ALBERT

Description: Although the Indo-Gangetic basin is adjacent to rupture areas of large Himalayan earthquakes (M 〉 or = 6), a quantitative study of the amplification of seismic waves in the region is still lacking. To obtain a first estimate of the amplification, for two years we operated an array of 10 broadband seismographs that crossed the central Indo-Gangetic basin in a north-south direction. Using earthquake recordings of shallow earthquakes at soft sites and hard reference sites, we computed standard spectral ratios (SSRs). SSRs at sites near the Himalayan foothills, where the sediment thickness is approximately 4 km, reveal a broadband amplification with a fundamental frequency of 0.13 Hz. The amplification at this frequency varies between 20 and 60. The fundamental frequency increases to the south as the thickness of the sediments decreases, becoming approximately 0.8 Hz at the southernmost site. The amplification at the fundamental frequencies exceeds 10 at all eight soft sites. Calculations based on reasonable earthquake source and attenuation models and application of random vibration theory suggest that peak ground acceleration and peak ground velocity at soft sites near the foothills, located 100 km from the epicenter, would be amplified by a factor of 2-4 and 6-12, respectively. All our results assume linear behavior of the sediments. Although this assumption would not be valid during intense motions resulting from large earthquakes, our results, nevertheless, provide a basic building block for incorporating nonlinear behavior.

Description: 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 0.72-2.5 km/s with thicknesses varying from 0.5 to 3.7 km 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.

Description: 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〉 or =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 approximately 1 km/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 3.4 km/s, akin to global observations. The mountain ranges that constitute the Himalaya and southern Tibet have intermediate velocities primarily ranging from 2.8 to 3.0 km/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.

Description: 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 70 km depth in this region, compared to an average focal depth of 15-20 km 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+ or -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.