Strike-slip Earthquakes Research Articles

Field observations of surface ruptures accompanying a tsunami and supershear earthquake along a plate boundary strike-slip fault

AbstractStrike-slip earthquakes near major subduction zones have received less attention than thrust or reverse earthquakes in subduction zone areas. The occurrence of the 2018 Palu Mw 7.5 earthquake in eastern Indonesia provides an unprecedented opportunity to investigate the characteristics of one of these events. The Palu earthquake occurred on the left-lateral, north–south-striking Palu–Koro fault, which is the main plate boundary structure accommodating the convergence between blocks in a triple junction area. It excited a significant tsunami, which unusually is associated with strike-slip earthquakes, and also ruptured at a supershear speed, which is mostly observed on strike-slip faults in continents. Based on our fieldwork, we speculate that the normal slip component of the offshore rupture section in Palu bay on the middle segment probably favours tsunami genesis. Our field investigation has revealed evidence of a simple geometry as well as slip partitioning of dip-slip and strike-slip motion on two subparallel strands on the main segment, both of which may have contributed to the supershear of the rupture propagation. Instead of only a transtensive behaviour of the middle segment, our results also illustrate the transpressional property of the northern and southern rupture segments, which shows more complex behaviour than that of a common continental strike-slip fault.

Microwave Brightness Temperature (MBT) Background in Bayan Har Block, Qinghai-Tibet Plateau and Its Importance in Searching for Seismic MBT Anomalies

The abnormal behaviors of microwave brightness temperature (MBT) before and after some strong inland earthquakes have been studied for more than 15 years, but the normal features of MBT background in the investigated regions still lack essential attention. This study focused on the extremely seismically active Bayan Har block on the Qinghai-Tibet Plateau in China, and revealed the spatiotemporal variations of monthly mean background and monthly standard deviation (STD) of MBT by using data of 10.65 and 89 GHz from AMSR-2 instrument. In terms of space, the results revealed that the MBT backgrounds at the two frequencies both basically exhibited a negative correlation with regional altitude but were more pronounce at high frequency. They also showed different response characteristics to the properties of soil and vegetation. In terms of time, the low-frequency background exhibited a complex month-to-month variation, with auxiliary data suggesting a joint contribution of surface soil moisture (SSM) and seasonal temperature; while the high-frequency background presented good agreement only with the variation in surface temperature. Meanwhile, the monthly STD of MBT was discovered being affected by SSM at the low-frequency and by snowfall events at the high-frequency. By employing MBT data of 10.65 GHz from AMSR-E and AMSR-2 sensors, the spatiotemporal evolutions of MBT anomalies before, during and after the Ms 7.1 Yushu earthquake on 13 April 2010 and the Ms 7.4 Maduo earthquake on 21 May 2021 were obtained referring to dynamic monthly mean background. A typical strip-shaped positive MBT anomaly just covering the Bayan Har block was found occurring prior to the two earthquakes, and the time series of average MBT anomaly inside the block was analyzed by using multiple datasets. The typical abnormal MBT strip was discriminated being independent of non-seismic factors and regarded as a possible precursor for both earthquakes. This research uncovered the normal features of MBT background and demonstrated the common characteristics of MBT anomalies preceding two strike-slip earthquakes inside the Bayan Har block. It has instructive significance for studying, understanding and searching for seismic MBT anomalies on Qinghai-Tibet Plateau.

Piezomagnetic Anomalies Associated with the 2021 MW 7.3 Maduo (China) Earthquake

Stress changes due to earthquake rupture can disturb geomagnetic fields significantly. In order to investigate the impact of the 2021 MW 7.3 Maduo earthquake on geomagnetic fields, a piezomagnetic model is constructed based on the coseismic slip to calculate the static coseismic piezomagnetic anomalies (PMs). The PMs are considerable in near-field. However, the PMs are negligible in regions tens of kilometers from the fault rupture. The PMs of our model are consistent with those of other strike-slip earthquakes, indicating that our piezomagnetic model is reasonable. The east component of observed coseismic geomagnetic changes and calculated PMs on a geomagnetic repeat station located about 6 km from fault trace are +4.8 ± 2.2 nanotesla and +4.3 nanotesla, respectively. It seems that the piezomagnetic model can explain the observed data. The PMs are up to 10 nanotesla in the near-field with the initial magnetization of 3 A/m and stress sensitivity of 2 × 10−3 MPa−1. Consequently, considerable coseismic geomagnetic changes that are above error could be observed along the fault, especially at locations with geometrical complexities.

