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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]

NW Iran-SE Turkey is a tectonically active zone related to the Arabia-Eurasia convergence, but the active stress state in this zone has not yet been clearly studied. To improve the knowledge of present-day stress state in this region, optimum reduced stress tensor was analysed. For this, a large number of earthquake focal mechanisms (277) were collected. The analyses show most mechanisms exhibit strike-slip to thrust faulting. These data indicate that this region is dominated by an N158° maximum horizontal compressive stress (SHmax) belonging to a transpressional tectonic regime. In the scale of the study area, the relative magnitude of the intermediate and minimum principal stress axes do not differ much (ϕ = 0.09). Brittle deformation in this area is dominantly accommodated by a combination of strike-slip and thrust faulting (Aϕ = 1.82 to 2.30). The analyses reveal that two sets of faults show a high tendency to slip and reactivate. These sets contain NW-SE-striking right-lateral and NNE-SSW-striking left-lateral faults. The results of this study may help to study the active seismicity, tectonic activity, and seismic risk in this region. [ABSTRACT FROM AUTHOR]

The Mexican subduction zone, the Gulf of California spreading center, as well as the triple junction point around the Jalisco and the Michoacán Blocks, represents the most active seismogenic belts inducing seismic hazard in the Jalisco-Colima-Michoacán region. Herein, considering such seismotectonic setting, we develop a new seismic source model for the surrounding of Jalisco-Colima-Michoacán to be used as an input in the assessment of the seismic hazard of the region. This new model is based on revised Poissonian earthquake (1787–2018) and focal mechanism (1963–2015) catalogs, as well as crustal thickness data and all information about the geometry of the subducting slabs. The proposed model consists of a total of 37 area sources, comprising all the three different possible categories of seismicity: shallow crustal, interface subduction, and inslab earthquakes. A special care was taken during the delimitation of the boundaries for each area source to ensure that they represent a relatively homogeneous seismotectonic region and to include a relatively large number of earthquakes that enable us to compute as reliable as possible seismicity parameters. Although the sources were delimited following the standard criteria of assessing the probabilistic seismic hazard, they are also characterized in terms of their seismicity parameters (annual rate of earthquakes above Mw 4.0, b-value, and maximum expected magnitude), mean seismogenic depth, as well as the predominant stress regime. The proposed model defines and characterizes regionalized potential seismic sources that can contribute to the seismic hazard at the Jalisco-Colima-Michoacán region, providing the necessary information for seismic hazard estimates. [ABSTRACT FROM AUTHOR]

The paper represents a multidisciplinary investigation of the relation between seismicity, active stress field and geological structure for the West of Romania. The study is based on revised and updated catalogues of earthquakes (3572 events) and focal mechanisms (89 solutions). The large spatio-temporal variations of the stress tensor parameters, b-value (b=0.74-0.81) and fractal dimensions of seismicity (D=1.05-2.34) highlight both clustering patterns along active fault systems and scattered epicentres within planes or volumes. A tectonic model with at least three blocks bordered by active faults is proposed controlling SHmax trend: it rotates clockwise in time and 3D space by up to 900 along with the changes of the stress regime from the compressional to extensional and activity migration between two major and complex fault systems. Low b-values characterize the regardless of the size of data source, but the smaller source/dataset, with or without aftershocks included, ensures a more realistic estimation of seismic hazard that is confirmed by seismic history (Tr_Mw5.5 = 76 years). Key words: active tectonics, earthquakes, focal mechanisms, stress regime, seismic hazardThe paper represents a multidisciplinary investigation of the relation between seismicity, active stress field and geological structure for the West of Romania. The study is based on revised and updated catalogues of earthquakes (3572 events) and focal mechanisms (89 solutions). The large spatio-temporal variations of the stress tensor parameters, b-value (b=0.74-0.81) and fractal dimensions of seismicity (D=1.05-2.34) highlight both clustering patterns along active fault systems and scattered epicentres within planes or volumes. A tectonic model with at least three blocks bordered by active faults is proposed controlling SHmax trend: it rotates clockwise in time and 3D space by up to 90° along with the changes of the stress regime from the compressional to extensional and activity migration between two major and complex fault systems. Low b-values characterize the regardless of the size of data source, but the smaller source/dataset, with or without aftershocks included, ensures a more realistic estimation of seismic hazard that is confirmed by seismic history (Tr_Mw5.5 = 76 years). [ABSTRACT FROM AUTHOR]

