Seismotectonic Implications Research Articles

Source parameters and scaling relations for small to moderate earthquakes in theIndo‐BurmaRanges,North‐east India, and its seismotectonic implications

The Indo‐Burma Ranges (IBRs) and its surrounding North‐east India is one of the seismotectonically active subduction systems in the world, where the Indian Plate is subducting beneath the Sunda Plate. This has resulted in major earthquakes in the past. In this study, spectral analysis of S‐wave has been used to investigate the source parameters for local earthquakes (3.3 ≤ MD ≤ 5.8) exclusively in the Indo‐Burma region fusing a network of six stations by adopting the Brune model. The corner frequencies (fc) of the events are varied from 0.6 to 3.2 Hz. The estimated source parameters lie in the range from 9 × 1013–3.7 × 1017 N m, 0.2–35.1 MPa, 410–1,956.4 m, 0.002–1.196 m, and 2.9–13.5 Hz for seismic momentM0, stress drop (Δσ), source radius, source dislocation, and maximum cut‐off frequency (fmax), respectively. The scaling relation betweenM0andfchas been derived asM0 = 1.622 × 1016 fc−5.298. The scaling relations ofM0withfmaxand stress drop have also been derived. The high‐stress drops (>10 MPa) were at a depth zone of ~40–60 km, while the stress drops were less than 4.0 MPa beyond this depth, indicating the shallow and deeper portion of the lithosphere to be relatively brittle. We postulate that this could occur due to a complex detachment process as a result of slab tearing in the region. The upper boundary of stress drop was found to have an increasing trend with the focal depth of the earthquakes. The empirical relations betweenM0 − MDandMW − MDhave also been derived for the IBRs. The estimated source parameters and scaling relations will be useful in estimations of lithospheric strength, simulation of strong ground motion in the IBRs, and calibrating the coefficient of the local earthquakes in the IBR and its surrounding region.

Source characteristics and seismotectonic implications of the 26 September 2019Mw 5.7 Silivri High-Kumburgaz Basin earthquake and evaluation of its aftershocks at the North Anatolian Fault Zone (Central Marmara Sea, NW Turkey)

SUMMARYThe Central Marmara Sea region hosts the northwestern branch of the North Anatolian Fault Zone (NAFZ) with its known seismic gap between the 1912 Ganos (Mw 7.2) and 1999 Izmit (Mw 7.4) major devastating earthquakes and thus poses a significant seismic hazard potential for the megacity Istanbul. The 26 September 2019 Mw 5.7 Silivri High-Kumburgaz Basin (central Marmara Sea) earthquake ruptured a thrust fault with a minor strike-slip component at the north of the eastern end of this gap relatively in the shallow depth (h= 8 km) range. Thus, in this study, we examine source properties of the main shock activity and coseismic behaviour of the failure, and the pattern of post-seismic deformation based on the aftershock distribution to have an insight into the role of the subsidiary and main fault structures on the crustal kinematics along this complicated branch of the NAFZ. The relocated epicentres are aligned in the E–W direction and tend to propagate towards the segments to the east of the main shock. The detected aftershock activity appears to focus on the east side of the main shock and almost no seismic activity was observed to the west of the epicentre. Independent investigations from coda-wave fitting, point-source, and finite-fault slip modelling agree on the moment magnitude of Mw5.7 for the 26 September 2019 main shock. The kinematic rupture model of this event implied that the main rupture nucleated around the hypocentre, and then propagated bilaterally along the E–W direction but with significant progress towards the east. The distribution of the slip vectors indicates that the rupture evolved on a dextral thrust fault plane. The spatio-temporal behaviour of the overall aftershocks sequence, their focal mechanism solutions and our kinematic slip model clearly shows that the existing secondary structures developed in simple shear dextral deformation are likely responsible for the main shock activity. We conclude that such type of deformation model results in a motion in response to the thrust faulting with strike-slip component with an N89°W (271°) orientation and 33°NE dipping at left stepover transpressional region on the NAFZ.

