D. Srinagesh

Transverse Tectonics in the Sikkim Himalaya: Evidence from Seismicity and Focal-Mechanism Data

Abstract In the present study, about 356 local earthquakes in the region of the Sikkim Himalaya have been accurately located and analyzed using 2181 P travel times and 2161 S travel times from a network of 11 broadband seismic stations operated by the National Geophysical Research Institute during January 2006 to November 2007. Further refinement of the hypocentral parameters using the hypoDD relocation program resulted in 198 well-constrained locations. Interestingly, this study reveals several characteristic features that distinguish Sikkim from the rest of the Himalaya. The seismicity distribution is found to be confined mostly between the main boundary thrust (MBT) and the main central thrust (MCT) but not quite associated with either. While the entire Himalayan front is generally characterized by shallow-angle thrust faulting, focal mechanisms in this region are predominantly of strike-slip type in conformity with a right-lateral strike-slip mechanism along the northwest-trending Tista and Gangtok lineaments. The P -axis trends of earthquake focal mechanisms are clearly oriented north-northwest, marking a clear transition from the ambient north-northeast trending direction of Indian plate motion with respect to the Eurasian plate all along the Himalayan front. Moderate-sized earthquakes occur down to 70xa0km depth in this region, compared to an average focal depth of 15–20xa0km in the rest of the Himalaya. Also, a high average crustal P velocity of 6.66 km/sec and a fairly low b value of 0.83±0.04 are obtained indicating the probability of occurrence of a higher magnitude earthquake in the future. A north–south section in the Sikkim region shows a relatively flat topography, unlike in the rest of the Himalayan mountain chain and suggestive of lower rates of convergence in the recent geologic past. It is proposed that crustal shortening in the Sikkim Himalaya has been substantially accommodated by transverse tectonics rather than underthrusting in recent times.

Sedimentary Thickness Variations in the Indo‐Gangetic Foredeep from Inversion of Receiver Functions

Abstract We utilize receiver functions from ten broadband seismic stations deployed along a north–south profile traversing the Indo‐Gangetic plains in northern India to investigate the sedimentary thickness variations in this hitherto less‐studied foreland basin south of the Himalaya. Parameterization of the velocity structure adopting the neighborhood algorithm approach shows that the data can be satisfactorily modeled for low‐velocity sediments having shear‐wave velocities in the range of 0.72–2.5u2009u2009km/s with thicknesses varying from 0.5 to 3.7xa0km beneath the individual stations. The velocity–depth functions obtained in this study are important for earthquake‐hazard assessment of the densely populated urban centers spread over this region, in terms of predicting strong ground motions due to large earthquakes in the Himalaya.

Continental scale body wave tomography of India: Evidence for attrition and preservation of lithospheric roots

We assemble P and S waveforms of 2301 teleseismic earthquakes registered at 413 broadband seismic stations spanning the Indian plate from the southern tip of India to the Himalayan collision belt and generate an accurate data set of 52,050 P and 30,423 S arrival times through the multichannel cross-correlation approach. These traveltimes are then inverted to obtain 3-D P and S velocity structures of the subcontinent at a 2° × 2° lateral resolution. The heterogeneous nature of the Indian lithospheric mantle revealed in this study suggests that the lithospheric roots are not uniformly thick on a regional scale. The key cratonic segments of the Indian shield are characterized by pockets of high velocity anomalies (∼3%) at shallow depths (<300 km), with the diamondiferous regions like Wajrakarur revealing high shear wave anomalies down to ∼300 km. In contrast to the southern Deccan volcanic province (DVP), the northwestern DVP is underlain by low velocity anomalies at similar depths suggesting that the upper mantle retains imprints of Deccan volcanism which was facilitated by the reactivation of the rift systems.

