Figure 8. A tectonic model proposed for the Sikkim Himalaya based on the lithospheric geoelectric structure obtained by the present MT experiment and available seismic and density information as well as a kinematic model of the wedge structure. (From Pavankumar and Manglik, 2021)  (Reprinted from Physics and Chemistry of the Earth, Parts A/B/C, 124, G. Pavankumar, Ajay Manglik, Complex tectonic setting and deep crustal seismicity of the Sikkim Himalaya: An electrical resistivity perspective, 103077, Copyright (2021), with permission from Elsevier.)

5.1.2  Gravity Modeling of the Kopili and Bomdila fault regions

An integrated approach based on seismotectonics, gravity and magnetic data is utilized to understand the tectonic activity of the Kopili and Bomdila Faults bounding the Mikir Hills (Sharma S., et al., 2018). The Kopili Fault dips NE at 75\(^{\circ}\) whereas the Bomdila Fault dips NNE at 50-55\(^{\circ}\) angle. The bottom of seismogenic zones is inferred to be 45±2 km and 50±2 km for the Kopili Fault and the Bomdila Fault regions, respectively. The low gravity values over the Bomdila Fault area indicate presence of thick alluvial deposits while lesser sediment thickness is observed along the Kopili Fault.

5.2 Indian Shield Region

5.2.1 Magnetotelluric investigations in the vicinity of the Delhi Seismic Zone

The Delhi Seismic Zone (DSZ) in the northwest Indian shield is one of the seismically active intraplate regions with frequent occurrence of small-to-moderate magnitude earthquakes. These earthquakes occur mainly along the NE-SW oriented Proterozoic Aravalli-Delhi Fold Belt (ADFB) and NNW-to-NW trending Delhi-Sargodha Ridge (DSR). However, the detailed subsurface architecture of DSZ and its surrounding areas concealed by alluvial sediments is still unclear. Pavankumar et al. (2021) and Manglik et al. (2022) conducted magnetotelluric (MT) experiments across the DSR and the ADFB, respectively, to image this region in terms of electrical resistivity structure. The results yield a northward dipping electrical conductor (< 10 Ωm) down to 20-25 km for the DSR whereas the ADFB buried beneath the alluvial sediments of the western Ganga Basin consists of a collage of nearly vertical conductive and resistive blocks and a sharp resistive contrast. These blocks appear to continue beneath the Kumaun-Garhwal Himalaya. These results have significant implications for earthquake hazard potential of both the region, the DSZ and the Uttarakhand Himalaya.

5.2.2 Pre-eruptive crustal electrical structure and tectonics of the recent Palghar earthquake swarm activity region, Maharashtra

The DVP is a major geological domain of the Indian peninsular shield. Though the region is considered to be seismically stable, it has experienced some significant intraplate earthquakes, as well as swarm type earthquake activity at Bhatsa, Silvasa, Navsari, Nasik, Valsad and more recently at Palghar. Pavankumar et al. (2020) carried out MT studies along two profiles covering the Bhatsa and the Palghar earthquake swarm regions. Geoelectric models of these two regions depict strong resistivity contrast suggesting that the crust below the basalt cover is fragmented. The zones of fragmentation coincide with the major pre- and post-eruptive tectonic structures, e.g. the West Coast fault, the Panvel Flexure and the Kurudwadi rift.

5.2.3 Seismogenesis of intraplate Kachchh rift in western India by Magnetotellurics

The Kachchh basin comprises a set of E-W trending faults and fault bounded uplifts. Since historic past, the region is experiencing moderate to large magnitude earthquakes and is considered as one of the most active intraplate regions of the world. Magnetotelluric studies carried out in various segments of the region yield well resolved electrical images of the deep crust, enabling delineation of the broad geometry of various active faults (Nagar et al., 2021). The MT models in conjunction with 3-D relocated hypocenters provide better constraints on the geometry of the seismogenic segments of the faults at depth. Among the various intra- basin faults, the South Wagad Fault (SWF) has a downward lower crustal extension that possibly connects to a fluid reservoir in the upper mantle. However, the Kachchh Mainland, Katrol Hill, Gedi and Allahbund faults are limited to the upper crustal depths. The North Wagad Fault is inferred as an antithetic splay of the SWF.

6. Seismic Hazard, Risk and Strong Ground Motion Studies 

6.1 Strong Ground Motion and Earthquake Hazard Assessment

6.1.1 Plate boundary regions