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The stress regime patterns of high-seismically active regions within the western part of the India–Eurasia collision, spanning from 67° E to 83° E and 27° N to 39° N, are elucidated through analysis of 684 Focal Mechanism Solutions from 1962 to 2021. Eighteen seismically active zones used for the stress tensor inversion, are defined based on the spatial extent of the seismicity, the depth distribution of seismic events, focal mechanism studies, and seismotectonics of the region. The defined regimes are: (1) Sulaiman Ranges and Lobe Region, (2) Hindukush, (3) Pamir, (4) Nanga Parbat Syntaxis, (5) Hazara Syntaxis, (6) Kashmir–Zanskar region, (7) Kangra–Chamba, (8) Kinnaur and Kaurik–Chango fault zone (KCFZ), (9) Garhwal, (10) Kumaon, (11) Karakoram fault zone, and (12) Gozha–Ashikule fault zone. Seismicity is reported only in the crust or up to mid-crust in most of the regions, except for the Pamir and Hindukush, where the seismicity can be observed down to 160 and 280 km, respectively. We report a clockwise rotation of the maximum horizontal stress (SHmax) of about 42° and 21° in the Hindukush and Pamir regions, respectively. with increasing focal depths from north west to north. The region where major and strong earthquakes occur indicates pure compressive regimes. Most of the zones support transpressive and transtensional tectonics with a few zones by normal and strike-slip fault regimes. Regions like Nanga Parbat syntaxis, Kinnaur, KCFZ, and Zanskar are exceptions, where extensional and transformational tectonic features dominate. Plate convergence force has less effect on defining the stress regime in the Karakoram fault zone and Gozha–Ashikule regions, which display transtensional and pure extensional regimes, respectively. Underthrusting of the Indian plate through complex tectonics is indicated by dominant compression stresses with evidences of normal, strike-slip, and oblique fault mechanisms. [ABSTRACT FROM AUTHOR]

The M s 7.3 1948 Aşgabat earthquake was one of the most devastating earthquakes of the 20th century, yet little is known about its location, style and causative fault. In this study, we bring together new seismic and geomorphic observations with previously published descriptions of surface rupture and damage distributions to determine the likely source of the earthquake. We determine the epicentre and focal mechanism of this earthquake from digitized historical seismograms and the relocation of regional seismicity to show that the earthquake most likely nucleated close to the city of Aşgabat. The earthquake ruptured a right-lateral strike-slip fault to the southeast of the city, which has a clear long-term expression in the landscape, and also likely reactivated a subparallel concealed thrust along the Gyaursdag anticline east of the city. The earthquake potentially also ruptured a right-lateral segment northwest of Aşgabat, which does not have an identifiable expression in the landscape. Using high-resolution satellite imagery and digital elevation models we investigate the geomorphology of active faulting around Aşgabat and adjacent parts of the Köpetdag (Kopeh Dagh) mountain range front, showing that there are significant strike-slip and oblique strike-slip segments adjacent to the city that apparently did not rupture in 1948, and yet show clear geomorphic expression and potential right-lateral displacement of Parthian-era (∼2000 yr) and post-Sassanian era (∼1500 yr) archaeological remains. Luminescence dating of displaced fluvial terraces west of Aşgabat yields a vertical displacement rate of 0.6 mm yr−1, though the strike-slip rate remains undetermined. [ABSTRACT FROM AUTHOR]

Marmara Region hosts a substantial part of the inhabitants in Turkey, more than 30 per cent of the total population in the cities of Istanbul, Bursa and Kocaeli. This region has experienced several large earthquakes in the past and is still under threat of destructive earthquakes in the future. There, subparallel strands of the North Anatolian Fault (NAF) expand into the region, distributing the earthquake hazards across the whole region. Thus, it is a key issue to investigate how the tectonic process is distributed between these subparallel strands to discriminate their individual earthquake hazards. In this context, we used GPS slip rates to quantify the slip partitioning of the subparallel strands of the fault system. In addition to available slip rates compiled by Bulut et al. and Kreemer et al. we analysed 71 GPS sites (51 continuous and 20 campaign-based) to intensify the GPS network in the region. GPS observations reveal 81.7, 7.7 and 10.7 per cent slip ratios for northern, middle and southern strands. These ratios are also verified by the time series analysis of continuous GPS stations, leading us to the conclusion that the northern strand of the NAF hosts substantially shorter recurrence periods and, therefore, in the long term, the highest earthquake hazard in the Marmara Region. [ABSTRACT FROM AUTHOR]