Coseismic slip distributions and stress interactions of the November 12, 2017, Sarpole-Zahab (Western Iran) earthquake (MW=7.3) and its aftershocks: seismotectonic implications

Coseismic slip distributions and stress interactions of the November 12, 2017, Sarpole-Zahab (Western Iran) earthquake (MW=7.3) and its aftershocks: seismotectonic implications

Correction to: Seismotectonic implications of the 2020 Samos, Greece, Mw 7.0 mainshock based on high‐resolution aftershock relocation and source slip model

The 30 October 2020, Mw 7.0 Samos mainshock took place in the offshore north of Samos Island in eastern Aegean area, previously struck in 1904 with a comparable magnitude earthquake offshore the southern coastline. The investigation of the aftershock seismicity evolution and the properties of the activated fault network was accomplished with aftershock relocation performed with the double-difference and cross-correlation techniques. The highly accurate relocated seismicity illustrates a well-defined E–W activated structure located deeper than 5 km with an average depth of ~ 12 km. Moment tensor solutions indicate mostly normal faulting with an average T-axis ~ 185ο. Strong-motion waveform modeling revealed a N-dipping fault plane with a coseismic slip patch of 36 km $$\times$$ 22 km and a maximum slip equal to 1 m at 12 km depth. The slip is mainly concentrated in a single asperity implying a rupture mode of asperities breaking in isolated earthquakes rather than to cooperate to produce a larger rupture. Coulomb stress calculations unveil increased positive static stress changes values at the locations of the majority of the aftershocks and activation of minor fault segments by stress transfer.

Ongoing Inversion of a Passive Margin: Spatial Variability of Strain Markers Along the Algerian Margin and Basin (Mediterranean Sea) and Seismotectonic Implications

Subduction initiation is an important but still poorly documented process on Earth. Here, we document one of a few cases of ongoing transition between passive and active continental margins by identifying the geometrical and structural signatures that witness the tectonic inversion of the Algerian continental margin and the deep oceanic domain, located at the northern edge of the slow-rate, diffuse plate boundary between Africa and Eurasia. We have analyzed and tied 7900 km of deep seismic reflection post-stacked data over an area of ∼1200 km long and ∼120 km wide. The two-way traveltime lines were converted into depth sections in order to reconstruct and map realistic geometries of seismic horizons and faults from the seafloor down to the acoustic basement. Along the whole length of this young transitional domain, we identify a clear margin segmentation and significant changes in the tectonic signature at the margin toe and in the deep basement. While the central margin depicts a typical thick- and thin-skinned tectonic style with frontal propagation of crustal thrust ramps, the central-eastern margin (Jijel segment) reveals a higher strain focusing at the margin toe together with the largest flexural response of the oceanic lithosphere. Conversely, strain at the margin toe is limited in the western margin but displays a clear buckling of the oceanic crust up to the Spanish margin. We interpret these contrasting, segmented behavior as resulting from inherited heterogeneities in (1) the geometry of the Algerian continental margin from West to East (wrench faulting in the west, stretched margin elsewhere) and (2) the Miocene thermal state related to the diachronous opening of the Algerian basin and to the magmatic imprint of the Tethyan slab tearing at deep crustal levels. The narrow oceanic lithosphere of the Western Algerian basin is assumed to favor buckling against flexure. From the dimension and continuity of the main south-dipping blind thrusts identified at the margin toe, we reassess seismic hazards by defining potential lengths for ruptures zones leading to potential magnitudes up to 8.0 off the central and eastern Algerian margins.

Seismotectonic implications of the 2020 Samos, Greece, Mw 7.0 mainshock based on high-resolution aftershock relocation and source slip model

Seismotectonic implications of the 2020 Samos, Greece, Mw 7.0 mainshock based on high-resolution aftershock relocation and source slip model

Deformation Pattern of the Northern Sector of the Malta Escarpment (Offshore SE Sicily, Italy): Fault Dimension, Slip Prediction, and Seismotectonic Implications

