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.
5.2.1 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