Co-Seismic Ionospheric Disturbances Following the 2016 West Sumatra and 2018 Palu Earthquakes from GPS and GLONASS Measurements

The study of ionospheric disturbances associated with the two large strike-slip earthquakes in Indonesia was investigated, which are West Sumatra on 2 March 2016 (Mw = 7.8), and Palu on 28 September 2018 (Mw = 7.5). The anomalies were observed by measuring co-seismic ionospheric disturbances (CIDs) using the Global Navigation Satellite System (GNSS). The results show positive and negative CIDs polarization changes for the 2016 West Sumatra earthquake, depending on the position of the satellite line-of-sight, while the 2018 Palu earthquake shows negative changes only due to differences in co-seismic vertical crustal displacement. The 2016 West Sumatra earthquake caused uplift and subsidence, while the 2018 Palu earthquake was dominated by subsidence. TEC anomalies occurred about 10 to 15 min after the two earthquakes with amplitude of 2.9 TECU and 0.4 TECU, respectively. The TEC anomaly amplitude was also affected by the magnitude of the earthquake moment. The disturbance signal propagated with a velocity of ~1–1.72 km s−1 for the 2016 West Sumatra earthquake and ~0.97–1.08 km s−1 for the 2018 Palu mainshock earthquake, which are consistent with acoustic waves. The wave also caused an oscillation signal of ∼4 mHz, and their azimuthal asymmetry of propagation confirmed the phenomena in the Southern Hemisphere. The CID signal could be identified at a distance of around 400–1500 km from the epicenter in the southwestern direction.

Insights on the small tsunami from January 28, 2020, Caribbean Sea MW7.7 earthquake by numerical simulation and spectral analysis

A major left-lateral strike-slip Mw7.7 earthquake occurred in the vicinity of the Caribbean Sea on January 28, 2020. As a result, a small-scale tsunami was generated. The properties of the seismogenic source were described using observational data gathered for the earthquake and tsunami, as well as information on the regional tectonic setting. The tsunami was simulated with the COMCOT model and Okada’s dislocation model from finite fault solutions for MW7.7 Caribbean Sea earthquakes published by the United States Geological Survey. The simulation results were compared to tide gauge records to validate whether the seafloor’s vertical displacement generated by the strike-slip fault caused a small-scale tsunami. We conducted a spectral analysis of the tsunami to better understand the characteristics of tsunami records. The tsunami simulation results showed that the co-seismic vertical displacement caused by a strike-slip MW7.7 earthquake could have contributed to the small-scale tsunami, but the anomalously large high-frequency tsunami waves recorded by the George Town tide gauge 11 min after the earthquake were unrelated to the earthquake-generated tsunami. According to the spectrum analysis, the predominant period of noticeable high-frequency tsunami waves recorded by the George Town tide gauge occurred only two minutes after the earthquake. This indicates that the source of the small-scale tsunami was close to the George Town station and the possible tsunami source was 150 km away from George Town station. These facts suggest that a submarine landslide was caused by the strike-slip earthquake. The comprehensive analysis showed that the small-scale tsunami was not caused solely by co-seismic seafloor deformation from the strike-slip event but that an earthquake-triggered submarine landslide was the primary cause. Therefore, the combined impact of two sources led to the small-scale tsunami.

