Crustal Structure Beneath Sumatra and Northeast India Inferred from Receiver Function Modeling

Goyal, Ayush (2016) Crustal Structure Beneath Sumatra and Northeast India Inferred from Receiver Function Modeling. Masters thesis, Indian Institute of Science Education and Research Kolkata.

PDF (MS dissertation of Ayush Goyal (11MS080)) AyushGoyal_11MS080_MS-Thesis.pdf - Submitted Version Restricted to Repository staff only Download (2MB)

Abstract

We estimated crustal shear wave velocity structure beneath five and ten broadband seismic stations of Sumatra and Northeast India, respectively, by using non-linear direct search approach Neighbiurhood Algorithm (NA) followed by joint inversion of Rayleigh wave group velocity and H-K stacking method on receiver functions of teleseismic earthquakes data. Inversion results from Northeast India show significant variations of thickness, shear velocities and Vp/Vs ratio in the crust. The inverted shear wave velocity models show crustal thickness variations from South, 32-36 km in Shillong Plateau, 38-40 in Assam Valley to North, ~44 km beneath Lesser Himalaya. Average Vp/Vs ratio in Shillong Plateau is less (1.7-1.76) compared to Assam Valley and Lesser Himalaya (~1.80). Average crustal shear velocity beneath the study region varies from 3.4 to 3.5 km/s. Sediment structure beneath Shillong Plateau and Assam Valley shows 1-2 km thick sediment layer with low V s (2.5-2.9 km/s) and high Vp/Vs ratio (1.8-2.1), while it is observed to be of greater thickness (4 km) in Lesser Himalaya. For both Shillong Plateau and Assam Valley shows thick upper and middle crust (10-20 km) and thin (4-9 km) lower crust. Vp/Vs ratio in Assam valley is higher (~1.73-1.84) compared to Shillong Plateau (~1.7-1.8). Lower crust velocity for both the region is observed to be high ~3.8-3.96 km/s (Vp/Vs 1.7-1.8). On the other hand, inversion results from Sumatra show significant variations of sediment thickness from 1 km beneath the backarc basin (station BKNI and PMBI) to 3-7 km beneath the coastal part of Sumatra region (station LHMI and MNAI) and Nias island (station GSI). Crustal thickness beneath Sumatra region varies between 27-35 km, with exception of 19 km beneath Nias island, with average crustal Vs ~3.2-3.4 km/s (Vp/Vs ~1.8). It is well known that thick sediments with low Vs (high Vp/Vs) amplify seismic wave even from a small-magnitude earthquake which can cause huge damage in the zone. Since, Sumatra is an earthquake prone zone; this study can also be helpful for seismic hazard assessment.

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