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The June 21, 2022 strong earthquake Mw 6.2 occurred nearly 165 km southeast of Kabul city, Afghanistan, and caused more than 1000 casualties and huge property losses. An untimely occurrence of the shallow focus event (~ 10 km, USGS report) at mid-night when people were asleep and the poor constructed houses caused so many casualties. It is one of the most devastating earthquake in Afghanistan in the recent years. The GCMT solution shows a strike-slip faulting at a centroid depth ~ 15 km; the NNE-SSW nodal plane is comparable with the trend of Chaman-Gardez fault system, though the eipcenter is ~ 50 km away from the surface fault traces. Afghanistan is under tectonic stresses from the Hindu-Kush collision zone to the north, Makran subduction zone to the south and from the transpressional zones to the east and west. Stress inversion study shows a NNW-SSE compressional stress and NNE-SSW extensional stress in the study area, the central intra-plate zone of Afghanistan. A seismic cross section of the main shock, aftershocks and the past events shows shallow near vertical source zones. The aftershock trend and the maximum PGA trend are parallel to the Chaman-Gardez fault system. Coulomb stress-change images after the main shock show that the aftershocks occurred in the increased stress zone of the rupture area.

The first stage of seismic impact assessments for engineering calculations of structural earthquake resistance is seismic hazard assessment. For this, seismological and seismotectonic studies identify seismogenic structures, then determine the maximum credible magnitudes, recurrence periods, faulting types, distances between selected faults and constructions site, and ground conditions at said sites. After which, it is necessary to evaluate changes in the acceleration amplitudes with distance in order to reveal the regularities in the attenuation of peak ground accelerations. The article discusses various methods for estimating the regularities of acceleration attenuation with distance. It is shown that empirical estimates of the mean amplitudes for various distances have the highest accuracy. No a priori expressions are applied to describe the amplitude attenuation curves. At the Schmidt Institute of Physics of the Earth (Moscow), using global data, it is shown that in the engineering range of accelerations, there are three zones distinguished by attenuation and dependences on the focal mechanism and soil conditions. Each zone has its own attenuation equation. For each equation, the corresponding standard deviation is estimated. The empirical method for estimating attenuation has been applied to the Garhwal region of the Himalayan seismic belt. Records of strong movements acquired during strong earthquakes in this region are used: Dharamsala (April 26, 1986, MW = 5.5), Uttarkashi (October 20, 1991, MW = 6.8), and Chamoli (March 28, 1999, MW = 6.5). An attenuation curve has been plotted for the Garhwal region. The empirical data are approximated by the estimated attenuation equations with a standard deviation of about 50% versus 100% by the semiempirical method. The question of regional attenuation features in this Himalayan region is considered. The results are useful in assessing seismic hazard/seismic risk in the Himalayas.

The northwestern Deccan Volcanic Province (NWDVP) in India has undergone major tectonic changes during the Eocene–Paleocene, when voluminous eruptions took place due to interaction of the Indian plate with the Reunion plume. The intense seismic activity makes this region as the most vulnerable intraplate earthquake zones, world over. In this study, we utilize arrival times of 40,499 P and 39,581 S phases from waveforms of 5653 events registered at 93 seismic stations to obtain high-resolution tomographic images of the crust and uppermost mantle beneath the NWDVP. The images shed light on the relation between seismicity and seismic structure in the source regions of two large earthquakes (Mw ≥ 7.7) at depths >20 km in the rift basins, shallower moderate (Mw ∼5) quakes and swarm induced activity in the horst region. Higher Vp/Vs ratios are observed for the seismogenic rock matrix with entrapped magma fluid that played an important role in triggering large and moderate tectonic earthquakes. High Vp/Vs ratios are also observed in the uppermost crust, indicating that fluid in sediments/fractures triggers the swarm and induced seismicity. The source zones of the shallow, mining induced seismicity are imaged as low Vp, Vs zones. The vertical and horizontal intrusions of the Deccan volcanic magma in the uppermost mantle and lower crust are also well imaged as high Vp and Vs anomalies. Variations in the depth to the Moho and lower crustal velocities signify alterations due to tectonic processes during different time scales.

Agartala is one of the fastest developing cities in Northeast Region (NER) of India and is also the capital city of Tripura state. The whole NER is in zone V in the seismic zoning map of India, one of the most seismic-prone regions in the world. The region is buttressed between the Himalayan collision zone to the north and Indo-Burma subduction zone to the east, and has experienced two (1897 and 1950) great earthquakes (Mw > 8.0) and several large earthquakes (Mw ≥ 7.0) since 1897. The Agartala area lies in an intraplate zone and most recently experienced a well-felt shallow (depth 30 km) earthquake of Mw 5.7 on January 3, 2017 that occurred at a distance ~75 km northeast of the city. Some evidence of liquefaction was identified along the Manu River in Kanchanbari village. In that context, this study is attempted to evaluate the liquefaction potential of the Agartala area. Dynamic properties of soil are determined using data of some 97 standard penetration test (SPT) boreholes. The cyclic shear stress of the soil layers is estimated considering a peak surface ground acceleration of 0.36 g. It is observed that according to the liquefaction potential index (LPI) scale, the central part of the city shows high to moderate, the northern part moderate to non-liquefiable and the southern part low to non-liquefiable potential. The results are presented in maps on a geographical information system (GIS) platform using the QGIS software. The liquefaction potential maps are very useful for professional engineers, government agencies and disaster management authorities for future development and planning of the city against liquefaction failure.

We present crustal anisotropy estimates constrained by shear wave splitting (SWS) analysis using local earthquakes in the Shillong plateau and Assam valley area, North East India (NE India) region. Splitting parameters are determined using an automated cross-correlation (CC) method. We located 330 earthquakes recorded by 17 broadband seismic stations during 2001–2014 in the study area. Out of these 330 events, seismograms of 163 events are selected for the SWS analysis. Relatively small average delay times (0.039–0.084 s) indicate existence of moderate crack density in the crust below the study area. It is found that fast polarization directions vary from station to station depending on the regional stress system as well as geological conditions. The spatial pattern of crustal anisotropy in the area is controlled mostly by tectonic movement of the Indian plate towards NE. Presence of several E-W and N-S trending active faults in the area also play an important role on the observed pattern of crustal anisotropy.