Can Oceanic Core Complexed Faults Be Reactivated by Strike-Slip Earthquakes? Rupture and Coulomb Stress Changes from the 2020 Mw 6.6 Vernadsky Earthquake, Equatorial Atlantic

Can Oceanic Core Complexed Faults Be Reactivated by Strike-Slip Earthquakes? Rupture and Coulomb Stress Changes from the 2020 Mw 6.6 Vernadsky Earthquake, Equatorial Atlantic

EFFECTS OF SHEAR WALLS ON A TYPICAL FOUR-STORY REINFORCED CONCRETE STRUCTURE SUBJECTED TO SEVERE EARTHQUAKE EVENTS

Various seismic-resistant design methods are used to ensure the stability of multi-story buildings against lateral forces caused by earthquakes. Utilization of reinforced concrete shear walls is one of the most reliable methods of design and construction of earthquake-resistant buildings because it increases structural resistance to lateral loads and stiffens and strengthens the structure, thereby minimizing earthquake-induced damages. This paper investigates the beneficial effects of using shear walls in the structural design of a typical low-rise building to improve its resistance to earthquake events. To this end, a four-story reinforced concrete structure is modeled first without shear walls, then with the addition to shear walls. The 2002 Denali Alaska earthquake is used as an example of a severe seismic excitation because it is considered the most massive strike-slip earthquake in North America in almost 150 year. SAP2000 is used to perform the dynamic analysis. In order to obtain an accurate representation of the structure’s behavior, response modal nonlinear time-history dynamic analysis is utilized to analyze and compare the response of the building with and without shear walls. Study results showed that shear walls are very effective in achieving compliance with seismic design codes. In addition, the use of shear walls significantly reduces the shear stresses, bending moments, and displacements of the various members of the structure.

Absence of Near-Trench Early Triggering during the 2012 Mw 7.2 Indian Ocean Strike-Slip Earthquake: Evidence from One-Day Aftershocks

AbstractDynamic earthquake triggering is a widely accepted mechanism of earthquake interaction, which plays a vital role in seismic hazard estimation, although its efficacy at regional distances is under debate. The 2012 Mw 7.2 Indian Ocean event is one of the first reported events to produce dynamic stress triggering at regional distances using backprojection (BP) techniques. Alternatively, the coherent radiators in BP images can be interpreted as localized water reverberation phases. We present further evidence against near-trench triggering during this event. We collected 24 hr seismic recordings of two nearby stations located near the trench. We adopted a waveform denoising algorithm and detected 125 aftershocks using two regional seismic stations with a minimum magnitude of ML∼2.7 and completeness magnitude of ML∼3.6, whereas none of these aftershocks occurred near the trench. The absence of immediate (within one day) aftershocks near the trench suggest the absence of dynamic triggering during the offshore mainshock.

A Stochastic Model for Simulating Vertical Pulseless Near-Fault Seismic Ground Motions

ABSTRACTThe vertical near-fault seismic ground-motion component can cause significant structural deformation and damage, which can be evaluated from time history analysis using actual or synthetic ground-motion records. In this study, we propose a new stochastic model for the vertical pulseless near-fault ground motions that depends on earthquake magnitude, rupture distance, and site condition. The proposed model is developed based on the time–frequency characteristics of 606 selected actual vertical record components in strike-slip earthquakes. The use and validation of the model are presented using simulated records obtained by two simulation techniques. For the validation, the statistics of time–frequency-dependent power spectral acceleration estimated from the simulated records using the proposed stochastic model are compared with those from the actual records and the ground-motion models available in the literature.

Forensic investigation of flowslides triggered by the 2018 Sulawesi earthquake

The Sulawesi earthquake with a moment magnitude of Mw 7.5 struck the Central Sulawesi region of the Sulawesi Island, Indonesia, on September 28, 2018. The epicenter of the earthquake was located in the mountainous region of Donggala Regency, in the neck of the Minahasa Peninsula in the Central Sulawesi Province of Indonesia. Although the epicenter was located in Donggala Regency, the greatest devastating effects were observed about 70 km south of the epicenter in the Palu Valley. The event was the first of its kind to cause large-scale flowslides simultaneously at four key locations such as Balaroa, Petobo, Jono Oge, and Sibalaya with extensive ground displacements ranging from several hundred meters to more than 1 km. This article reviews the field observations of geotechnical failures and infrastructure damage caused by liquefaction resulting from the shallow strike-slip earthquake at Palu City, Donggala Regency, and Sigi Regency. A geo-spatial analysis was performed on data collected from aerial drone imagery, along with portable dynamic cone penetration testing (PDCPT) in the field. The investigation revealed a highly stratified ground with alternating soil layers of varying permeability and very low bearing resistance at shallow depths. The investigation also helped in assessing the extent of damage caused by geotechnical failure to the residential infrastructures, irrigation structures, and roads.Graphical

