New insights on the tectonic framework using EIGEN6C4 gravity data, seismicity, and finite element stress analysis: An attempt to map earthquake vulnerable zones in parts of North-East India and surroundings

Shillong Plateau and its surroundings experience frequent seismic activities characterised by the complex tectonic setup covering different parts of the Eastern Himalaya, Indo-Burma ranges and their conflux at the Assam syntaxis; which need further multivariate investigations. The present study deals with a combined analysis of EIGEN6C4 Bouguer gravity data, 56 years (1964–2020) seismicity data, 11 stations GPS velocity data (1996–2015) and focal mechanism of twelve earthquake events (1996–2017). We have enhanced Bouguer gravity data using the Theta derivative. Upward continuations of the Bouguer gravity were estimated at the heights of 5 km, 10 km, 20 km, 40 km, 60 km, and 100 km, which helped to infer the approximate persistent depth of the structures. Theta derivatives for the upward continued maps have also been estimated to enhance structures and fault zones at different approximated depths. Large numbers of smaller lineaments and shallow crustal smaller blocks are delineated at ∼2.5 km depth, whereas medium grade lineaments are mostly limited within the depth range of 2.5–25 km. However, relatively longer crustal-scale lineaments are highlighted at a greater depth of ∼30 km or more. Euler depth solutions of Bouguer gravity indicate a density heterogeneity interface in the depth range of 12–23 km that corroborates well with upward continuation results and past seismicity data. We have estimated Moho depth using the Parker-Oldenburg algorithm from the Bouguer gravity data. It delineates nine thin crustal structural features (S1–S9), twelve thick crustal structural features (D1-D12) and eleven crustal-scale lineaments (L1-L11) based on the distinct variations of Moho depths and their extensions. The b-value analysis indicates the highest order of b-value in the 15–30 km depth range, mainly in Mikir hill (∼1.7), the eastern part of Shillong Plateau (1.5) and the Indo-Burma ranges (2.3) that perhaps correlate well with the heterogeneity interface delineated by Euler depth solutions. The Eastern Himalaya generally exhibits characteristic low-Bouguer anomaly, high lineament density, low to moderate b-value, and deep-Moho/thick crust. The seismic stress model estimated using the finite element approach exhibits higher-order stress (∼34 MPa) around the central part of Shillong Plateau and at the northern part of Mikir hill with an SW-NE trend. Seismic stress correlates well with the regional lineament trend, GPS velocity/strain direction and low b-value, the joint analysis help to delineate a possible zone of future large earthquakes.