SUMMARY We investigated sedimentary thickness and shear wave velocity structure in the western part of the Indo-Gangetic Plain (Punjab and Haryana Plain) and adjoining Siwalik Himalaya with the help of receiver function inversion at 20 broad-band seismological stations. This region is one of the most seismically vulnerable zones of the world due to the presence of thick surface sediments in the foreland basin that can amplify seismic waves and cause huge damage due to the earthquakes of the Himalaya. The study reveals a progressive thickening of sediments from southwest to northeast. The basement depth varies from ∼1.5 to 1.7 km in the Central Alluvium Plain, ∼1.8 to 2.8 km in the Zone of Terminal Fans, and attains a maximum of ∼3.8 km near the Himalayan Frontal Thrust. The inverted models show the presence of soft alluvial with extremely low Vs (< 0.5 km s−1) and high Vp/Vs (∼2.5–3.0) at the top ∼400–700 m of the surface at most of the stations. A comparatively higher velocity of surface sediments observed at northern stations suggests the presence of compact sediments at the surface. The layered sedimentary structure revealed by the S-wave velocity models supports the previous geophysical investigations using borehole data. The velocity–depth structure obtained in this study is important for evaluating the seismic hazard of the densely populated urban areas spread over this region.
Since 1934, two great earthquakes (Mw ≥8.0) and some eight large/strong earthquakes (Mw 6.5–7.8) occurred in the eastern Himalayas, from eastern Nepal to the Eastern Himalayan Syntaxis (EHS) Zone. All these events are instrumentally recorded by local as well as by the global network. From west to east these events are the 2015 two large/devastating earthquakes (Mw 7.8 and 7.3, respectively) in eastern Nepal, the 1934 and 1988 Bihar–Nepal earthquakes (Mw 8.1 and 6.8, respectively), the 2011 Sikkim (Mw 6.9), the 1967 and 2009 Bhutan (Mw 6.5 and 6.3, respectively), 1964 and 1947 Arunachal–Tibet (Mw 6.5 and 7.0, respectively), and the 1950 Assam–Tibet great (Mw 8.4) earthquakes. Most recently, on 28 April 2021, an earthquake Mw 6.1 occurred in the Assam foredeep. These events are documented as shallow (0–20 km) Main Himalayan Thrust (MHT) earthquakes in the International Seismological Center and National Earthquake Information Center — USGS catalogues. With the advent of local digital seismic networks and better precision of hypocentre locations, the eastern Himalayan earthquake source zones are found to be much different; these are not typically MHT events. Precise hypocentre locations indicate bimodal seismicity at shallower (0–20 km) as well as at deeper depths (40–70 km) in the eastern Himalayas. Seismic cross‐sections and fault plane solutions of the earthquakes do not support a uniform seismotectonic model for the entire Himalayas. All the instrumentally recorded past great earthquakes in the Himalayas and the recent destructive earthquakes in the eastern Himalayas study region are critically examined in view of the known seismotectonic model, and we suggest that each large/great earthquake occurred in its own unique complex tectonic environment. The transverse structures play a major role in generating the deeper earthquakes in the eastern Himalayas and the EHS; the shallower MHT may be flattened.
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