Marine seismic reflection data coupled with on-land structural measurements improve our knowledge about the active deformation pattern of the northern sector of the Malta Escarpment, a bathymetric and structural discontinuity in the near-offshore of Eastern Sicily. As favourably oriented to be reactivated within the Neogene Africa–Europe convergence, it is believed that the Malta Escarpment has a significant role in the recent seismotectonic framework of the Western Ionian Basin and the Hyblean foreland domain of SE Sicily, where some of the largest and most destructive Mediterranean earthquakes are located according to available historical catalogs. Offshore seismic data along with bathymetric grids illuminate the shallow subseafloor setting and allow more accurate mapping of the seafloor expression of previously identified faults in the area. The seismic interpretation and the near-fault sediment pattern analysis provide constraints on fault 3D geometries as well as on their through-time tectonic activity, suggesting also that part of the observed deformation may have been caused by nontectonic processes. Identified faults form currently an E-dipping, roughly N–S trending, and 60 km-long extensional belt deforming the seafloor with a significant displacement amount in the Ionian offshore between Catania and Siracusa. 3-dimensional parameters of faults were then used to derive expected magnitudes and their reactivation propensity. Empirical scaling relationships and forward methods point to a high seismic potential for the detected fault as well as predict the fault slip behavior according to the field-derived differential stress. This combined analysis along with faults displacement measurements pointed out how the longest and most continuous fault could be capable of generating M > 7 seismic events, putting forward strong seismotectonic implications for the adjacent and densely populated Hyblean Plateau. The expected magnitude and the estimated recurrence time interval are compatible with those inferred for large historical earthquakes in the area even if other offshore seismic sources cannot be ruled out.

Active faults of El Salvador

In this work we present a review of the current state of knowledge of the active faults in El Salvador and its seismo-tectonic implications. An updated map of active fault traces is combined with the spatial analysis of shallow seismicity, focal mechanisms, recent geodetic GPS velocities, local strain estimations and morpho-tectonic features to provide a synoptic view of the active tectonics of the El Salvador. The major faults selected as potential seismic sources bound tectonic blocks or regions that include GPS sites with consistent relative velocity vectors. We propose several active tectonic domains along El Salvador controlled by three current deformation regimes: a crustal block in the forearc sliver dominated by rigid westward translation with the faster and more homogeneous GPS velocities; three zones dominated by E-W distributed extensional coaxial deformation; and two bands with a deformation compatible with transtensional regime. In the Western sector of the ESFZ, GPS velocities and local structure suggest that E-W extension concentrates along the NNW-SSE oriented Santa Ana volcanic axis and it could connect to the north with the extensional region of the Ipala Graben. This is consistent with to the eastward shift of the North America-Cocos-Caribbean triple junction and “the closing of the zipper” proposed in recent models that progressively slows down the strike-slip movement along the northern limit of the forearc sliver. In the central sector large GPS velocity gradient parallel to the volcanic arc may be associated to complex and discontinuous structure of the ESFZ driving slowdown of the westward movement of local tectonic blocks. The southeaster sector of the ESFZ is an incipient large pull-apart structure affecting a pre-existing extensional N-S oriented fabric that induces two tectonic subdomains, the eastern one undergo E-W coaxial extension and the western one characterized by transtensional strain regime. We propose a new structural segmentation of the ESFZ considering that this fault zone includes all those structures that accommodate the relative velocity between the forearc sliver and the Chortís block. The 38 active faults with surface traces that are mapped for more than 5 km long proposed in this work, with maximum potential magnitude Mw ranging from 5.98 to 7.94, will contribute to improve regional and local seismic sources databases and seismic hazard assessment.

The Mw = 5.6 Kanallaki Earthquake of 21 March 2020 in West Epirus, Greece: Reverse Fault Model from InSAR Data and Seismotectonic Implications for Apulia-Eurasia Collision

We identify the source of the Mw = 5.6 earthquake that hit west-central Epirus on 21 March 2020 00:49:52 UTC. We use Sentinel-1 synthetic aperture radar interferograms tied to one permanent Global Navigation Satellite System (GNSS) station (GARD). We model the source by inverting the INSAR displacement data. The inversion model suggests a shallow source on a low-angle fault (39°) dipping towards east with a centroid depth of 8.5 km. The seismic moment deduced from our model agrees with those of the published seismic moment tensors. This geometry is compatible with reverse-slip motion along the west-verging Margariti thrust fault that accommodates part of the convergence within the collision zone between Apulia and Eurasia. We also processed new GNSS data and estimate a total convergence rate between Apulia and Eurasia of 8.9 mm yr−1, of which the shortening of the crust between the Epirus coastal GNSS stations and station PAXO in the Ionian Sea (across the Ionian Thrust) is equivalent to ~50% of it or 4.6 mm yr−1. By back-slip modelling we found that a 60-km wide deformation zone takes up nearly most of the convergence between Apulia-Eurasia, trending N318°E. Its central axis runs along the southwest coast of Corfu, along the northeast coast of Paxoi, heading toward the northern extremity of the Lefkada island. The island of Paxoi appears kinematically as part of the Apulian plate.