Seismic anisotropy beneath the Eastern Dharwar craton

The southeastern Indian Shield, an assemblage of several Precambrian geological terranes, carries imprints of major tectonic events, including those related to rifting contemporaneous with India-Antarctica continental separation, volcanism, and sedimentation in Gondwana. In this study, we investigate the character of seismic anisotropy underneath 14 broadband stations spanning this region, utilizing the SK(K)S and direct S waves from earthquakes deeper than 400 km. In total, 113 high-quality splitting measurements reveal that the delay times (δ t ) between the fast and slow axes of anisotropy range from 0.32 s to 1.62 s for direct S waves and from 0.31 s to 1.80 s for SK(K)S phases. The fast polarization directions at a majority of the stations are in accordance with shear at the base of the lithosphere, coinciding with the present-day motion of the Indian plate with respect to the fixed Eurasian plate as defined through the NUVEL1A plate model. The coast-parallel splitting trends in the vicinity of the Eastern Ghat mobile belt can be reconciled by invoking a combination of anisotropy frozen in the lithosphere due to continental rifting along the eastern margin of the Indian plate and active asthenospheric anisotropy.

Structure and Tectonics of the Bay of Bengal through Waveform Modeling of the 21 May 2014 Earthquake of Magnitude 6.0

On 21 May 2014, an earthquake of magnitude 6.0 occurred within the Bay of Bengal at 16:21:54 UTC. This event, with latitude 18.2046°xa0N, longitude 88.0298°xa0E, and a depth of 47xa0km (U.S. Geological Survey [USGS], 2014), is an unusually strong earthquake in a region of relatively low seismicity. The region is bordered by the tectonically active Burma and Andaman arcs to the east (Curray, 2005) and a nascent, diffuse plate boundary zone separating the Indian plate from the Australian plate in the south (Rao and Kumar, 1996). On the western and northern sides lies the Indian continental margin. The diffuse deformation zone is identified as the plate boundary where the Australian plate has an anticlockwise rotation with respect to the Indian plate (DeMets etxa0al. , 1990), due to smooth subduction of the former beneath the Sunda arc as compared with a resistant India–Eurasia collision in the north (Stein and Okal, 1978). This complex rotational kinematics enables both convergence and divergence on the same plate boundary, as evidenced by thrust‐type focal mechanisms in the east and normal‐fault earthquakes in the west on the Chagos‐Laccadive ridge. The scenario is further complicated by accommodation of the deformation through a peculiar strike‐slip motion along the 5000xa0km long Ninetyeast ridge which runs into the Burma–Andaman arc. Subrahmanyam etxa0al. (2008) based on gravity data and Gahalaut etxa0al. (2010) using Global Positioning System (GPS) data have suggested the Ninetyeast ridge subducts obliquely beneath the Andaman arc.nnThe Burmese arc to the east marks the eastern boundary of the Indian plate and is characterized by a recent cessation of subduction and transformation into a strike‐slip zone (Le Dain etxa0al. , 1984; Kumar and Rao, 1995; Kumar etxa0al. , 1996; Rao and Kumar, 1999; Vigny etxa0al. , 2003; Rao and Kalpna, 2005; Gahalaut and …

Near‐Surface Shear Velocities in Diverse Geological Segments of India

Abstract In this paper we investigate the near‐surface shear velocities beneath 144 broadband seismic stations of India that span diverse geological terrains, using nearly 37,635 good quality (SNR≥2.5) three‐component waveforms from 3849 earthquakes. The results suggest lower shear velocities beneath regions of large sedimentation, with the lowest in the range of ∼1u2009u2009km/s observed for the Indo‐Gangetic plains. These low velocity estimates show a dependence on frequency, implying velocity changes with depth. Segments that represent the Precambrian shield reveal high shear velocities in the range of 3.2 to 3.4u2009u2009km/s, akin to global observations. The mountain ranges that constitute the Himalaya and southern Tibet have intermediate velocities primarily ranging from 2.8 to 3.0u2009u2009km/s. Overall, the near‐surface shear velocities seem to be correlated with the local geology and provide inputs for site‐specific hazard assessment in terms of predicting strong ground motions due to scenario earthquakes.