We have evolved 3–D seismic velocity structures in northeast India region and its adjoining areas to understand the geodynamic processes of Indian lithosphere that gently underthrusts under the Himalayas and steeply subducts below the Indo–Burma Ranges. The region is tectonically buttressed between the Himalayan arc to the north and the Indo–Burmese arc to the east. The tomographic image shows heterogeneous structure of lithosphere depicting different tectonic blocks. Though our results are limited to shallower depth (0–90 km), it matches well with the deeper continuation of lithospheric structure obtained in an earlier study [Koulakov, 2011]. We observe low velocity structure all along the Eastern Himalayas down to ~ 70 km depth, which may be attributed to deeper roots/thicker crust developed by underthrusting of Indian plate. Parallel to this low velocity zone lies a high velocity zone in foredeep region, represents the Indian lithosphere. The underthrusting Indian lithosphere under the Himalayas as well as below the Indo–Burma Ranges is well reflected as a high velocity dipping structure. The buckled up part of bending Indian plate in study region, the Shillong Plateau–Mikir Hills tectonic block, is marked as a high velocity structure at shallower depth. The Eastern Himalayan Syntaxis, tectonic block where the two arcs meet is identified as a high velocity structure. The Bengal Basin, tectonic block to the south of Shillong Plateau shows low velocity due to its thicker sediments. Based on the tomographic image, a schematic model is presented to elucidate the structure and geodynamics of Indian lithosphere in study region.

Tectonic stress regime in the Shillong plateau, northeast region of India, is examined by stress tensor inversion. Some 97 reliable fault plane solutions are used for stress inversion by the Michael and Gauss methods. Although an overall NNW-SSE compressional stress is observed in the area, the stress regime varies from western part to eastern part of the plateau. The eastern part of the plateau is dominated by NNE-SSW compression and the western part by NNW-SSE compression. The NNW-SSE compression in the western part may be due to the tectonic loading induced by the Himalayan orogeny in the north, and the NNE-SSW compression in the eastern part may be attributed to the influence of oblique convergence of the Indian plate beneath the Indo-Burma ranges. Further, Gravitational Potential Energy (GPE) derived stress also indicates a variation from west to east.

Globally, one of the largest intraplate earthquakes of M w 7.7 occurred on 26 January 2001 in the Kachchh rift basin (KRB), western India. The continuing long aftershock sequence over decades has generated much debate on the seismogenic fault(s). We have analyzed more than 10,000 aftershocks ( M w >1.0) recorded by a 50‐station broadband network in the region during 2006–2014. A total of 834 aftershocks ( M w >2.4), each recorded by at least eight broadband seismic stations with a minimum of eight P and six S phases, are relocated in this study by double‐difference tomography (tomoDD) method. The relocated aftershocks and velocity images reveal a near‐vertical or steeply south‐dipping deeper structure as the source zone of the mainshock and aftershocks; the structure correlates well with the geologically mapped South Wagad fault (SWF). Among the other geologically known faults, the Kachchh Mainland fault (KMF) and the Gedi fault (GF) are also well identified in the seismic sections. Further, fault‐plane solutions of 109 aftershocks having M w ≥3.5 corroborate well with the known faults. The geological model and seismological observations suggest that the SWF overstepped the KMF and intersects it at depth. The intersecting fault zone is the source area for the deeper (10–35 km) reverse faulting earthquakes in the KRB. At the fault end of the SWF, a heterogeneous velocity structure is imaged, which is attributed to a fluid‐filled rock matrix that triggered the mainshock. On the other hand, the GF is reported to be a later‐activated fault to the north of SWF; it generated some shallower aftershocks (0–20 km) mostly by strike‐slip mechanisms.

The attenuation of seismic wave energy in and around the source area of the Andaman earthquake of 13 September 2002 is estimated using high-quality aftershock data. Thirteen aftershocks, duration magnitude (M d) ranging between 2.9 and 4.1, depth

A partial support has been given by Italy INGV-DPC (Istituto Nazionale di Geofisica e Vulcanologia and Dipartimento di Protezione Civile) Projects UNREST and SPEED, and by Italy’s Ministry of Education PRIN project (Seismic Hazard in Central Apennines, UR Del Pezzo).

Seismic source characteristics in the Kachchh rift basin and Saurashtra horst tectonic blocks in the stable continental region (SCR) of western peninsular India are studied using the earthquake catalog data for the period 2006–2011 recorded by a 52-station broadband seismic network known as Gujarat State Network (GSNet) running by Institute of Seismological Research (ISR), Gujarat. These data are mainly the aftershock sequences of three mainshocks, the 2001 Bhuj earthquake (M w 7.7) in the Kachchh rift basin, and the 2007 and 2011 Talala earthquakes (M w ≥ 5.0) in the Saurashtra horst. Two important seismological parameters, the frequency–magnitude relation (b-value) and the fractal correlation dimension (D c) of the hypocenters, are estimated. The b-value and the D c maps indicate a difference in seismic characteristics of these two tectonic regions. The average b-value in Kachchh region is 1.2 ± 0.05 and that in the Saurashtra region 0.7 ± 0.04. The average D c in Kachchh is 2.64 ± 0.01 and in Saurashtra 2.46 ± 0.01. The hypocenters in Kachchh rift basin cluster at a depth range 20–35 km and that in Saurashtra at 5–10 km. The b-value and D c cross sections image the seismogenic structures that shed new light on seismotectonics of these two tectonic regions. The mainshock sources at depth are identified as lower b-value or stressed zones at the fault end. Crustal heterogeneities are well reflected in the maps as well as in the cross sections. We also find a positive correlation between b- and D c-values in both the tectonic regions.