Geodetic networks enable us to investigate interseismic crustal deformation along the northwest Himalaya. Using 144 GNSS surface velocities and a Bayesian inversion model, we estimate the slip rate and fault geometry of the Main Himalayan Thrust (MHT) along six arc-normal transects in the northwest Himalaya. We consider that the fault plane consists of three sections along the décollement, namely the locking zone (0−12 km), the transition zone (10−22 km) and the creeping zone (≥22 km). The MHT is found to be completely locked from the surface down to an average depth of 6 ± 2 km. The locking-to-creeping transition zone along the décollement extends from the edge of the fully locked area to a deeper depth (14 ± 3 km) to the tip of the creeping zone of the MHT (17 ± 2 km) with a slip rate of 1.6 ± 0.9 to 3.7 ± 1.1 mm yr−1. Considering the range of uncertainties between 1−2 mm yr−1 for the GNSS velocities, the inverted slip rate along the transition zone of MHT turns out to be insignificant. Thus, the locking zone along the northwest Himalaya extends from the MFT to ∼111 ± 6 km in the north with a locking depth of ∼17 ± 2 km. The deeper part of the MHT is inferred to be creeping with an average slip rate of ∼19.1 ± 1.9 mm yr−1 along the northwest Himalaya. In addition, we have also illustrated a splay-fault model to account for the fault kinematics along the splay faults and the main décollement. The splay-fault model indicates a distributed slip rate at the locking-to-creeping transition zone and about ∼15 per cent smaller slip rate of the MHT than that of the single-fault model. Further, the checkerboard test and the uniform slip model exhibit the reliability of the current GNSS network and the inversion model (single- and splay-fault models). Overall, the updated fault kinematics inevitably contribute to the improvement of seismic hazard evaluation along the northwest Himalaya. [ABSTRACT FROM AUTHOR]

The North Anatolian Fault is the ∼1200-km-long active continental transform boundary between Anatolia and Eurasia. This strike-slip system initiated around 10–12 Ma and experienced diachronous episodes of strain localization along its strike. The structural evolution of the ∼350-km-long fault segments crossing the North Aegean Sea remains to be accurately investigated. There, the modern North Anatolian Fault is localized along two main branches: the northern branch ends at the North Aegean Trough and the southern branch ends at the Edremit-Skyros Trough. The Evia Basin is located in the North Aegean Domain between the North Anatolian Fault and the Corinth Rift. This study presents seismic reflection lines crossing the aforementioned structures of the North Aegean Domain, which document their subsurface structure and the sedimentary record of their activity since the Messinian. The seismic-reflection data set is tied to regional-scale stratigraphic markers, which constrains the age of main tectonic events related to the formation of the North Anatolian Fault. The seismic-reflection lines show that the two main branches of the North Anatolian Fault became localized structures at 1.3–2 Ma, coevally with the formation of the Evia Basin. Since 2 Ma, the North Aegean Troughs developed as a series of horsetail basins propagating westwards at the termination of the branches of the North Anatolian Fault. On a regional scale, the wide and diffuse North Anatolian transtensive shear zone active from Serravalian to Late Pliocene turned into a narrower shear zone at the two main branches of the North Anatolian Fault since the Early Pleistocene. This abrupt episode of strain localization occurred in the frame of the major Early Pleistocene change in stress regime from NE–SW to N–S extension, which has been observed throughout the Aegean Sea. [ABSTRACT FROM AUTHOR]

Earthquakes in the Korean Peninsula often occur in subsurface hidden faults that are hardly identified before the seismic activity. We investigate a midcrustal subsurface hidden fault in a seismically quiescent region of the central Korean Peninsula that produced the 28 October 2022 M L 4.1 earthquake. A combined analysis of seismicity, geophysical properties and geological features constrain the subsurface fault geometry. The midcrustal fault may extend to the lineament on the surface that presents low gravity anomalies. The focal mechanism solutions and seismicity distribution suggest that the lineament may correspond to the surface trace of the fault. Further, the responsible fault is a left-lateral strike-slip fault with dip of 88° and bilateral rupture at depths of ∼12–14 km. The lateral extent of the fault may reach >25 km. A series of foreshocks formed the source zone to induce the main shock. The main shock nucleated at a location of lateral contrasts of heat fluxes and magnetic anomalies in the fault. The strong ground motions by the main shock are amplified by the sedimentary layers in the event site, inducing aftershocks to spread along the fault with time. The fault orientation is laid to respond to the ambient stress field. The Coulomb stress changes induced by the main shock load stress the fault, increasing the possibility of another earthquake occurrence. The study suggests that a combined analysis of seismicity, geophysical properties and geological features may provide constraints on seismogenic subsurface hidden faults. [ABSTRACT FROM AUTHOR]

We use machine-learning and cross-correlation techniques to enhance earthquake detectability by two magnitude units for the earthquake sequence near Musreau Lake, Alberta, which is induced by wastewater disposal. This deep catalogue reveals a series of en echelon ∼N–S oriented strike-slip faults that are favourably oriented for reactivation. These faults require only ∼0.6 MPa overpressure for triggering to occur. Earthquake activity occurs in bursts, or episodes; episodes restricted to the largest fault tend to have earthquakes starting near the southern end (distant from injectors) and progressing northwards (towards the injectors). While most events are concentrated along these ∼N–S oriented faults, we also delineate smaller faults. Together, these findings suggest pore pressure as the triggering mechanism, where a time-dependent increase in pore pressure likely caused these faults to progressively reawaken. Analysis of the 'next record-breaking event', a statistical model that forecasts the sequencing of earthquake magnitudes, suggests that the next largest event would be M L ∼4.3. The seismically illuminated length of the largest fault indicates potential magnitudes as large as M w 5.3. [ABSTRACT FROM AUTHOR]