Northwestern Iran is one of the seismically active regions with a high seismic risk in the world. This area is a part of the complex tectonic system due to the interaction between Arabia, Anatolia and Eurasia. The purpose of this study is to deduce the stress regimes in the northwestern Iran and surrounding regions from stress inversion of earthquake focal mechanisms. We compile 92 focal mechanisms data from the Global CMT catalogue and other sources and also determine the focal mechanisms of 14 earthquakes applying the moment tensor inversion. We divide the studied region into 9 zones using similarity of the horizontal GPS velocities and existing focal mechanisms. We implement two stress inversion methods, Multiple Inverse Method and Iterative Joint Inversion Method, which provide comparable results in terms of orientations of maximum horizontal stress axes SHmax. The similar results of the two methods should make us more confident about the interpretations. We consider zones of exclusion surrounding all the earthquakes according to independent focal mechanisms hypothesis. The hypothesis says that the inversion should involve events that are far enough from each other in order that any previous event doesn't affect the stress field near the earthquake under consideration. Accordingly we deal with the matter by considering zones of exclusion around all the events. The result of exclusion is only significant for eastern Anatolia. The stress regime in this region changes from oblique to strike slip faulting because of the exclusion. In eastern Anatolia, the direction of maximum horizontal stress is nearly north-south. The direction alters to east-west in Talesh region. Errors of σ 1 are lower in all zones comparing with errors of σ 2 and σ 3 and there is a trade-off between data resolution and covariance of the model. The results substantiate the strike-slip and thrust faulting stress regimes in the northwest of Iran. [ABSTRACT FROM AUTHOR]

The increasing number of earthquakes in recent decades in Northwestern Iran and the determination of the epicenters of these events makes possible to estimate accurately the changing tectonic regime using the Win-Tensor inversion focal mechanism program. For this purpose focal mechanism data were collected from various sources, including the Centroid Moment Tensor catalog (CMT). The focal mechanism and fault slip data were analyzed to determine change in the stress field up to the present day. The results showed that two stages of brittle deformation occurred in the region. The first stage was related to Eocene compression in NE-SW direction, which created compressional structures with NW-SE strike, including the North and South Bozgush, south Ahar and Gushedagh thrust belts. The second brittle stage began in the Miocene with NW-SE compression and caused developing thrusts with N-S trends that were active presently. These stress regimes were created by the counter-clockwise rotation of the Azerbaijan plateau caused by movement on strike slip faults and continuous compression between the Arabian plate, the south Caspian basin and the Caucasus region. Pliocene-Quaternary activity of the Sabalan and Sahand volcanoes as well as recent earthquakes occurred as a result of this displacement and rotational movement. The abundance of hot springs in the Ardebil, Hero Abad and Bostanabad areas also bore witness to this activity. [ABSTRACT FROM AUTHOR]