The 2019–2020 Southwest Puerto Rico Earthquake Sequence: Seismicity and Faulting

Abstract The 2019–2020 Southwest Puerto Rico earthquake sequence ruptured multiple faults with several moderate magnitude earthquakes. Here, we investigate the seismotectonics of this fault system using high-precision hypocenter relocation and inversion of the near-field strong motions of the five largest events in the sequence (5.6≤Mw≤6.4) for kinematic rupture models. The Mw 6.4 mainshock occurred on a northeast-striking, southeast-dipping normal fault. The rupture nucleated offshore ∼15 km southeast of Indios at the depth of 8.6 km and extended southwest–northeast and up-dip with an average speed of 1.55 km/s, reaching the seafloor and shoreline after about 8 s. The 6 January 2020 (10:32:23) Mw 5.7 and the 7 January 2020 (11:18:46) Mw 5.8 events occurred on two east–southeast-striking, near-vertical, left-lateral strike-slip faults. However, the 7 January 2020 (08:34:05) Mw 5.6 normal-faulting aftershock, which occurred only 10 min after the Mw 6.4 normal-faulting mainshock, ruptured on a fault with almost the same strike as the mainshock but situated ∼8 km farther east, forming a set of parallel faults in the fault system. On 11 January 2020, an Mw 6.0 earthquake occurred on a north–northeast-striking, westing-dipping fault, orthogonal to the faults hosting the strike-slip earthquakes. We apply template matching for the detection of missed, small-magnitude earthquakes to study the spatial evolution of the main part of the sequence. Using the template-matching results along with Global Positioning System analysis, we image the temporal evolution of a foreshock sequence (Caja swarm). We propose that the swarm and the main sequence were a response to a tectonic transient that most affected the whole Puerto Rico Island.

Fault Geometry and Slip Distribution of the 2021 Mw 7.4 Maduo, China, Earthquake Inferred from InSAR Measurements and Relocated Aftershocks

Abstract A nearly 70 yr hiatus of major seismic activity in the central eastern Bayan Har block (BKB) ended on 22 May 2021, when a multislip-peak sinistral strike-slip earthquake struck western Maduo County, Qinghai. This earthquake, which ruptured the nearly 170 km long Kunlun Pass–Jiangcuo fault, is a rather unique event and offers a rare opportunity to probe the mechanical properties of the intraplate lithosphere of the central eastern BKB. Here, we inferred the fault geometry associated with the Maduo earthquake using Interferometric Synthetic Aperture Radar (InSAR), and relocated aftershocks and inverted the slip distribution through InSAR radar phases and range offsets. Our analysis revealed that the geometry of the fault varies along the strike: the southeastern end of the fault dips steeply to the northeast, whereas the northwestern end dips southwestward. Using the combined datasets to constrain a coseismic slip, we found that the 2021 Maduo event was dominated by sinistral strike-slip movement, with a slight normal-slip component at a shallow depth, rupturing the steep-dipping fault for nearly 170 km in length. Five asperities were detected along the fault strike in the shallow crust (0–12 km) with a peak slip of ∼4.2 m corresponding mostly to simple structures, namely, continuous and straight rupture segments, suggesting that the rupture propagated across geometrical barriers in a multiasperity way. Based on an analysis of the strain field and the focal mechanisms of both the 2021 Maduo earthquake and historical earthquakes that have occurred in the BKB, we propose that the fault zones within the BKB can also generate large earthquakes and have the ability to accommodate the ongoing eastward and northeastward penetration of the Indian plate into the Eurasian plate.