Driving stress and seismotectonic implications of the 2013 Mw5.8 Awaji Island earthquake, southwestern Japan, based on earthquake focal mechanisms before and after the mainshock

A driving stress of the Mw5.8 reverse-faulting Awaji Island earthquake (2013), southwest Japan, was investigated using focal mechanism solutions of earthquakes before and after the mainshock. The seismic records from regional high-sensitivity seismic stations were used. Further, the stress tensor inversion method was applied to infer the stress fields in the source region. The results of the stress tensor inversion and the slip tendency analysis revealed that the stress field within the source region deviates from the surrounding area, in which the stress field locally contains a reverse-faulting component with ENE–WSW compression. This local fluctuation in the stress field is key to producing reverse-faulting earthquakes. The existing knowledge on regional-scale stress (tens to hundreds of km) cannot predict the occurrence of the Awaji Island earthquake, emphasizing the importance of estimating local-scale (< tens of km) stress information. It is possible that the local-scale stress heterogeneity has been formed by local tectonic movement, i.e., the formation of flexures in combination with recurring deep aseismic slips. The coseismic Coulomb stress change, induced by the disastrous 1995 Mw6.9 Kobe earthquake, increased along the fault plane of the Awaji Island earthquake; however, the postseismic stress change was negative. We concluded that the gradual stress build-up, due to the interseismic plate locking along the Nankai trough, overcame the postseismic stress reduction in a few years, pushing the Awaji Island earthquake fault over its failure threshold in 2013. The observation that the earthquake occurred in response to the interseismic plate locking has an important implication in terms of seismotectonics in southwest Japan, facilitating further research on the causal relationship between the inland earthquake activity and the Nankai trough earthquake. Furthermore, this study highlighted that the dataset before the mainshock may not have sufficient information to reflect the stress field in the source region due to the lack of earthquakes in that region. This is because the earthquake fault is generally locked prior to the mainshock. Further research is needed for estimating the stress field in the vicinity of an earthquake fault via seismicity before the mainshock alone.

Foreland seismicity associated with strike-slip faulting in southeastern Sicily, Italy: Seismotectonic implications and seismic hazard assessment

Between February 6 and 12, 2016, an earthquake sequence affected the western sector of the Hyblean foreland domain in SE Sicily (Italy), a region characterized by several disastrous events, as the seismic history describes, of which little is still known about the seismogenic sources. Despite the low magnitude of most events (MLmax 4.3), the sequence represents one of the largest episodes of seismic strain release in the area over the last 30 years since the December 13, 1990 earthquake (ML = 5.4).Accurate seismic phase arrival times related to the seismic sequence were obtained by using a wave-correlation method which enabled a significant improvement in precision of earthquake locations, allowing to detect a N-S striking rupture area about 8 km deep. The N-S trending discontinuity well matches, in the near-surface, with the observed geological structures, in rather good agreement with the computed focal solutions which evidence a roughly N-S elongated structure characterized by a left-lateral kinematics.The detailed knowledge learned on the geometry of the fault surface activated by the seismic sequence, together with the characterization of earthquake source parameters, gave important constraints for a realistic estimation of seismic ground motion throughout stochastic simulations. The modeling technique was based on the comparison between instrumental signals of the strongest event of the sequence and the associated synthetic ones. This permitted to properly define the set of input parameters for the construction of a realistic scenario for a MW 6.1 earthquake, which provides the largest contribution to the seismic hazard for the investigated area.

