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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.

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.

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.

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).

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).

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

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 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.

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.

The Uttarkashi earthquake ( m b = 6.6) of October 20, 1991 occurred in a complicated geological/tectonic setting at the Main Central Thrust (MCT) zone of the Himalaya. The main shock was preceded by a precursory earthquake activity, which continued for about eight years, and then there was a quiescent period for about three years. Three foreshocks, M > 4.0, occurred 48 h before the main shock, two aftershocks, M > 4.0, occurred within 12 h of the main shock which was then followed by another two aftershocks, M > 4.0, after about seven days, on October 27, 1991. The largest aftershock, M 5.2, occurred within 12 h. The aftershocks, magnitude less than 4.0, continued for about two months, and it decayed at a rate t −0.87 . About 1000 earthquakes, magnitude less than 1.0 and above, were recorded by a closely spaced five-station temporary microearthquake (MEQ) network. Out of these, 124 aftershocks which occurred inside the network are located by Homogeneous Station Method. Spatial variation of the aftershocks with time was observed. Fault-plane solutions of the main shock and aftershocks indicate dominantly thrust faulting. Based on the hypocentral data, depth estimate of the plane of detachment is made, and a fault-interaction model is presented. The model shows that the main shock occurred at the juncture of MCT zone, plane of detachment and the basement thrust. The north-dipping Uttarkashi fault, a parallel fault at the MCT zone, has been activated by the tectonic stress generated due to relative movement of the Tibetan plate over the Indian plate.

Two microearthquake surveys were carried out at the Main Boundary Thrust (MBT), Eastern Himalaya. Temporal as well as spatial variation in microseismicity was observed. The vertical section of the hypocentres shows subcrustal earthquakes down to a depth of 80 km to the south of the MBT and composite focal mechanisms show strike-slip solutions or transverse tectonics which do not agree with the conceptual tectonic model.

A combination of resistivity and gamma-count-rate data from coal seams may lead to the estimation of ash content of coal in a fairly easy way provided we have a few chemical analyses of ash content in the coal seams logged. An attempt has been made to draw nomograms between resistivity, gamma-count-rate and ash content for two different coal basins in India.

Electrical and gamma-ray logging have been very useful for identification and accurate determination of depth and thickness of coal seams in Gondwana and Tertiary coal fields of India. The characteristic resistance/resistivity peaks of coal seams in a particular area have been correlated, thus providing a picture of the subsurface structure. Physico-chemical properties of layers or sections of coal seams are responsive to electrical logs. Gammaray logs are found to be very useful for correlation and have sometimes been the only logs used in cased and dry boreholes for detection of coal seams. Under favourable conditions a single-point resistance log reveals a detailed picture of the formations and picks up thin coal seams as well as thin shale bands within the coal seam. But in some cases it fails to differentiate between coal and sandstone beds in spite of high contrast in true resistivities. Multi-electrode long-normal and lateral logs are found to be more useful in differentiating such formations because of higher penetration in this system. Long-normal and lateral curves can also be used to determine true resistivity of the formation. But long normal logs cannot pick up thin coal bands and/or thin shale partings within the coal seam because of the “adjacent bed effect”. Gamma-ray logging can be done in both cased and uncased boreholes or even in a dry borehole but its resolution for shaly coal or thin coal is not sufficient. Combined study has been found to yield the best results.

Reversed Pn velocities have been determined from regional earthquake sources for the Wairarapa region in the southeast North Island, New Zealand. Values were 8.64 (±0.13) km/s in the easternmost part of the region and 8.90 (±0.13) km/s in the central Wairarapa, which are consistent with subducted Pacific plate lying at shallow depth under the region. A value of 8.22 km/s between Wellington and the Wairarapa region is interpreted as a possible difference in the velocity structure above the refractor between the two localities.