Improved Scaling Relationships for Seismic Moment and Average Slip of Strike-Slip Earthquakes Incorporating Fault-Slip Rate, Fault Width, and Stress Drop

ABSTRACT We develop a self-consistent scaling model relating magnitude Mw to surface rupture length (LE), surface displacement DE, and rupture width WE, for strike-slip faults. Knowledge of the long-term fault-slip rate SF improves magnitude estimates. Data are collected for 55 ground-rupturing strike-slip earthquakes that have geological estimates of LE, DE, and SF, and geophysical estimates of WE. We begin with the model of Anderson et al. (2017), which uses a closed form equation for the seismic moment of a surface-rupturing strike-slip fault of arbitrary aspect ratio and given stress drop, ΔτC. Using WE estimates does not improve Mw estimates. However, measurements of DE plus the relationship between ΔτC and surface slip provide an alternate approach to study WE. A grid of plausible stress drop and width pairs were used to predict displacement and earthquake magnitude. A likelihood function was computed from within the uncertainty ranges of the corresponding observed Mw and DE values. After maximizing likelihoods over earthquakes in length bins, we found the most likely values of WE for constant stress drop; these depend on the rupture length. The best-fitting model has the surprising form WE∝logLE—a gentle increase in width with rupture length. Residuals from this model are convincingly correlated to the fault-slip rate and also show a weak correlation with the crustal thickness. The resulting model thus supports a constant stress drop for ruptures of all lengths, consistent with teleseismic observation. The approach can be extended to test other observable factors that might improve the predictability of magnitude from a mapped fault for seismic hazard analyses.

Within- and Between-Event Variabilities of Strong-Velocity Pulses of Moderate Earthquakes within Dense Seismic Arrays

ABSTRACT Ground motion with strong-velocity pulses can cause significant damage to buildings and structures at certain periods; hence, knowing the period and velocity amplitude of such pulses is critical for earthquake structural engineering. However, the physical factors relating the scaling of pulse periods with magnitude are poorly understood. In this study, we investigate moderate but damaging earthquakes (Mw 6–7) and characterize ground-motion pulses using the method of Shahi and Baker (2014) while considering the potential static-offset effects. We confirm that the within-event variability of the pulses is large. The identified pulses in this study are mostly from strike-slip-like earthquakes. We further perform simulations using the frequency–wavenumber algorithm to investigate the causes of the variability of the pulse periods within and between events for moderate strike-slip earthquakes. We test the effect of fault dips, and the impact of the asperity locations and sizes. The simulations reveal that the asperity properties have a high impact on the pulse periods and amplitudes at nearby stations. Our results emphasize the importance of asperity characteristics, in addition to earthquake magnitudes for the occurrence and properties of pulses produced by the forward directivity effect. We finally quantify and discuss within- and between-event variabilities of pulse properties at short distances.

Multicycle Simulation of Strike-Slip Earthquake Rupture for Use in Near-Source Ground-Motion Simulations

ABSTRACTRealistic dynamic rupture modeling validated by observed earthquakes is necessary for estimating parameters that are poorly resolved by seismic source inversion, such as stress drop, rupture velocity, and slip rate function. Source inversions using forward dynamic modeling are increasingly used to obtain earthquake rupture models. In this study, to generate a large number of physically self-consistent rupture models, rupture process of which is consistent with the spatiotemporal heterogeneity of stress produced by previous earthquakes on the same fault, we use multicycle simulations under the rate and state (RS) friction law.We adopt a one-way coupling from multicycle simulations to dynamic rupture simulations; the quasidynamic solver QDYN is used to nucleate the seismic events and the spectral element dynamic solver SPECFEM3D to resolve their rupture process. To simulate realistic seismicity, with a wide range of magnitudes and irregular recurrence, several realizations of 2D-correlated heterogeneous random distributions of characteristic weakening distance (Dc) in RS friction are tested. Other important parameters are the normal stress, which controls the stress drop and rupture velocity during an earthquake, and the maximum value of Dc, which controls rupture velocity but not stress drop. We perform a parametric study on a vertical planar fault and generate a set of a hundred spontaneous rupture models in a wide magnitude range (Mw 5.5–7.4).We validate the rupture models by comparison of source scaling, ground motion (GM), and surface slip properties to observations. We compare the source-scaling relations between rupture area, average slip, and seismic moment of the modeled events with empirical ones derived from source inversions. Near-fault GMs are computed from the source models. Their peak ground velocities and peak ground accelerations agree well with the ground-motion prediction equation values. We also obtain good agreement of the surface fault displacements with observed values.

