Estimation of site-specific amplification factors and expected peak ground motion at Shillong city, Meghalaya: A deterministic approach

An attempt has been made to estimate the site amplification factors of different geomorphological units in Shillong city, northeastern India, based on the distribution of peak ground acceleration (PGA) due to maximum credible earthquakes that originated in nearby major faults. Empirical relations are used to determine the amount of shaking at base rock as a function of the magnitude of the earthquake (M-w), source depth (km), fault dimension (intraplate/interplate), velocity structure of medium and the distance from the source (km). The computed PGA values are convolved with the average shear wave velocity (V-s30) values at various geomorphological units of Shillong to ascertain the degree of amplification while travelling from base rock to the surface. The amplification for the highly dissected land in the city is found to be the maximum within a range of 2.77-2.92, while the high plateau segment is characterized by least values (2.01-2.16). Simultaneously, the effective ground motion mapped on the surface indicates the maximum value of 0.6-0.94 g for a probable earthquake of M-w 8.1 due to Dauki fault. Similarly, Dhubri and Kopili faults might produce a ground motion of 0.05-0.08 and 0.22-0.33 g for a maximum credible earthquake of M-w similar to 7.0, respectively.