After the occurrence of unusual upper crustal seismic activity within the eastern sector of the Trans-Mexican Volcanic Belt (TMVB) in 2019, an area of generally low seismicity, an effort was made to interpret the origin. Concerns leading to the careful analysis presented here, were manifold: Shallow clustered seismicity can indicate active volcanism or related processes, as well as hydrothermalism. Additionally, the area was struck by a major earthquake (Mw 6.4) on January 4, 1920, which had devastating effects on the area's complex geological features and caused hundreds of deaths and significant material damage. As a result, several events of the 2019 sequence of unusual seismic activity have been relocated to precisely know the location. The 34 earthquakes of the 2019 seismic sequence occur in a relatively small area of less than 1600 km2. The seismic sequence can be further subdivided into 2 clusters, separated by the Pico de Orizaba – Cofre de Perote volcanic chain, which itself separates two tectonostratigraphic terranes, is a topographic divide and represents the limit to a considerable topographic drop. We also used the seismic sequence to constrain further the state of local stress and henceforth observe the tectonic regime of the area. For this purpose, focal mechanisms of the events with Mc > 4 (this sequence and previous data) were determined and the inversion of the stress tensor of this group was carried out. Similarly, the state of stress for the central zone of the TMVB was recalculated to establish its possible variations towards the eastern part of the TMVB. We show that the state of stress of the Eastern/Easternmost TMVB presents a transtensional regime, similar to the central TMVB, which has been suggested is mainly caused and controlled by the geometry of the subduction of the Cocos plate beneath the North American plate. A review of Rayleigh wave dispersion curves, calculated through interferometry of ambient seismic noise, a change is observed that suggests some variation in the structure of the eastern and easternmost areas. This contribution is relevant because little is known about the seismotectonics of the TMVB's eastern sector, in part because of the low cortical seismicity reported along the TMVB, the reduced coverage of seismic stations in this region and the distance from the trench Middle American Trench (MAT), where most of the important tectonic processes in Mexico take place. Recently, with the incorporation of a regional seismic network, the detection of small-magnitude events in the study area has significantly been improved. • New seismic data for the Eastern Trans-Mexican Volcanic Belt (E-TMVB) in Mexico, a generally low seismicity zone. • Differences between Eastern and Easternmost TMVB using seismic noise data. • Analysis of two seismic clusters close to young volcanic products. • Preliminary results of the behavior of the crustal stress regime, crustal structure and tectonic regime. [ABSTRACT FROM AUTHOR]

The kinematic models and the associated orientation of compressional stress in northeastern Iran have long been the subject of debate. Previously proposed models have relied on the interpretation of overall fault geometry and geological fault slip data. These models generally suffer from low resolution of the temporal and spatial changes in the stress field and geodynamic regime. In the recent years there has been a significant increase in the number of focal mechanisms available for the study area, and it is now possible to estimate the present-day stress field in relative detail based on the formal stress inversion method. We compiled 59 earthquake focal mechanisms from the Global/Harvard CMT catalogue and various other sources. These we grouped into 7 separate regions (boxes). For each region reduced stress tensors have been obtained by formal stress inversion using the Win-Tensor program (Delvaux and Sperner, 2003). The present-day stress state obtained from earthquake focal mechanisms shows that the North of Central–East Iran Blocks (NCEIB) is presently subjected to a transpressional tectonic regime (R′ = 1.78 ± 0.26), with an N37 ± 7° E direction of horizontal principal compression. This stress state is consistent with the direction of convergence between the Arabian and Eurasian plates. In this part of northeastern Iran this convergence is accommodated by clockwise rotations. The Doruneh fault system (DFS) curves ESE–WSW, perpendicular to relative motion. This orientation results in oblique compression with a significant along-belt component of left-lateral shear in the western and central segments and belt-perpendicular compression in the eastern segment. [ABSTRACT FROM AUTHOR]

The North Tabriz fault is a segmented dextral fault in Northwest Iran, with a history of major destructive earthquakes that have repeatedly destroyed the city of Tabriz (current population 1.6 million). The quiescence of the fault (last major temblor in 1854) and a lack of outcrop study have hampered stress analysis. Resolution of the stress states on the fault could be used for seismotectonic study along the North Tabriz fault and for understanding the geodynamics of the Arabia-Eurasia collision zone. Using fault-slip data collected from 88 localities in the fault system, we conducted an inversion analysis of this fault-slip data and analysis of the stratigraphic, geometric, and structural information. As a result, we confirmed that transcurrent deformation is prevalent on the North Tabriz fault and adjacent areas and is generally accomplished by predominant NW-SE-trending dextral and NE-SW-trending sinistral faults. Specifically, three separate tectonic episodes are recognised from the stress inversion data, consistent with the geologic data: (i) a post-Cretaceous and pre-Early Miocene compressional (Laramian) stress regime, (ii) an Early Miocene extensional stress regime, and (iii) modern tectonic episode with different local stress regimes (compressional and extensional) along the different segments of this fault. [ABSTRACT FROM AUTHOR]