Inferring Critical Slip-Weakening Distance from Near-Fault Accelerogram of the 2014 Mw 6.2 Ludian Earthquake

Abstract To better assess potential earthquake hazards requires a better understanding of fault friction and rupture dynamics. Critical slip-weakening distance (Dc) as one of the key friction parameters, however, is hard to determine on natural faults. For strike-slip earthquakes, we may directly estimate the Dc from Dc″—the double near-fault ground displacement at the time of the peak velocity (Fukuyama and Mikumo, 2007). Yet near-fault observations are very few, and, thus, there were only limited earthquakes with such Dc″ estimation. In 2014, an Mw 6.2 strike-slip event—the Ludian earthquake—occurred in southwest China. The strong-motion station (LLT) that is ∼0.45 km from the fault recorded the earthquake and enabled us to estimate Dc″ from the accelerograms. We inspect the polarity of the accelerometers and compare the integrated velocities with waveforms of nearby broadband stations. We also analyze the particle motion at the LLT station and retrieve the earthquake initiation at the intersection of the conjugated faults. We then apply the baseline correction to the seismograms, recover the ground velocities and displacements, and obtain the value of Dc″=0.1 m at the station. The recovered final displacements are compared with the predicted ground displacements of a finite-fault model. The discrepancy of fault-parallel displacements might imply limited underestimation of Dc″, and the estimated upper limit is 0.3 m. Comparison between the Dc″ and final slip on the fault patch follows the scaling of previous larger earthquakes. Analysis of the near-fault accelerometer data enhances our understanding on the earthquake source of the Ludian earthquake. This case extends the lower magnitude boundary of the Dc″ values obtained from natural faults and opens a window into the friction property in the seismically active region.

Consideration of submarine landslide induced by 2018 Sulawesi earthquake and tsunami within Palu Bay

ABSTRACT On September 28 2018, an Mw 7.5 strike-slip earthquake occurred to the north of Palu Bay on the Sulawesi Island, Indonesia. This triggered a destructive tsunami within the bay, which reached Palu city. Simulation have been conducted to investigate the landslide source. However, the tsunami should be investigated considering a physical model in the area where detailed bathymetric survey had not been conducted. In this study, we investigated the impact of coastal landslides on the southern part of the bay using a two-layer model. Owing to the increasing collapse volume, the southern west coastal landslide could approximately explain the observations in Palu city. However, the calculated mass volume of the source largely overestimated the bathymetric survey data. Hence, we considered the possibility of submarine landslide in the southern part of the bay and the simulation results could approximately explain the maximum tsunami heights in the southern part of the bay. These results suggest that more detailed multibeam data will be required to investigate the possible submarine landslide in the southern area which could induce a destructive tsunami reaching Palu city within a few minutes after the collapse.

Characteristics of Seismicity in the Eagle Ford Shale Play, Southern Texas, Constrained by Earthquake Relocation and Centroid Moment Tensor Inversion

Abstract Analysis of earthquake locations and centroid moment tensors (CMTs) is critical in assessing seismogenic structures and connecting earthquakes to anthropogenic activities. The objective of this study was to gain insights into the seismotectonics of the Eagle Ford Shale play (EF), southern Texas, through relative relocation of earthquakes, assessment of CMT solutions, and investigation of the background stress field. Using Texas Seismological Network (TexNet) data from 2017 through 2019, we were able to relocate 326 earthquakes and obtain CMT solutions for 37 ML≥2.0 earthquakes. These earthquakes are located in the sedimentary basin and uppermost crust, with depths ranging from 2 to 10 km. The earthquake groups in the northeastern EF are linearly distributed along the Karnes fault zone, whereas the southern and western groups are spatially scattered around mapped or unmapped faults. CMT solutions identified 32 normal fault earthquakes and five strike-slip earthquakes. The orientation of the fault plane of most normal fault earthquakes is southwest–northeast, whereas the possible fault plane of the strike-slip fault is from north-northwest to south-southeast, which is roughly perpendicular to the normal faults. Normal and strike-slip faults in the EF are of high dip angles, with the dip angles of the most faults ranging from 60° to 80°. Stress inversion results show that the major orientation of maximum horizontal stress (SHmax) is southwest–northeast, with minor local stress-field rotations. We further estimated earthquake energy release in the EF region using moment magnitude from the CMT solutions, and the cumulative earthquake energy release curve reveals three notable increases in cumulative seismic moment, which occurred in January–July 2018 and January–March 2019, and May–August 2019. Whether these energy releases were caused by anthropogenic activities is a matter for further investigation.