Strong ground motion parameters for the Guwahati city area, the capital city of the state of Assam in northeast India, are examined with the help of data accrued from local as well as worldwide network. Empirical relations are proposed for the ground motion parameters as a function of earthquake magnitude, distance, fault type, source depth and velocity characteristics of medium. Seismotectonics of the study region is examined, and a maximum credible earthquake M S ~ 8.0 is presumed from the Brahmaputra fault, the nearest source zone in the city area. Such great/major event may cause intensity of the of 9.3 with a probability of 0,95 in the Guwahati city during time interval of 500 years. Further, the design spectrum with 67 % confidence level and the synthetic three-component accelerograms are constructed. These results are much relevant and useful for structural engineering to mitigate seismic hazards in the region.

An attempt is made to map the spatial variation of the tectonic stress pattern in northeast India and its adjoining south Asia region using stress tensor inversion of some 516 fault‐plane solutions. The Bhutan Himalaya and the Arunachal Himalaya are mapped with north–south to north‐northwest–south‐southeast compression. The eastern Himalaya syntaxis zone, on the other hand, shows a clockwise rotation; a north‐northeast compression is dominant. To the south, in the intraplate part of the region, the Shillong plateau, Assam valley, Bengal basin (Bangladesh), and Tripura fold belt exhibit north‐northwest to north‐northeast compression. Orthogonal horizontal extension is dominant in southern Tibet, Bhutan, and partly in the syntaxis zone, and the same is also observed in the Shillong plateau and Assam valley area of the intraplate region. The Indo–Burma ranges and the Sagaing fault in the Myanmar region show a northeast–southwest compression; an orthogonal horizontal northwest–southeast extension is also observed in the Sagaing fault zone. A depth variation of the tectonic stress is observed below the Indo–Burma ranges; it changes from north–south to northeast–southwest in the southern part, and from northeast–southwest to north‐northeast–south‐southwest in the northern part in the deeper seismogenic zone. The stress inversion results of clusters of events in individual zones, though mostly conformable with the average observations, indicate a variation in the Shillong plateau due to heterogeneity and tectonic complexity.

An attempt has been made to examine the empirical relationship between moment magnitude (M W) and local magnitude (M L) of earthquakes recorded in the Shillong-Mikir Plateau of Northeastern India. Moment tensor solutions of 106 earthquakes recorded during the period 1976–2009 are used. The focal mechanism solutions of these earthquakes include 1 Harvard-CMT solution (M W ≥ 4.0), 54 solutions from different publications and 51 solutions obtained for the local earthquakes (2.0 ≤ M L ≤ 5.0) recorded by a 20-station permanent broadband network during 2001–2009 in the region. The moment tensor solutions of these local earthquakes are obtained by the discrete wave number method. The M W –M L relationship in the region is determined by generalized orthogonal regression analysis, which is found to be M W = M L (1.00 ± 0.02) + (0.02 ± −0.05). It is observed that, on average, M W is equivalent to M L with an uncertainty of about (0.02 ± −0.05) magnitude units for earthquakes of the Shillong-Mikir Plateau. Conversio...

The aftershock sequence of the March 1999 Chamoli earthquake (M s 6.6) in the western Himalaya is analysed to examine the seismic characteristics of the active fault. About 350 aftershocks recorded by about 40 seismic stations are used to map the b-value and fractal correlation dimension (D c) in the earthquake source area. The maximum likelihood method is used to estimate the b-value, and the correlation integral method for the fractal correlation dimension. A comparatively higher b-value (0.7) is mapped to the north at the Main Central Thrust (MCT) zone with respect to a lower b-value (0.5) to the south at the Alakananda fault (ANF) zone. The cross section of the b-value imaged the seismically active ANF at depth. The fractal dimension map, on the other hand, identified the ANF with D c ∼0.8–0.9, that implies a near linear seismogenic structure at the ANF.

We examined seismic characteristics, b value and fractal dimension of the aftershock sequence of the January 26, 2001 Bhuj earthquake (Mw 7.7) that occurred in the Kutch failed rift basin, western margin of the Stable Continental Region (SCR) of India. A total of about 2,000 events (M ≥ 2.0) were recorded within two and a half months, immediately after the main shock. Some 795 events were precisely relocated by simultaneous inversion. These relocated events are used for mapping the frequency-magnitude relation (b value) and fractal correlation dimension (Dc) to understand the seismic characteristics of the aftershocks and the source zone of the main shock. The surface maps of the b value and Dc reveal two distinct tectonic arms or zones of the V-shaped aftershock area, western zone and eastern zone. The b value is relatively higher (~1.6) in the western zone compared to a lower value (~1.4) in the eastern zone. The Dc map also shows a higher value (1.2–1.35) in the western zone compared to a lower Dc (0.80–1.15) in the eastern zone; this implies a positive correlation between Dc and b value. Two cross sections, E–W and N–S, are examined. The E–W sections show similar characteristics, higher b value and higher Dc in the western zone and lower in the eastern zone with depth. The N–S sections across the fault zones, however, show unique features; it imaged both the b and Dc characteristics convincingly to identify two known faults, the Kutch Mainland fault and the South Wagad fault (SWF), one stepping over the other with a seismogenic source zone at depth (20–35 km). The source zone at depth is imaged with a relatively lower b and higher Dc at the ‘fault end’ of the SWF showing a negative correlation. These observations, corroborated with the seismic tomography as well as with the proposed geological/tectonic model, shed a new light to our understanding on seismogenesis of the largest SCR earthquake in India in the recent years.

An attempt has been made to examine an empirical relationship between moment magnitude (M W) and local magnitude (M L) for the earthquakes in the northeast Indian region. Some 364 earthquakes that were recorded during 1950–2009 are used in this study. Focal mechanism solutions of these earthquakes include 189 Harvard-CMT solutions (M W ≥ 4.0) for the period 1976–2009, 61 published solutions and 114 solutions obtained for the local earthquakes (2.0 ≤ M L ≤ 5.0) recorded by a 27-station permanent broadband network during 2001–2009 in the region. The M W–M L relationships in seven selected zones of the region are determined by linear regression analysis. A significant variation in the M W–M L relationship and its zone specific dependence are reported here. It is found that M W is equivalent to M L with an average uncertainty of about 0.13 magnitude units. A single relationship is, however, not adequate to scale the entire northeast Indian region because of heterogeneous geologic and geotectonic environments where earthquakes occur due to collisions, subduction and complex intra-plate tectonics.