Izmir metropolitan area is one of the biggest cities of Turkey in terms of population and industrial density. Amount of recordings and quality of located events increased since the new local seismic array, IzmirNET, was installed in 2008. Present study addresses to elucidate seismotectonics of Izmir vicinity by a new data set and reveal first results of seismological analysis obtained for 4.5 years. 930 small earthquakes are well located within less than 1.0 km horizontal error (RMS ≤ 0.15 s) over a total of 1421 recorded events. Data were processed by using SEISAN Earthquake Analysis Software implemented at SeismoLab of Dokuz Eylul University. The Orhanli–Tuzla Fault Zone and Izmir faults are capable of producing a destructive earthquake. Events show clear clusters around the Outer Bay of Izmir Gulf and Sigacik Bay. We also detected some dense clusters Sigacik Bay to the South, and in the outer bay of Izmir Gulf along NW–SE direction. We investigated kinematics of active faults around the Izmir and Foca areas by using 41 focal mechanism solutions from different algorithms. Foca is located 60 km in the NW direction of Izmir. Mechanisms generally resulted in pure normal or dominant normal faulting with minor strike-slip components in agreement with the kinematics of the other fault zones in Western Turkey. A stress tensor study using selected fault plane solution indicates that the study area is under extension regime in an N–S direction with 71° plunge for maximum compressive principal stress axis (S1), and closer to horizontals for intermediate (S2) and minimum compressive principal stress axis (S3). [ABSTRACT FROM AUTHOR]

Abstract: The state of stress and its implications for shear on fault planes during fluid injection are crucial issues for the HDR (Hot Dry Rock) or EGS (Enhanced or Engineered Geothermal System) concept. This is especially true for hydraulic stimulation experiments, aimed at enhancing the connectivity of a borehole to the natural fracture network, since they tend to induce the shearing of fractures, which is controlled by the local stress regime. During the 2000 and 2003 stimulation tests at Soultz-sous-Forêts, France, about 10,000 microearthquakes were located with a surface seismological network. Hundreds of double-couple (DC) focal mechanisms were automatically determined from first-motion polarities using the FPFIT program [Reasenberg, P.A., Oppenheimer, D., 1985. FPFIT, FPPLOT and FPPAGE: Fortran computer programs for calculating and displaying earthquake fault-plane solutions. US Geological Survey Open-File Report 85-739, 25 pp.]. The majority of these mechanisms indicate normal-faulting movement with a more or less pronounced strike-slip component. Some quasi-pure strike-slip events also occurred, especially in the deeper part of the stimulated rock volume, at more than 5km depth. Although we found a double-couple solution for all events, we tried to observe and quantify the proportion of the non-double-couple (NDC) component in the seismic moment tensor for several microseisms from the 2003 data. The study shows that the NDC is higher for the events in the vicinity of the injection well than for the events far from the well. We used the method of Rivera and Cisternas [Rivera, L., Cisternas, A., 1990. Stress tensor and fault-plane solutions for a population of earthquakes. Bull. Seismol. Soc. Am. 80, 600–614.] to perform the inversion of the deviatoric part of the stress tensor from P-wave polarities. This method was applied to different datasets from the 2000 test, taken from the shallower and deeper parts of the stimulated region. The results show a stable, horizontal, NE-SW-oriented trend of the minor horizontal stress, but a rotation of the major stress from a sub-vertical direction (top of the stimulated region) to a sub-horizontal one (bottom of the stimulated region). This implies a change from a normal-faulting to a strike-slip regime, in agreement with our fault-plane solutions. Finally, we applied the stress components to the nodal planes of several events and were able to determine their fault plane and obtain a 3D image of the fracture network, based on real data. [Copyright &y& Elsevier]

This is the first seismological investigation of the stress regime in the Jamaican crust. The fault plane solutions of 32 well-recorded local earthquakes were determined and used as inputs for determining stress directions. The results consistently show that a strike-slip fault regimeexists in three homogeneous crustal areas, namely the Blue Mountain—Wagwater restraining bend of eastern Jamaica, the Rio Minho—Crawle River (RMCR) fault of central Jamaica and the Montpelier–Newmarket belt (MNB) of western Jamaica. However, for the island as a whole, a thrust-faulting regimewas found, but inhomogeneity is implied by the higher level of misfit obtained. These results are in keeping with results of other recent studies in the Jamaica–Cuba region and with the strike-slip regime of the Gonave microplate boundaries. [ABSTRACT FROM AUTHOR]