SUMMARY: Large continental earthquakes are infrequent, but they are important for understanding active tectonics. Many pre-1990, magnitude greater than 7 earthquakes have not been well documented due to a lack of data. The declassification of KeyHole (KH) imagery provides opportunities for investigating those historical events. In this study, we present new geodetic observations of surface deformation from the 1976 November 24 Ms 7.3 Chaldiran earthquake in eastern Turkey. We use various historical KH imagery and modern Pleiades stereo imagery to determine the coseismic deformation, providing new constraints on the 1976 rupture. Based on the image measurements, we resolve the coseismic slip at depth. The inverted slip from a simple elastic dislocation model, ∼4.0 m at shallow depth of the Hidirmentes segment, is larger than the surface displacement, around 3 m from previous field surveys, suggesting that the coseismic deformation is a combination of slip on the primary fault with 30 ± 16 per cent off-fault deformation. We also measure cumulative offsets at two sites with well-preserved features, and find a constant ratio of horizontal to vertical offsets throughout the past few earthquake cycles. Both the observed constant slip ratio in this study and offset clusters from previous field measurements support that the Chaldiran fault exhibits a characteristic slip behaviour in the late Quaternary. By combining all the geodetic and geological observations in the broader region, we also present a brief analysis of the slip rate of the Chaldiran fault. [ABSTRACT FROM AUTHOR]

The Daliang Shan is a tectonic unit that connects the active southeastern Tibetan Plateau with the stable South China block. As a newly generated (formed later than the Xianshuihe–Xiaojiang active fault system) seismotectonic zone induced by the Tibetan tectonics, the detailed constraints of the crustal deformation are central to the understanding of the kinematics and dynamics of the Tibetan expansion. This paper establishes and analyses a high-spatial resolution global positioning system (GPS) velocity field from a dense GPS network in this region. Our modelling results indicate that, in contrast to the equivalent sinistral strike-slip rate of approximately 5 mm yr–1 on the Anninghe–Zemuhe and Daliangshan faults, their inferred interseismic locking depth varies within a large range. The southern segment of the Anninghe Fault and the middle segment of the Daliangshan Fault have deep locking depths of ∼13 km, indicating that the seismic risk is high in these areas. In addition, the detectable counter-clockwise rotation rate of 0.35 ± 0.12° Myr–1 of the Mabian block makes a significant contribution of ∼50 per cent to the strike-slip motion on its boundary faults. This counter-clockwise rotation may be induced by a left-lateral shear gradient with southeastward motion relative to the South China-fixed reference frame, indicating the significance of a simple-shear pattern in exploring the kinematics of the encroachment of the Tibetan tectonics upon a stable block (craton). [ABSTRACT FROM AUTHOR]

We combine Global Positioning System (GPS) velocity field with Interferometric Synthetic Aperture Radar (InSAR) results to study the interseismic deformation across the western Altyn Tagh fault (ATF) at longitude 86° E. GPS and InSAR data are consistent after correcting for the contribution from vertical deformation in the InSAR line-of-sight map. InSAR and GPS data identify an area of ∼2 mm a−1 sinistral shear and ∼6 mm a−1 of NS shortening located ∼150 km south of the ATF near the Manyi fault system. Excluding the data located in that area, Bayesian inversion of a 2-D profile across the ATF indicate a locking depth of 14.8 ± 3.5 km and a slip rate of 8.0 ± 0.4 mm a−1, lying at the lower range of previously published estimates. In addition, we find no significant offset between the fault at depth and the surface fault trace and no asymmetry of the interseismic profile that implicitly reveal lateral variations of the elastic strength across the ATF. Detailed analysis of InSAR profile across the fault show no distinguishable surface creep along the western ATF. Our study highlights how different data sets, data selection and model assumption might impact results on the ATF slip rate, locking depth and rheological contrast across the fault. [ABSTRACT FROM AUTHOR]

Although the Dead Sea Transform (DST) fault system has been extensively studied in the past, little has been known about the present-day kinematics of its southernmost portion that is offshore in the Gulf of Aqaba. Here, we present a new GPS velocity field based on three surveys conducted between 2015 and 2019 at 30 campaign sites, complemented by 11 permanent stations operating near the gulf coast. Interseismic models of strain accumulation indicate a slip rate of |$4.9^{+0.9}_{-0.6}~\mathrm{ mm}\,\mathrm{ yr}^{-1}$| and a locking depth of |$6.8^{+3.5}_{-3.1}~\mathrm{ km}$| in the gulf's northern region. Our results further indicate an apparent reduction of the locking depth from the inland portion of the DST towards its southern junction with the Red Sea rift. Our modelling results reveal a small systematic left-lateral residual motion that we postulate is caused by, at least in part, late post-seismic transient motion from the 1995 M W 7.2 Nuweiba earthquake. Estimates of the moment accumulation rate on the main faults in the gulf, other than the one that ruptured in 1995, suggest that they might be near the end of their current interseismic period, implying elevated seismic hazard in the gulf area. [ABSTRACT FROM AUTHOR]