The tectonic model of the Shillong plateau and Assam valley in the northeast India region, the source area for the 1897 great earthquake (Ms ~ 8.7) and for the four (1869, 1923, 1930 and 1943) large earthquakes (M. ≥ 7.0), is examined using the high precision data of a 20-station broadband seismic network. About 300 selected earthquakes M ≥ 3.0 recorded during 2001–2009 are analysed to study the seismicity and fault plane solutions. The dominating thrust/reverse faulting earthquakes in the western plateau may be explained by the proposed pop-up tectonics between two active boundary faults, the Oldham–Brahmaputra fault to the north and the Dapsi–Dauki thrust to the south, though the northern boundary fault is debated. The more intense normal and strike-slip faulting earthquakes in the eastern plateau (Mikir massif) and in the Assam valley, on the other hand, are well explained by transverse tectonics at the long and deep rooted Kopili fault that cuts across the Himalaya and caused the 2009 Bhutan earthquake (Mw 6.3). It is conjectured that the complex tectonics of the Shillong plateau and transverse tectonics at the Kopili fault make the region vulnerable for impending large earthquake(s).

Ground motion parameters for Shillong–Mikir Plateau of Northeastern India are examined. Empirical relations are obtained for ground motions as a function of earthquake magnitude, fault type, source depth, velocity characterization of medium and distance. Correlation between ground motion parameters and characteristics of seismogenic zones are established. Simultaneously, new empirical relations are derived for attenuation of ground motion amplitudes. Correlation coefficients of the attenuation relations depend on the site classifications that are identified based on average shear wave velocity and site response factors. The attenuation relation estimated for logarithmic width of Mikir Plateau found to be a little bit higher than that of Shillong Plateau both for soft and hard ground which accounts for geometrical spreading and anelastic attenuation. Simultaneously, validation are made studying the seismic microzonation process related to geomorphological, geological subsurface features for thickly populated Guwahati city of India under threat from scenario earthquake.

article i nfo The Shillong Plateau in northeastern India represents one of the most seismically active "pop-up" structures within the peninsular shield area. In order to constrain the role of the inferred Oldham Fault in the northern boundary of the plateau, we performed 2-D finite element method (FEM) simulations for convergent displacement caused by northeastward movement of the Indian plate with respect to the Eurasian plate. Various rock properties (density, Poisson's ratio, Young's modulus, cohesion, and angle of internal friction) and the Mohr-Coulomb failure criterion are used to evaluate failure and faulting patterns. Two plane strain models with appropriate boundary conditions were also calculated. The predicted maximum compressive stress (σ1) shows a preferred orientation that helps explain the tectonic environment and the fault pattern. The best-fit model suggests that a compressive stress regime is dominant in the study area everywhere except for the uppermost part of the crust where extensional stress dominates. With increased progressive convergent displacement, the modeled σ1 are predicted to rotate counterclockwise around the fault zones. The simulation results suggest that the Oldham Fault does not have a significant role in the development of stress and deformation distribution in the area. We also infer that the tectonically induced deformation in both the plateau and the adjoining areas is restricted to mainly within the crust (b30 km).

Ground motion parameters for the Shillong–Mikir plateau, northeastern India are examined. Empirical relations are obtained for ground motions as a function of earthquake magnitude, fault type, source depth, velocity characterization of medium and distance. A correlation between ground motion parameters and characteristics of seismogenic zones is established. Simultaneously, new empirical relations are derived for the attenuation of ground motion amplitudes. The logarithmic width is found to be independent of earthquake magnitude and distance. The attenuation relations estimated for the logarithmic width of the Mikir plateau are found to be a little bit higher than that of the Shillong plateau both for soft and hard ground, which accounts for geometrical spreading and anelastic attenuation.

A three-component broadband seismograph is in operation since January 2007 at the Indian School of Mines (ISM) campus, Dhanbad. We have used the broadband (BB) seismograms of 17 teleseismic events (M ≥ 5.8) recorded by this single BB station during 2008-09 to estimate the crust and upper mantle discontinuities in Dhanbad area which falls in the peninsular India shield. The converted wave technique and the Receiver function analysis are used. A 1-D velocity model has been derived using inversion. The Mohorovicic (Moho) discontinuity (crustal thickness) below the ISM observatory is estimated to be ~41 km, with an average Poisson ratio of ~0.28, suggesting that the crust below the Dhanbad area is intermediate to mafic in nature. The single station BB data shed new light to the estimate of crustal thickness beneath the eastern India shield area, which was hitherto elusive. Further, it is observed that the global upper mantle discontinuity at 410 km is delayed by ~0.6 sec compared to the IASP-91 global model; this may be explained by a slower/hotter upper mantle; while the 660 km discontinuity is within the noise level of data.