Subduction initiation and the rise of the Shillong Plateau

Discrepancies between geodetically and geologically estimated thrust fault slip rates are generally viewed as a methodological problem. Even when slip rate is steady over geological time, a discrepancy may exist because each method is sensitive to different deformation processes. However, this offers a tool to estimate the partitioning of convergence between footwall and hanging wall deformation, and therefore a way to discriminate among orogenic styles. Here we investigate one such discrepancy for the Shillong Plateau, a basement-cored contractional orogen within the Himalayan foreland basin. Using a regional block model to explain the modern geodetic velocity field and explicit uncertainty analysis of the geologic rates, we show that this discrepancy cannot be reconciled simply by invoking uncertainties in individual methods.Our results indicate that the Shillong Plateau is not an ongoing forward break of the Bhutan Himalayas, as was believed until recently. Instead, the observed inter-plate convergence and plateau uplift in this region may be driven primarily by an attempt of the negatively buoyant Indian passive margin lithosphere (the Surma Basin), south of the plateau, to initiate subduction. As a result, the uplift history of the plateau, which constrains the geologic rate, is significantly lower than expected given the geodetic convergence rate. We propose that this convergence is largely accommodated by the transport of the footwall into the mantle. This geodynamic scenario has important regional seismotectonic implications: (1) the cold and brittle sinking passive margin may have enabled the deep extent (∼30 km) and therefore large magnitude of the MW 8+ Shillong Earthquake of 1897; (2) the collapse of the Indian lithosphere into the mantle may have created the anomalously deep (∼20 km) Surma Basin; and (3) this subsidence may also drive accelerated post-Miocene westward propagation of the Indo-Burman Wedge. We propose that the Shillong Plateau is the only modern example of passive margin collapse, and can serve as a natural laboratory to study the earliest phase of subduction.

Conjugate ruptures and seismotectonic implications of the 2019 Mindanao earthquake sequence inferred from Sentinel-1 InSAR data

In 2019, four strong earthquakes of Mw>6.4 occurred successively in Mindanao, Philippines. Based on the reports from the USGS and PHIVOLCS, these earthquakes were dominated by strike-slip ruptures. Whether these earthquakes are temporally and spatially related remained unknown. We characterized the coseismic displacement fields during the earthquake sequence using an InSAR technique with Sentinel-1 SAR data. The InSAR deformation measurements convincingly reveal that the four earthquakes produced distinct coseismic displacement patterns. We estimated the source parameters of the earthquakes with a two-step inversion strategy. The optimal model suggests that the earthquake sequence resulted from the reactivation of a conjugate fault structure that involves two nearly vertical left-lateral strike-slip faults and two high-angle right-lateral strike-slip faults. We calculated Coulomb stress changes from the earthquake sequence, suggesting that the previous strong earthquakes had significant stress-encouraging effects on the following events. The regional velocities based on the GPS analysis suggest that the formation of this conjugate structure is mainly due to the westward movement of the subducting Philippine Sea Plate. This earthquake sequence provides a seismotectonic background for subsequent strong earthquakes and helps to better understand the formation mechanisms and seismotectonic implications of conjugate structure rupturing.

The 4 December 2015 Mw 7.1 Normal-Faulting Antarctic Plate Earthquake and Its Seismotectonic Implications

ABSTRACTOceanic plate seismicity is generally dominated by normal and strike-slip faulting associated with active spreading ridges and transform faults. Fossil structural fabrics inherited from spreading ridges also host earthquakes. The Indian Oceanic plate, considered quite active seismically, has hosted earthquakes both on its active and fossil fault systems. The 4 December 2015 Mw 7.1 normal-faulting earthquake, located ∼700 km south of the southeast Indian ridge in the southern Indian Ocean, is a rarity due to its location away from the ridge, lack of association with any mapped faults and its focal depth close to the 800°C isotherm. We present results of teleseismic body-wave inversion that suggest that the earthquake occurred on a north-northwest–south-southeast-striking normal fault at a depth of 34 km. The rupture propagated at 2.7 km/s with compact slip over an area of 48×48 km2 around the hypocenter. Our analysis of the background tectonics suggests that our chosen fault plane is in the same direction as the mapped normal faults on the eastern flanks of the Kerguelen plateau. We propose that these buried normal faults, possibly the relics of the ancient rifting might have been reactivated, leading to the 2015 midplate earthquake.

