Fundamental Frequency Anomaly Around Cimeta River, Padalarang, West Java

Abstract

Lembang Fault segment Cimeta and Cimandiri Fault segment Padalarang assumed to coincide at Padalarang, which lies at Ngamparah and Bojongkoneng district, Bandung, West Java. Padalarang is an area with high traffic and infrastructure conditions, such as freeway overpass named Cimeta bridge, rail bridge and soon-to-be tunnel of fast train Jakarta – Bandung. The presence of these faults becomes a threat to both residents and the infrastructures. We performed an investigation of seismic microzonation using a Horizontal Vertical Spectral Ratio in the area. Microtremor measurement has been done in 37 points widespread from the Cimandiri Fault segment Padalarang to Lembang Fault segment Cimeta. The method could provide information about the fundamental frequency and amplification factor of earthquake propagation to the site from the analysis of spectral ratio microtremor data. Fundamental frequency associated with the thickness of sedimentation meanwhile amplification factor indicates the impedance contrast at the site. The result shows that the fundamental frequency of Ngamprah and Bojongkoneng area around 1-6 Hz indicated deep sedimentation. However, we found an anomaly of the high fundamental frequency of 9 Hz at the Cimeta river. Therefore, we attempted to conduct another measurement around the area at different times and conditions. Three measurements of microtremor around the Cimeta river have shown that the area has high fundamental frequencies around 9-11 Hz. These results may indicate the presence of both bedrock and geological structure of Lembang Fault at the Cimeta river.