Base-rock elevation measurement using single and array microtremor investigation

Abstract

Site-specific ground motion analysis is a method for developing surface ground motion and surface spectral acceleration models. The analysis is performed by conducting propagation analysis from baserock to surface elevation. The important information needs for site-specific ground motion analysis is the position of base-rock or bedrock below the surface level. This paper presents the investigation results of baserock elevation at the Diponegoro University Semarang, Indonesia. The background reason for the base-rock investigation of this university area is because this area is located close to three seismic sources (Semarang, Lasem and Demak shallow crustal faults). Seismic microzonation caused by all three seismic source scenarios are still on-going. One of the important analysis used for seismic microzonation at the study area is site-specific ground motion analysis. The base-rock elevation prediction in the study area was performed using a combination of single and array microtremor tests and soil boring investigations. The base-rock elevation prediction developed using microtremor tests was verified using the soil profile developed from soil boring investigation. Based on soil boring and microtremor investigations, the base-rock elevation at the university area was predicted at 30 to 60 m below the surface level.