The Kinematics of the East Anatolian Fault Zone, Eastern Turkey and Seismotectonic Implications

One of most prominent and active faults in Eastern Turkey is the NE-SW oriented left-lateral strike-slip East Anatolian Fault Zone (EAFZ) with a length of approximately 500 km. In this study, we have examined the recent seismicity of EAFZ that was obtained from the records of 34 three-dimensional broad-band earthquake stations established around the fault zone within TURDEP project since 2006. Further the seismicity and fault mechanism solutions of EAFZ, Eastern Turkey have been examined. The new fault mechanism solutions in addition to previously published 220 earthquakes, with a magnitude of ML=2.0 or more were determined by a local moment tensor solution and P-wave first motion data. It was suggested that the recent tectonic deformation of EAFZ south of Türkoğlu was taken up by the left-lateral strike-slip active faults in between Amik and Adana Basins were young trans-tensional stress regime was also active.

Seismotectonic implications of the Berne earthquake swarms west-southwest of Albany, New York

AbstractFive earthquake swarms occurred from 2007 to 2011 near Berne, New York. Each swarm consisted of four to twenty-four earthquakes ranging from M 1.0 to M 3.1. The network determinations of the focal depths ranged from 6 km to 24 km, 77% of which were ≥14 km. High-precision, relative location analysis showed that the events in the 2009 and 2011 swarms delineate NNE-SSW orientations, collinear with NNE trends established by the distribution of the spatially distinct swarms; the events in the 2010 swarm aligned WNW-ESE. Focal mechanisms determined from the largest event in the swarms include one nodal plane that strikes NNE, collinear with the distribution of the swarms and relative events within the swarms.Two, possibly related explanations exist for the Berne earthquake swarms. (1) The swarms were caused by reactivations of proposed blind NE- and NW-striking rift structures associated with the NE-trending Scranton gravity high. These rift structures, of uncertain age (Proterozoic or Neoproterozoic/Iapetan opening), have been modeled at depths appropriate for the seismicity. (2) The NNE-trending swarms were caused by reactivations of NNE-striking faults mapped at the surface north-northeast of the earthquake swarms. Both models involve reactivation of rift-related faults, and the development of the NNE-striking surficial faults in the second model probably was guided by the blind rift faults in the first model. The Berne swarms may be evidence that these faults are seismically capable and, if so, could sustain a maximum event on the order of Mw 5.7–6.6, based on fault segment length.

Lateral variations of the midcrustal seismicity in western Nepal: Seismotectonic implications

The midcrustal seismicity along the Himalayas depicts some lateral variations. This seismicity is often related to stress build up, at depth, along the downdip extent of the fault segments producing large and great earthquakes. To better characterize these variations, we deployed the first temporary seismological network in West Nepal (HiKNet) as a complement to the Nepalese national network. We used automatic picks, manually validated, to perform absolute and relative location of the earthquakes. The resulting catalog contains 2190 local events, shallow to midcrustal (8–16 km). The comparison between the seismicity and the geologic balanced cross-sections shows that most earthquakes occur within the hanging wall of the Main Himalayan Thrust fault nearby ramps or suspected contacts between Lesser Himalayan slivers. Some of the events associated with these geological structures are clustered within weeks-long earthquake swarms with seismic envelopes migrating at 35–50 m/day. These transients are possibly related to fluid migrations or geodetically unresolved slow slip events. The lateral variations of some of the structures associated to this seismicity are likely partially controlling the extent of the coseismic ruptures during intermediate earthquakes that break partly the locked fault zone, in a similar way to what was reported after the Mw7.8 2015 Gorkha-Nepal earthquake. A better characterization of the faults segmentation, including a better location and extent of the seismogenic segments at depth, is an important input for seismic hazard studies.