We estimated frequency-dependent attenuation of coda waves (![Graphic][1] ) and body waves (![Graphic][2] and ![Graphic][3] ) in 1.5–24 Hz by applying the single isotropic scattering theory and the extended coda-normalization method, respectively, in the crust beneath the Andaman Sea. We used 43 aftershocks of the 13 September 2002 earthquake ( M w 6.5) in the Andaman Sea recorded by three stations installed in the Andaman Islands. The coda Q factors calculated from the amplitude decay rate of the S -wave coda show a dependence on frequency and lapse time. We found that with the increase in lapse time window from 10 to 40 s, Q ( Q C at 1 Hz) increases from 55 to 153, while the frequency-dependent coefficient n decreases from 1.1 to 0.94. The average frequency-dependent relations of ![Graphic][4] vary from 0.02 f -1.1 to 0.01 f -0.94 with an increase in lapse time window from 10 s to 40 s, respectively. The values of ![Graphic][5] and ![Graphic][6] corresponding to spectral amplitude decays show strong frequency dependence and are expressed as 0.02 f -1.01 and 0.01 f -1.0, respectively. Our results are consistent with those of other seismically active regions. The ratio ![Graphic][7] is found to be larger than unity for the whole frequency range. We separated intrinsic absorption (![Graphic][8] ) and scattering attenuation (![Graphic][9] ) using the independent estimates of ![Graphic][10] and ![Graphic][11] . The results show that ![Graphic][12] is close to ![Graphic][13] and both of them are larger than ![Graphic][14] , suggesting that coda decay is predominantly caused by intrinsic attenuation. [1]: /embed/inline-graphic-1.gif [2]: /embed/inline-graphic-2.gif [3]: /embed/inline-graphic-3.gif [4]: /embed/inline-graphic-4.gif [5]: /embed/inline-graphic-5.gif [6]: /embed/inline-graphic-6.gif [7]: /embed/inline-graphic-7.gif [8]: /embed/inline-graphic-8.gif [9]: /embed/inline-graphic-9.gif [10]: /embed/inline-graphic-10.gif [11]: /embed/inline-graphic-11.gif [12]: /embed/inline-graphic-12.gif [13]: /embed/inline-graphic-13.gif [14]: /embed/inline-graphic-14.gif

The Andaman-Sumatra subduction zone is seismically one of the most active and complex subduction zones that produced the 26 December 2004 mega thrust earthquake (Mw 9.3) and large number of aftershocks. About 8,000 earthquakes, including more than 3,000 aftershocks (M ≥ 4.5) of the 2004 earthquake, recorded during the period 1964–2007, are relocated by the EHB method. We have analysed this large data set to map fractal correlation dimension (Dc) and frequency-magnitude relation (b-value) characteristics of the seismogenic structures of this ~3,000-km-long mega thrust subduction zone in south-east Asia. The maps revealed the seismic characteristics of the Andaman-Sumatra-Java trenches, West Andaman fault (WAF), Andaman Sea Ridge (ASR), Sumatra and Java fault systems. Prominent N–S to NW–SE to E–W trending fractal dimension contours all along the subduction zone with Dc between 0.6 and 1.4 indicate that the epicentres mostly follow linear features of the major seismogenic structures. Within these major contours, several pockets of close contours with Dc ~ 0.2 to 0.6 are identified as zones of epicentre clusters and are inferred to the fault intersections as well as asperity zones along the fault systems in the fore arc. A spatial variation in the b-value (1.2–1.5) is also observed along the subduction zone with several pockets of lower b-values (1.2–1.3). The smaller b-value zones are corroborated with lower Dc (0.5–0.9), implying a positive correlation. These zones are identified to be the zones of more stress or asperity where rupture nucleation of intermediate to strong magnitude earthquakes occurred.

Seismotectonics of the two recent earthquakes, one Mw 6.3 in the Bhutan Himalaya on 21 September 2009 and the other Mw 5.1 in the Assam valley on 19 August 2009, are examined here. The recent seismicity and fault plane solutions of these two felt earthquakes suggest that both the events occurred on the Kopili fault zone, a known active fault zone in the Assam valley, about 300 km long and 50 km wide. The fault zone is transverse to the east–west Himalayan trend, and its intense seismicity indicates that it transgresses into the Himalaya. The geologically mapped curvilinear structure of the Main Central Thrust (MCT) in the Himalaya, where the epicentre of the Bhutan earthquake is located, is possibly caused by the transverse Kopili fault beneath the MCT. This intensely active fault zone may be vulnerable to an impending larger earthquake (M > 7.0) in the region.

We have imaged earthquake source zones beneath the northeast India region by seismic tomography, fractal dimension and b value mapping. 3D P-wave velocity (Vp) structure is imaged by the Local Earthquake Tomography (LET) method. High precision P-wave (3,494) and S-wave (3,064) travel times of 980 selected earthquakes, m d ≥ 2.5, are used. The events were recorded by 77 temporary/permanent seismic stations in the region during 1993–1999. By the LET method simultaneous inversion is made for precise location of the events as well as for 3D seismic imaging of the velocity structure. Fractal dimension and seismic b value has been estimated using the 980 LET relocated epicenters. A prominent northwest–southeast low Vp structure is imaged between the Shillong Plateau and Mikir hills; that reflects the Kopili fault. At the fault end, a high-Vp structure is imaged at a depth of 40 km; this is inferred to be the source zone for high seismic activity along this fault. A similar high Vp seismic source zone is imaged beneath the Shillong Plateau at 30 km depth. Both of the source zones have high fractal dimension, from 1.80 to 1.90, indicating that most of the earthquake associated fractures are approaching a 2D space. The spatial fractal dimension variation map has revealed the seismogenic structures and the crustal heterogeneities in the region. The seismic b value in northeast India is found to vary from 0.6 to 1.0. Higher b value contours are obtained along the Kopili fault (~1.0), and in the Shillong Plateau (~0.9) The correlation coefficient between the fractal dimension and b value is found to be 0.79, indicating that the correlation is positive and significant. To the south of Shillong Plateau, a low Vp structure is interpreted as thick (~20 km) sediments in the Bengal basin, with almost no seismic activity in the basin.

The best known conceptual tectonic model of the Himalayan Seismic Belt (HSB) suggests that the Basement Thrust Front (BTF) lies beneath the Main Central Thrust (MCT) with a prominent ‘ramp’. The ‘ramp’ is viewed as a geometrical asperity that accumulates the stress due to the Himalayan collision tectonics, and it has been suggested that the past great earthquakes occurred on the plane of detachment. The plane of detachment is the interface between the Indian shield and the Himalayan sedimentary wedge, also known as the Main Himalayan Thrust (MHT). The recent earthquake data from the local permanent and temporary networks and a re-examination of source processes of the great earthquakes in the Himalaya, however, do not support this model for the entire HSB. The four known instrumentally recorded great (M ∼8.0–8.7) earthquakes in the foothills of the Himalaya in India, from west to east – the 1905 Kangra, 1934 Bihar, 1897 Shillong and the 1950 Assam earthquakes – occurred by different tectonic processes, an...

