Indirect Estimation of Local Soil Response in Light of Past as well as Recent Earthquakes in the Shillong Plateau

Abstract

The northeastern region of India is a seismically active zone. The Shillong Plateau (SP) located in the southwestern portion of northeast India surrounded by an intense network of active tectonic faults. The 1897 Assam earthquake (EQ) (MW 8.1) and other major EQs (MW ≥ 7.0) had originated in the faults surrounding the SP, thus highlighting the SP as a zone of high seismicity. During these past major to great EQs, widespread damages in the form of destruction to buildings, excessive ground shaking, uneven settlements, occurrence of ground fissures, and sand vents were reported across the SP and its adjoining regions. Such large-scale catastrophes during an EQ are the combined effect of the ground motions generated during an EQ and its modification by the subsoil at a site. Presence of local soil influences the frequency content, duration, and amplitude of the ground motions generated during an EQ. To understand the ground motion amplification potential at a site due to local soil, recorded ground motions, dynamic soil properties, in situ subsoil characteristics, etc., are required to be known at the site of interest. For majority of location, however, regional dynamic soil properties and in situ subsoil properties are not readily available. In the absence of regional dynamic soil properties, site response studies considering available dynamic soil properties from other regions are followed worldwide. In the present study, response of local soil in the SP is assessed considering the observed ground motion scenario during different EQs at selected sites. Ground motion amplification during each EQ is determined considering the peak horizontal acceleration (PHA) at the bedrock level and the peak ground acceleration (PGA) at the ground surface level. The PHA during each EQ is estimated using regional ground motion prediction equations (GMPEs). The PGA on the other hand is derived based on felt intensities during various EQ at considered sites from isoseismal maps. Thus, the response of in situ soil over a wide range of ground motions is assessed based on actual scenario developed at the surface. This work will be helpful to understand the response of soil during probable future EQs in the absence of regional dynamic soil properties for the SP.