Shorelines Changes in Bengkulu City and Their Relation With Ground Shear Stain Based on Microseismic Data

Coastal erosion that occurred in the west part of Bengkulu city since 1917 has significantly changed the shoreline.Those signif­ icant changes are supposed having a relaation with the earthquake activities in the Indian Osean and around Bengkulu province. Earthquakes are frequent in this region resulting a variation ofthe Ground Shear Strain's values in some coastal areas and hence have impact on the environment. This study aims to (I) determine the rate of beach erosion that occurred in Bengkulu City , (2) determine the value ofthe Seismic Vulnerability Index of coastal segments, (3) know the Ground Shear Strain values in some coastal zones. For each zone, the microseismic data was acquired using a portable short period digital seismograph. We utilized the Horizontal to Vertical Spectral Ratio (HVSR) method to obtain the dominant frequency (f.,) and peak amplitude of HVSR spectra (A) to calculate the seismic vulnerability index values (K1 and the Ground-Strain Shear value (y). From this research we found that rate of coastal erosion on various types of beach was ranging from 4.4 to 16.1 m yr. The K1values are from 3.6 to 48.1. dr values are from 0.0026 to 0.0161