Aswan reservoir lake in Egypt is the world's second largest reservoir to trigger induced seismicity. It produced an induced earthquake Ms 5.3 in the reservoir area in 1981. Since then the area is experiencing reservoir triggered seismicity, and large number of earthquakes including several swarms are recorded. A 13-station three component digital network is in operation in the area, and we have analysed about 1000 events that are recorded during 2004–2007. These data are analysed using the Local Earthquake Tomography (LET) method to image the 3D velocity structure and to relocate the events for better understanding the seismic processes and active faults in the area. The results depicted heterogeneous velocity structure with low and high velocity blocks at different depth ranges and revealed deep rooted active faults. It is observed that the upper shallow (depth

A three-component broadband seismograph is in operation since January 2007 at the Indian School of Mines (ISM) campus. We have used the broadband seismograms of two local earthquakes M

Strong ground motion parameters for Shillong plateau of northeastern India are examined. Empirical relations are obtained for main parameters of ground motions as a function of earthquake magnitude, fault type, source depth, velocity characterization of medium and distance. Correlation between ground motion parameters and characteristics of seismogenic zones are established. A new attenuation relation for peak ground acceleration is developed, which predicts higher expected PGA in the region. Parameters of strong motions, particularly the predominant periods and duration of vibrations, depend on the morphology of the studied area. The study measures low estimates of logarithmic width in Shillong plateau. The attenuation relation estimated for pulse width critically indicates increased pulse width dependence on the logarithmic distance which accounts for geometrical spreading and anelastic attenuation.

We analyse aftershocks of the 26 January 2001 Bhuj earthquake, India, that were recorded for 10 weeks following the mainshock. We calculate undrained or instantaneous pore pressure and change in Coulomb stress due to the earthquake and their poroelastic relaxation in the following 10 weeks period. Almost all aftershocks occurred in the region of coseismic dilatation. In the subsequent period, pore pressure increased through relaxation in the dilatation region which further modified coseismic Coulomb stress. Maximum increase in pore pressure is estimated to be about 0.7 MPa in 60 days time following the mainshock. Correlation between the zones of increased pore pressure and postseismic Coulomb stress with that of aftershocks, suggests a definite role of fluid diffusion in their delayed triggering.

In the present study the attenuation of seismic-wave energy in and around the source area of the Chamoli Earthquake of 29 March 1999 is estimated using aftershock data. Most of the analyzed events are from the vicinity of the main central thrust (MCT), which is a well-defined tectonic discontinuity in the Himalayas. The method of a single backscattering model is employed to calculate frequency dependent values of coda Q ( Q c ). A total of 30 aftershock events are used for Q c estimation at central frequencies 1.5, 3, 6, 9, 12, 18, and 24 Hz through five lapse-time windows from 10 to 50 sec starting at double the travel time of the S wave. The observed Q c is strongly dependent on frequency, which indicates that the region is seismically and tectonically active with high heterogeneities. The variation of Q c has also been estimated at different lapse times to observe its effect with depth. The variation of Q c with frequency and lapse time shows that the lithosphere becomes more homogeneous with depth. Q c -values for higher frequencies increase very fast with depth within about the top 63 km of the lithosphere and then become more or less constant beyond this depth. This indicates that turbidity at higher frequency decays very fast with depth, and the mantle may be transparent to high-frequency waves. The variation of Q c at 1.5 Hz with lapse time matches quite well with those predicted by Gusev (1995). However, the frequency parameter n in the relation Q c = Q 0 f n , where Q 0 = Q c at 1 Hz, does not follow the expected pattern given in his model. This could be due to faster depth decay of turbidity as mentioned previously.

In this study we attempted to estimate 3-D P-wave velocity structure of northeast India region using the first arrival data of local earthquakes that were recorded by about 77 temporary/permanent local seismic stations. The data set, the published bulletins, include 3494 P-wave travel times and 3064 S–P travel times from 980 local earthquakes that were located with a minimum of six observations. The located earthquakes having a travel-time root mean square (RMS) residual ⩽0.49 s and azimuthal gap ⩽180° are selected to compute a 1-D velocity model for the region, which is used as initial model for the subsequent 3-D inversions. Our results demonstrate that the computed 3-D velocity model has significantly improved hypocenter locations of the selected 980 earthquakes by reducing the RMS error (⩽0.06) by about 88% with respect to that by the 1-D velocity model. The reconstructed P-wave velocity ( V p ) structure, with relocated events, reveal strong heterogeneity in lateral as well as in vertical direction corresponding to the local and regional geology/tectonics of the region. High V p is mapped beneath the Shillong Plateau–Mikir hills and in the vicinity of Indo-Burma ranges at shallower crust ( V p structure is imaged between the Shillong Plateau and Mikir hills at 20 km depth, which corresponds to the Kopili fault. The Kopili fault system extends down to 30 km depth as evidenced by the low V p . A high V p is imaged below the Mikir hills at 40 km depth, which is possibly the stress concentrator for high seismic activity along the Kopili fault, particularly at the fault end. The Bengal basin, south of the Shillong Plateau, is identified as a low V p zone extending down to a depth of about 20 km, that indicates the thick alluvium sediments.

More than 5000 high precision seismic phases of 560 selected aftershocks ( M ≥ 2.0) of the January 26, 2001 Bhuj earthquake ( M W 7.7) in western India are used for joint determination of the hypocentral parameters and for 3D inversion of P-wave velocity and V p / V s structures in the source area. The aftershocks are located with an average rms of 0.19 s, and average error estimates of latitude, longitude and depth are 1.2, 1.1 and 2.3 km respectively. Most of the aftershocks occurred in an area 70 × 35 sq km; the intense activity was observed at a depth range 12–37 km. A bimodal distribution of aftershocks indicates that the main shock rupture propagated in upward and downward directions. Further, the best located aftershocks show two trends, one in northeast, parallel to Anjar Rapar Lineament, and the other in northwest parallel to the Bhachau Lineament. Fault-plane solutions of the northeast trending aftershocks indicate reverse faulting with left-lateral strike-slip component. These solutions are com...

