Three-Dimensional Seismic Velocity Structure of the Lithosphere Beneath the Northeast Indian Region Based on Tomographic Inversion Using Local and Regional Earthquake Data

English Abstract:

This study is an attempt to clarify the tectonic interactions and the underlying dynamics based on estimation of considerable lithospheric seismic velocity anomalies persisting in the Northeast Indian region. This in turn allows us to arrive at a common consensus on the geodynamics of the study region. We have analysed the data obtained from ISC bulletin for the years, September, 1998 to July, 2013, contributed by regional seismic networks in the northeast India (run by the India Meteorological Department, IMD, under the Ministry of Earth Sciences, Govt. of India). In total, we have used the data from available 25 seismic stations installed in the selected study region. In total, 11357 of picks including 6157 P- and 5200 S-arrival times from 1459 events in the study region (lying between 21⁰-32⁰ N to 87⁰-101⁰E) were selected for this study. The results are well resolved up to the depth of 90 km.

The tomographic image shows very heterogeneous and complicated structure of the lithosphere. The Shillong-Mikir massif region shows up as a high in both P and S velocity anomaly patterns and low Vp/Vs region surrounded by lower velocity and higher Vp/Vs material. Both of these bodies have geological affinity with the Indian craton. From the tomographic image we interpret that they are a part of the Indian craton at present detached from it. The Eastern Himalayan region has lower P as well as S wave velocity compared to its surroundings from near surface up to about 70 km depth. This correlates well with the concept of isostatic model of the Himalayas, where it is expected that high mountains have low density roots. Low density material has low seismic velocity. The P and S wave velocity anomaly pattern for the Indo-Burma ranges show complicated pattern at all depth ranges. In some places along these ranges Vp/Vs ratio is very high. The P and S velocity anomalies are very low and Vp/Vs ratio is very high in the Tripura Fold Belt region. The observed seismic anomalies pattern show obvious correlation with the subsurface and fit well with the existing tectonic elements in the study region. There is some plausible corroboration between the observed seismic anomalies and the persisting gravity anomalies in the study region.