Horizontal-to-Vertical Spectral Ratio (HVSR) Method for Earthquake Risk Determination of Jakarta City with Microtremor Data

Abstract

Jakarta is the capital of the Republic of Indonesia which lies above a thick sedimentary basin. Geographically, Jakarta is 200 km away from the Indo-Australian subduction zone that sinks under the island of Java. There are many vital buildings and with the thick sediments underlying the city of Jakarta. Therefore, this region has a considerable seismic vulnerability. This will be dangerous if there is an earthquake that has the same frequency as the natural frequency of the building. It will cause a resonance resulting in amplification of seismic waves in the area. Each building has a different natural frequency, one that affects is the height of the building. To characterize the subsurface structure of the Jakarta Basin, microtremor data processing was obtained from the recording of 95 stations which was operated in October 2013 - February 2014 using Horizontal-to-Vertical Spectral Ratio (HVSR) method. HVSR is a method for obtaining subsurface information from single station measurements by comparing the Fourier spectrum of horizontal components to its vertical components. This ratio is a function of the frequency that will produce the H/V curve. The dominant frequency value on the HVSR curve represents the natural frequency of the area. The Seismic Vulnerability Index (Kg), which serves to determine the soil weak zone, can be calculated from the H/V curve. The dominant frequency value maps generated for the Jakarta area range from 0.2-0.22 Hz for low frequencies and 1-8.6 Hz for high frequencies. The large dominant frequency correlates with the thin sediment layer. Based on the frequency range, the south and northwest regions of Jakarta have relatively shallow basement depths compared to other regions. The resulting amplification value map can be divided into 4 maps with different period ranges. From the four maps, the North Jakarta area or the area around Jakarta’s coastline is most at risk of amplification with an H/V value up to 11 because the area is associated with alluvial deposits and coastal sediments. The seismic vulnerability distribution map in Jakarta City ranges from 15-850 relatively high in northeast and north of Jakarta.