Crustal Configuration and Seismic Stability of the Eastern Indian Shield and Adjoining Regions: Insights for Incidents of Great Earthquakes in the Nepal-Bihar-Sikkim Himalaya

Rashmi Singh,Prosanta Kumar Khan

doi:10.3389/feart.2021.586152

Rashmi Singh, Prosanta Kumar Khan

Open Access

https://doi.org/10.3389/feart.2021.586152

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Abstract

The Eastern Indian Shield (EIS) is comprised of the intracratonic (coal-bearing) Damodar Gondwana basin, rift-controlled extensional Lower Gangetic basin (LGB), and the downward flexed Indo-Gangetic basin (IGB). The present study involves the computations and mapping of the basement configuration, sediment thickness, Moho depth, and the residual isostatic gravity anomaly, based on 2-D gravity modeling. The sediment thickness in the area ranges between 0.0 and 6.5 km, and the Conrad discontinuity occurs at ∼17.0–20 km depth. The depth of the Moho varies between 36.0 and 41.5 km, with the maximum value beneath the Upper Gangetic basin (UGB), and the minimum of ∼36 km (uplifted Moho) in the southeastern part beneath the LGB. The maximum residual isostatic anomaly of +44 mGal in the southern part indicates the Singhbhum shear zone, LGB, and Rajmahal trap to be under-compensated, whereas the northern part recording the minimum residual isostatic anomaly of –87.0 mGal is over-compensated. Although the region experienced a few moderate-magnitude earthquakes in the past, small-magnitude earthquakes are sparsely distributed. The basement reactivation was possibly associated with a few events of magnitudes more than 4.0. Toward the south, in the Bay of Bengal (BOB), seismic activities of moderate size and shallow origin are confined between the aseismic 85 and 90°E ridges. The regions on the extreme north and south [along the Himalaya and the equatorial Indian Ocean (EIO)] are experienced moderate-to-great earthquakes over different times in the historical past, but the intervening EIS and the BOB have seismic stability. We propose that the two aseismic ridges are guiding the lithospheric stress fields, which are being further focused by the basement of the EIS, the BOB, and the N-S extended regional fault systems into the bending zone of the penetrating Indian lithosphere beneath the Himalaya. The minimum obliquity of the Indian plate and the transecting fault systems in the Foothills of the Himalaya channelize and enhance the stress field into the bending zone. The enhanced stress generates great earthquakes in the Nepal-Bihar-Sikkim Himalaya, and on being reflected back through the apparently stable EIS and BOB, the stress field creates deformation and great earthquakes in the EIO.

Highlights

Occasional incidents of major damaging earthquakes, such as 2015 (Mw 7.8) Nepal-Bihar, 2011 (Mw 6.9) Sikkim, 1988 (Mw 6.8) north Bihar, 1934 (Mw 8.0) north Bihar-Nepal, and 1833 (ML 7.6) Nepal-Bihar, account continued accumulation and occasionally release of strain energy in the Nepal-Bihar-Sikkim part of the Himalayas (Ansari et al, 2014; Khan et al, 2017a; Singh et al, 2020)
The Bouguer gravity anomaly map indicates that the study area is characterized by low (−96 mGal) to moderate (+33 mGal) regional anomalies with the maximum value over the Rajmahal trap, the Lower Gangetic basin (LGB) in the east, and over the well-defined shear zone in the south (Figure 3)
The minimum Bouguer gravity anomaly values of −50 to −96 mGal are associated with the Upper Gangetic basin (UGB), elongated along east-west between 24.5◦ and 25.5◦N in the northern part of the study area

Summary

Introduction

Occasional incidents of major damaging earthquakes, such as 2015 (Mw 7.8) Nepal-Bihar, 2011 (Mw 6.9) Sikkim, 1988 (Mw 6.8) north Bihar, 1934 (Mw 8.0) north Bihar-Nepal, and 1833 (ML 7.6) Nepal-Bihar, account continued accumulation and occasionally release of strain energy in the Nepal-Bihar-Sikkim part of the Himalayas (Ansari et al, 2014; Khan et al, 2017a; Singh et al, 2020). A few of these features, along with other transecting faults, have shown strike-slip dominated movements during the Holocene period (Sastri et al, 1971; Valdiya, 1976; Ansari et al, 2014; Khan et al, 2014; Singh et al, 2020) Such operative dynamics and kinematics toward the south of the Himalayan Foothills, compatible with the back propagation of the compressive stress field, facilitated deformation in the EIS, and the effects were likely extended up to the Central Indian basin (McKenzie and Sclater, 1971; Minster and Jordan, 1978; Stein and Okal, 1978; Gordon et al, 1990; DeMets et al, 1994)

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Discussion

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