Evidence for reactivation of new faults and seismicity migration away from the causative fault of the 2001 MW 7.7 Bhuj earthquake, western India

SUMMARY The Kachchh Rift Basin of western India that hosted the devastating 2001 MW 7.7 Bhuj earthquake has been witnessing minor to moderate seismicity since then. The genesis of these intraplate earthquakes and geometry of the causative faults is still elusive. In this study, we relocated all the earthquakes recorded between 2006 and 2018 and utilized a set of 4285 best located earthquakes to provide rare insights into the orientation and depth distribution of the currently active faults. We also analysed previously computed source mechanisms, in perspective of our location results. It is revealed that the present seismic activity is primarily aligned along two planes—one steeply dipping (∼60°) to the northeast with a NW–SE strike and the other gently (∼34°) dipping to the SSW, striking WNW–ESE. The traces of these fault surfaces coincide with the Kachchh Mainland fault (KMF) and the North Wagad Fault (NWF) when extrapolated to the surface, respectively. Activity along the NWF has been shown in earlier studies, however, clear evidence of activity along the steep north-dipping KMF is presented for the very first time. Thrust earthquakes dominate the NWF while strike-slip earthquakes are seen across the KMF. Our results show that the two fault surfaces converge between 70.30°E and 70.43°E longitudes in the depth range 22–32 km, however, there is large E–W offset between the northern and southern extremities of the fault-system. This convergence zone hosted the largest earthquakes (ML 4.7–5.1) between 2006 and 2018 and the hypocentre of 2001 main shock also coincide with it. The earthquakes occurring in the interfault region show major strike-slip motion and are probably influenced by the relative motion between the NWF and KMF. Consistent seismicity, assisted probably by high conducting material, is seen across the NWF while intermittent seismicity is revealed along the KMF. Scrutiny of earlier studies revealed that the 2001 main shock and first few weeks of aftershocks were hosted on another south-dipping fault, the South Wagad Fault (SWF), however, activity along the SWF is found to be meagre during 2006–2018. Instead, seismicity has currently migrated to faults north and south of the SWF. These currently active faults most likely were reactivated during/after the 2001 main shock.

Anatomy of strike-slip fault tsunami genesis

Tsunami generation from earthquake-induced seafloor deformations has long been recognized as a major hazard to coastal areas. Strike-slip faulting has generally been considered insufficient for triggering large tsunamis, except through the generation of submarine landslides. Herein, we demonstrate that ground motions due to strike-slip earthquakes can contribute to the generation of large tsunamis (>1 m), under rather generic conditions. To this end, we developed a computational framework that integrates models for earthquake rupture dynamics with models of tsunami generation and propagation. The three-dimensional time-dependent vertical and horizontal ground motions from spontaneous dynamic rupture models are used to drive boundary motions in the tsunami model. Our results suggest that supershear ruptures propagating along strike-slip faults, traversing narrow and shallow bays, are prime candidates for tsunami generation. We show that dynamic focusing and the large horizontal displacements, characteristic of strike-slip earthquakes on long faults, are critical drivers for the tsunami hazard. These findings point to intrinsic mechanisms for sizable tsunami generation by strike-slip faulting, which do not require complex seismic sources, landslides, or complicated bathymetry. Furthermore, our model identifies three distinct phases in the tsunamic motion, an instantaneous dynamic phase, a lagging coseismic phase, and a postseismic phase, each of which may affect coastal areas differently. We conclude that near-source tsunami hazards and risk from strike-slip faulting need to be re-evaluated.