Site Characterisation in the Proximity of Active Indo-Burma Subduction Zone Using Geological and Geophysical Methods: A Case Study from Champhai Town Area of Mizoram State in Northeast India

ABSTRACT The Mizoram state of the North East Region (NER), India, located in proximity to the Indo-Burma subduction zone, recently experienced an earthquake swarm; eight severely felt earthquakes (Mw 5.0–5.9) occurred during April-October, 2020, around the Champhai town area, which caused damage to several buildings and ground cracks with maximum reported MSK intensity VIII. The largest event, Mw 5.9, at depth 15 km, occurred on April 16, 2020. The present study deals with site characterisation of the Champhai town area by geological and geophysical investigations. The geological investigation involves detailed mapping of the lithounits, and the geophysical investigation includes shallow (down to 30 m) shear wave velocity (Vs30) and site amplification (SA) measurements over about 50 km2 in the town. Some 23 sites are selected for Vs30 measurements using the Multichannel Analysis of Surface Wave (MASW) technique. The observed Vs30 ranges from 200 m/s to 1060 m/s, classifying the area into three categories: Class-B (hard rock), Class-C (soft rock), and Class-D (stiff soil) as per National Earthquake Hazard Reduction Program (NEHRP) guidelines. Geologically, the class B and C sites are represented by arenaceous and argillaceous Tertiary rocks, respectively, whereas the class D is represented by the Quaternary sediments. Site amplification (SA) studies, on the other hand, are carried out by making ambient noise surveys at some 52 selected stations using a digital seismograph. The ambient noise data are analysed by the Nakamura method of H/V spectral ratio. The overall variation of peak amplification of the sites ranges from 1.26 to 6.65. A good correlation is obtained; the observed Vs30 and site amplification (SA) maps are fairly comparable with the detailed geological or lithounit map. These maps are very useful for seismic hazard micro-zonation of the town area to mitigate seismic hazard risk.