Estimation of optimum velocity model and precise earthquake locations in NE Aegean: Implications for seismotectonics and seismic hazard

This study relocates the seismicity in NE Aegean during the period 2011–2017 using data recorded by the Hellenic Unified Seismic Network (HUSN) in order to elucidate the relationship between seismicity and active faults in this area. P- and S-phase travel times of well-recorded events were first inverted in order to derive a minimum 1D velocity model with station delays using VELEST. Absolute locations of 4450 events were obtained by use of the nonlinear probabilistic algorithm NLLOC and the newly derived velocity model. Precise relative locations with horizontal and vertical uncertainties that do not exceed 1.2 km were calculated for 3354 events using the double-difference algorithm. The relocated seismicity delineates active faults to the south of Lesvos island, at the tip of Biga peninsula in Turkey and along the parallel strands of strike-slip faults that accommodate the westward motion of Anatolia. The comparison of the seismicity distribution with known active faults and the regional stress field shows that the strike-slip faults represent either principal shear zones, or Riedel shears oriented obliquely to the minimum stress axes. Normal faults are oriented almost perpendicular to the direction of the minimum stress axes in accordance with the transtensional deformation model. The seismogenic layer thickness derived from the depth distribution of relocated seismicity was found to be in the range of 14.8–15.8 km. By combining this thickness with geometrical characteristics of active faults and with a relationship that connects moment magnitude with rupture area, it is possible to estimate expected magnitudes of earthquakes. These magnitudes range from 6.3 to 7.2 depending on the rupture scenario that is considered for each fault. Of particular concern are the faults of Agia Paraskevi in Lesvos and Mastichochoria in Chios island that traverse densely populated areas and can produce large events with magnitudes from 6.4 to 6.9. Very little seismicity can be observed along these faults in the past 7 years, which may indicate either that they are creeping, or that they are locked and accumulate strain energy.

A unified earthquake catalogue for the Sea of Marmara Region, Turkey, based on automatized phase picking and travel-time inversion: Seismotectonic implications

The Marmara section of the North Anatolian Fault Zone (NAFZ) is late in its seismic cycle and can be expected to produce a magnitude M up to 7.4 earthquake during the next decades in direct vicinity to the 15-million population center Istanbul. This setting translates the seismic hazard into very high risk and makes a thorough understanding of the current seismotectonic setting of this NAFZ section a pressing task. The absence of near-fault stations along the most part of the offshore Marmara section limits the reliability of existing seismicity catalogues for this region. For the first time we combine the different regional permanent networks thereby optimizing azimuthal coverage and present a refined hypocenter catalogue for the Sea of Marmara on this basis. Compared to the original locations, adoption of a refined automated technique to determine precise onset times for the different body waves and an iterative travel-time inversion scheme, lead to substantial improvement of 6812 absolute earthquake locations, particularly in the epicentral distribution. The automated processing is explained in detail. Further optimization is achieved through relative relocation of 4407 earthquakes. Our catalogue covers more than a decade (2006–2016) with a regional moment-magnitude of completeness of Mc = 2.1. The epicentral distribution delineates the Marmara Section, i.e. the northern NAFZ branch, as the seismically most active fault strand. We identify several aseismic fault patches that are interpreted to represent locked parts of the fault. Seismic activity in the past decade predominantly occurs off the main fault on the edges of the aseismic patches, supporting previous studies that the Marmara section of the NAFZ contains both locked and creeping fault portions. Single-event focal mechanisms (2.7 ≤ Mw ≤ 4.5) indicate that currently both strike-slip and normal faulting occur, confirming the transtensional setting of the region.

Earthquake swarm of Himachal Pradesh in northwest Himalaya and its seismotectonic implications

On the 27th of August 2016, a seismic swarm activity consisting of 58 earthquakes (1.5 ≤ ML ≤ 4.4), which occurred in Rampur area of the Kullu-Rampur Tectonic window of Himachal Pradesh in Northwest Himalaya. The epicenters of these events are located at the northern front of the Berinag Thrust in its hanging wall. To better understand the seismotectonics of this region, we analyzed the spectral source parameters and source mechanism of this swam activity. Spectral analysis shows the low stress drop values (from 0.05 to 28.9 bars), suggesting that the upper crust has low strength to withstand accumulated strain energy in this region. The Moment Tensor solutions of 12 earthquakes (≥2.7ML) obtained by waveform inversion yield the shallow centroid depths between 5 and 10 km. All these events are of dominantly thrust fault mechanism having an average dip angle of ∼30°. The P-axes and the maximum horizontal compressive stresses are NE-SW oriented; the relative motion of the Indian Plate. The present study reveals that the swarm activity in the Himachal region of NW Himalaya is related to the out-of-sequence thrusting or the Lesser Himalayan Duplex system.