Public panic prevailed in every part of the Andaman and Nicobar Iislands of India following the megathrust Sumatra earthquake ( Mw 9.3) on 26 December 2004. In this article, we present a very brief analysis of our continuous three-month (January-March 2005) monitoring and recording of aftershock data following the main earthquake to show how this endeavor reduced public panic and constituted an important ingredient to a disaster management program for the Andaman-Nicobar region. Monitoring was conducted using six short-period three-component temporary digital seismograph stations set up in different parts of the Andaman and Nicobar islands. Our findings demonstrate that 1) there was no aftershock gap zone as recorded by the far-distant seismographic network, hence negating the possibility of immediate strong quakes in the Andaman and Nicobar islands; 2) there was no strong shaking at full moon (26 January 2005) due to tidal stresses, although the rate of aftershock activity increased by about 31% from the events of the preceding day; 3) there is a north-south trending 850 × 350-km2 rupture area beneath the Andaman and Nicobar islands; and 4) eruptions of mud at the Baratang volcanic zone and lavas at the Barren volcanic zone occurred because of strong shaking due to the mainshock and a series of aftershock clusters within 100 km of the individual volcanic zones. These eruptions may continue for a couple of years, until the aftershock sequence ceases. We show that this new dataset from the local seismographic network constituted an important factor in reassuring the coastal people and islanders about imminent dangers from future earthquakes and tsunamis, and we propose geochemical tests of the erupted mud samples from the Baratang volcanic zone, with geophysical exploration followed by drilling, to ascertain the presence of oil and gas reserves and the phenomenon of gas seepage under the …

Six three-component short-period digital seismograph (four Reftek and two Kinemetrics) stations were established in different parts of the Andaman– Nicobar Islands following the 26 December 2004 devastating Sumatra–Andaman mainshock (Mw 9.3). Here, we analyze about 18,000 aftershocks (M ≥3.0) recorded from 6 January to 16 March 2005 to better understand the seismotectonics of the region. A sudden burst of aftershock activity with irregular trend in the month of January 2005 was observed at almost all seismograph stations. The estimate of P- value = 0.9532 from aftershocks (M ≥4.5) is near to normal value of 1.0, which suggests a slow decay sequence of aftershock with complex and nonuniform stress change in a fault system (creep effects and history-dependent stress changes). The frequency–magnitude relation of the aftershocks followed the power law with average b-value of 0.7723 and it varies from 0.49 to 1.03, indicating the compressive stress state of the region and its heterogeneous structure with possible variations in frictional conditions along the fault. The distribution of the located aftershocks by a multistation method shows a north–south-trending aftershock cluster in an area of about 800 × 300 km2, which reflects an approximate rupture dimension of the mainshock beneath the Andaman–Nicobar Islands. Most of aftershocks occurred in a depth range of 5–65 km. The determination of composite fault-plane solutions of the best located aftershock clusters at three different depth ranges (0–15, 16–30, and >31 km) in the ten blocks of the region suggests that the mainshock rupture propagated through normal, reverse, and strike-slip earthquakes. The areas in the vicinity of Barren and Narcondum volcanic zones show a dominantly normal fault mechanism due to predominant tensional forces, suggesting the facilitation of brittle failure in the weakened crust by the process of underheating. The zone near Baratang mud volcano is associated predominantly with thrust fault at depth of 0–30 km, suggesting a high compressive force beneath the Baratang that ejected a huge amount of mud and slurry materials to the surface after the 26 December 2004 Sumatra–Andaman earthquake (Mw 9.3). Here, we propose that the relocation of our large aftershock dataset using converted sP-depth-phase technique for precise depth control is needed, and its sharing with global earthquake data can render more information on the subduction dynamics and geodynamical processes of the region.

The Latur earthquake (Mw 6.1) of 29 September 1993 is a rare stable continental region (SCR) earthquake that occurred on a previously unknown blind fault. In this study, we determined detailed three-dimensional (3-D) P- and S-wave velocity (Vp, Vs) and Poisson's ratio (σ) structures by inverting the first P- and S-wave high-quality arrival time data from 142 aftershocks that were recorded by a network of temporary seismic stations. The source zone of the Latur earthquake shows strong lateral heterogeneities in Vp, Vs and σ structures, extending in a volume of about 90 × 90 × 15 km3. The mainshock occurred within, but near the boundary, of a low-Vp, high-Vs and low-σ zone. This suggests that the structural asperities at the mainshock hypocenter are associated with a partially fluid-saturated fractured rock in a previously unknown source zone with intersecting fault surfaces. This might have triggered the 1993 Latur mainshock and its aftershock sequence. Our results are in good agreement with other geophysical studies that suggest high conductivity and high concentration of radiogenic helium gas beneath the source zone of the Latur earthquake. Our study provides an additional evidence for the presence of fluid related anomaly at the hidden source zone of the Latur earthquake in the SCR and helps us understand the genesis of damaging earthquakes in the SCR of the world.

A positive correlation between areas of increased coulomb stress changes, induced by an earthquake, and the spatial distribution in the occurrence of aftershocks have been reported in recent years. We consider such a correlation between the aftershocks of March 29, 1999 Chamoli earthquake (Ms 6.6) that occurred in the Garhwal Himalaya, India, and the mainshock induced coulomb stresses, to constrain some of the rupture parameters of the mainshock. Most of the aftershocks occurred above and near the updip edge of the rupture implying reactivation of pre-existing thrust fault of the Himalayan wedge. The 15 × 12 km2 rupture appears to be lying above the detachment, the plate boundary fault between the underthrusting Indian shield rocks and the Himalayan wedge rocks, in the Basement Thrust Front (BTF) zone.

