search method.

Seismic Sources in Stress‐Induced Anisotropic Media

Rocks in the upper mantle are extensively subjected to initial stress. The rock exhibits elastic anisotropy under the deviatoric stress, which affects the seismic response of underground structures. This paper aims to study the effect of stress-induced anisotropy on the seismic source. The seismic moment tensor generated by a shear faulting in a homogeneous isotropic medium with initial stress is derived by using the motion description in the intermediate configuration, the constitutive relation of third-order elasticity, and the boundary condition considering the effect of initial stress. Then the seismic moment tensor is decomposed to investigate seismic source characters quantitatively. Our results show that shear faulting in a stress-induced anisotropic medium can produce significant non-double-couple (non-DC) mechanisms, including compensated linear vector dipole (CLVD) and isotropic (ISO) components. ISO and CLVD components vary linearly with the initial shear stress on faults. In addition, stress-induced anisotropy causes the double-couple (DC) to deviate from the plane defined by the fault normal and slip direction. The initial stress not only affects the source intensity but also affects the propagation of seismic waves. The radiation patterns of longitudinal waves have non-uniform lobes, which shows the characteristic of anisotropy. Stress-induced anisotropic parameters are smaller than intrinsic anisotropy at high-stress levels and a nearly linear function of the increased initial stress. These results underscore the importance of considering stress-induced anisotropy in the inversion of focal mechanisms and provide a new perspective for monitoring underground stress.

Focal Mechanism Inversion on the Aftershock Sequence of the 25 October, 2010 Mw 7.8 Mentawai Earthquake: A Preliminary Result

An Mw 7.8 tsunami earthquake ruptured the shallow part of the Mentawai megathrust on 25 October 2010 and generated a large tsunami that hit the Mentawai Islands. The earthquake continued a partial rupture sequence of the 2007 Mw 8.5 and 7.9 Bengkulu earthquake and the 2008 Mw 7.2 North Pagai earthquake. The aftershock activity gives the opportunity to investigate the geometry of the subduction interface, which is critical to understanding the propagation and termination of a large earthquake. However, it is necessary to have a precise location and mechanism of the aftershock distribution to clearly image the fault geometry. In this preliminary study, we have applied the Cut-and-Paste (CAP) focal mechanism inversion method to 20 aftershocks (M ≥ 4.9) of the 2010 Mentawai earthquake. The seismic waveforms recorded by the BMKG broadband seismic network on the regional distance were used in the inversion. They successfully obtained ten focal mechanisms more than that provided in the GCMT catalog. The result exhibits a distinctly different thrust and normal faulting aftershocks activity. The thrust faulting aftershocks are mainly distributed in the downdip of the Mentawai 2010 mainshock rupture, indicating the stresses that have not been released during the mainshock. In the shallow region near the trench, the aftershocks are dominated by a normal faulting mechanism which shows the tensile stress caused by the bending of the subducting plate.

Present-day stress field pattern in the Vrancea seismic zone (Romania) deduced from earthquake focal mechanism inversion

Vrancea seismogenic zone in the South-Eastern Carpathians is characterized by localized intermediate-depth seismicity. Due to its complex geodynamics and large strain release, Vrancea represents a key element in the Carpatho-Pannonian system. Data from a recently compiled catalogue of fault plane solutions (REFMC) are inverted to evaluate stress regime in Vrancea on depth. A single predominant downdip extensive regime is obtained in all considered clusters, including the crustal layers located above the Vrancea slab. The prevalent stress regime confirms previous investigations and requires some mantle-crust coupling. The S3 principal stress is close to vertical, while S1 and S2 are horizontal, oriented perpendicularly and respectively tangentially to the Carpathians Arc bend. This configuration is present at any depth level. According to seismicity patterns, there are two main active segments in the Vrancea intermediate-depth domain, at 55 – 105 km and 105 – 180 km, both able to generate major events. The configuration of the tectonic stresses as resulted from inversion is similar in both segments. Also, high fault instability (I &gt; 0.95) is characterizing the segments. The only notable difference is given by the friction and stress ratio parameters which drop down in the bottom segment from μ = 0.95 to μ = 0.55 and from R = 0.51 to R = 0.29. This variation is attributed to possible weakening processes activated below 100 km depth and can explain the intensification of seismicity production as earthquake rate and average energy release in the lower segment versus the upper segment.