Ground Shear Strain and Rate of Erosion in The Coastal Area of North Bengkulu, Indonesia

摘要

North Bengkulu district has an area prone to earthquakes, because this area is very closed to the Sumatra subduction zone to the Eurasian tectonic plate. Coastal area in this region is experiencing with the very fast erosion. This is thought to have relation with earthquake-prone zones. This study aims to find the relationship between Ground Shear Strain (GSS) and rate of erosion in the coastal area of North Bengkulu. The data of coastal erosion rate was obtained by overlaying shoreline in 1947 and 2012. The GSS can be obtained by multiplying Seismic Vulnerability Index (SVI) and Peak Ground Acceleration (PGA) values around the shoreline. Seismic Vulnerability Index was obtained by processing microseismic data acquired using three component in short period of seismometers. The PGA was obtained from the historical earthquake and calculated by using Fukushima-Tanaka equation. The results show that the value of GSS varies between 0.0001 to 0.0055, and the SVI values is ranging from 1.2 to 16.1. In addition, we estimated that PGA value is 92 to 120 gal and the rate of erosion between 3.6 up to 5.8 m/yr. GSS value for each type of shore is 0.00046 for the shore of fine sandy flat, 0.0043 for shore of muddy flat, 0.0001 for shore of rocky flat, 0.0006 for shore of sandy rugged, 0.0003 for shore of steep lava rocky, 0.0014 for shore of steep rocky clay, 0.0011 for shore of tufa steep sandy stones, and 0.0014 for the shore of steep rocky tuffaceous clay. It is found that the GSS value depends on the type of coastal. In this case, flat coastal muddy show highest effect on the GSS. Both SVI and GSS can be estimated to be a quadratic relation to the erosion rate.