A microearthquake survey in the Sikkim Himalaya raised a question whether the north–south segment of the Main Central Thrust (MCT) in this part of the Himalaya is seismically active(?). Fault-plane solution of a cluster of events occurred below this segment of the MCT shows right-lateral strike-slip motion. The seismic observations and the geological evidences suggest that a NNE–SSW trending strike-slip fault, beneath this segment, caused right lateral movement on the MCT, and is seismically active.

A medium-size earthquake mainshock ( M w 6.5) occurred on 13 September 2002 (22 h 28 m 31 s UTC) in the Andaman Sea, about 20 km offshore Diglipur, north Andaman Island. The epicenter at 13.087° N and 93.112° E and centroid depth at 31 km were estimated by the U.S. Geological Survey (USGS). The main shock was a reverse-faulting event. Four digital seismograph stations were established in the Andaman Islands to monitor aftershocks. About 200 aftershocks ( M d ≥1.0) were recorded during a period from 19 September to 7 October 2002. The epicenter map shows a northwest-southeast-trending aftershock cluster in an area of about 140 × 70 km2, which reflects the rupture area of the mainshock. The aftershocks occurred mostly at a depth range of 5-20 km, except one at 38 km. The moment tensor solutions (USGS) of the mainshock and the largest aftershock ( M w 5.8), which occurred 22 hr after the mainshock, revealed consistent reverse faulting. The northeast-dipping northwest-southeast-trending nodal plane, which is comparable with the aftershock trend, is inferred to be the fault plane. The northeast-southwest compressional stress ( P axis), obtained by the fault-plane solutions, is compatible with the north-northeast movement of the Indian plate. The mainshock and the two well-located largest aftershocks, located by the USGS, occurred within the subducted plate. One composite fault-plane solution is obtained for the shallower aftershocks recorded by the temporary network. The solution shows normal faulting with a northeast-southwest tensional ( T ) axis. These aftershocks occurred off the subducted plate at a much shallower depth (

A 12-station temporary microearthquake network was established by the Geological Survey of India for aftershock monitoring of the January 26th, 2001 Bhuj earthquake (Mw 7.6) in the Kutch district of Gujarat state, western India. The epicentres of the aftershocks show two major trends: one in the NE direction and the other in the NW direction. Fault-plane solutions of the best-located and selected cluster of events that occurred along the NE trend, at a depth of 15–38 km, show reverse faulting with a large left-lateral strike-slip motion, which are comparable with the main-shock solution. The NW trending upper crustal aftershocks at depth

The Geological Survey of India (GSI) established a twelve-station temporary microearthquake (MEQ) network to monitor the aftershocks in the epicenter area of the Bhuj earthquake (Mw7.5) of 26th January 2001. The main shock occurred in the Kutch rift basin with the epicenter to the north of Bhachao village, at an estimated depth of 25 km (IMD). About 3000 aftershocks (Md ≥ 1.0), were recorded by the GSI network over a monitoring period of about two and half months from 29th January 2001 to 15th April 2001. About 800 aftershocks (Md ≥ 2.0) are located in this study. The epicenters are clustered in an area 60 km × 30 km, between 23.3‡N and 23.6‡N and 70‡E and 70.6‡E. The main shock epicenter is also located within this zone.

The seismotectonic model in the Sikkim Himalaya does not fit well with the proposed steady state or evolutionary models. The main boundary thrust (MBT) is seismogenic and is a mantle-reaching fault. The earthquakes are not confined to shallower depths (25 km) above the plane of detachment as proposed in the models, the seismic activity continues from surface to the lower crust (0-45 km) to the north of MBT, and earthquakes are produced by a thrust mechanism. The earthquakes to the east of Sikkim, in the Bhutan Himalaya, on the other hand, are produced along a 200-km-long northwest-southeast-trending lineament by transverse tectonics; the seismogenic lineament cuts across the Himalayan major thrusts and extends to the Goalpara wedge in the southeast. The earthquakes occur by strike-slip mechanism in the midcrust, at a depth range of 10-25 km, along this long active lineament.

The aftershocks of the Chamoli earthquake ( m b 6.3) of 28 March 1999 are analyzed to understand the seismotectonic structure of the Garhwal Himalaya. The permanent as well as temporary microearthquake network data are used. The mainshock occurred on the Basement Thrust by thrust faulting. The aftershocks, on the other hand, are triggered at the seismogenic faults to the south of the Main Central Thrust (MCT) by thrust faulting as well as by strike-slip faulting, and all the events occurred above the plane of detachment/basement thrust. The MCT is not the seismogenic fault.

Site response in the Garhwal Himalaya is studied using digital seismograms recorded by a five-station 24-bit digital microearthquake network established to monitor the aftershocks of the 28 March 1999 Chamoli earthquake ( m b 6.3). Fifteen aftershocks ( M d ≥2.0) are chosen for the site response estimation using horizontal-to-vertical spectral ratio and generalized inversion techniques. Site response curves at all the five sites show station-to-station variation of the site factor reflecting the changes in geologic/geotectonic/soil conditions. A comparison of the site response values obtained by the inversion with those computed using receiver function technique show a large scatter even though the pattern of the curves remains more or less similar. The peaks yielded by both the methods have been observed to occur at the same frequencies. The variation of site response corroborates the abrupt changes in intensity from one location to another due to local site condition. Manuscript received 22 September 2000.

The aftershock sequence of the September 30th, 1993 Killari earthquake in the Latur district of Maharashtra state, India, recorded by 41 temporary seismograph stations are used for estimating 3-D velocity structure in the epicentral area. The local earthquake tomography (LET) method of Thurber (1983) is used. About 1500P and 1200S wave travel-times are inverted. TheP andS wave velocities as well asV P/VSratio vary more rapidly in the vertical as well as in the horizontal directions in the source region compared to the adjacent areas. The main shock hypocentre is located at the junction of a high velocity and a low velocity zone, representing a fault zone at 6–7 km depth. The estimated average errors ofP velocity andV P/VSratio are ±0.07 km/s and ±0.016, respectively. The best resolution ofP and S-wave velocities is obtained in the aftershock zone. The 3-D velocity structure and precise locations of the aftershocks suggest a ‘stationary concept’ of the Killari earthquake sequence.