Estimation of Earthquake Ground Motion in Padang, Indonesia

Several powerful earthquakes have struck Padang during recent years, one of the largest of which was an M 7.6 event that occurred on September 30, 2009 and caused more than 1000 casualties. Following the event, we conducted a questionnaire survey to estimate the shaking intensity distribution during the earthquake. About 500 residents of Padang were interviewed. The residents received explanations for each item on the questionnaire from the interviewers, and answers were filled in directly on the answer sheets. From this survey we produced a map of the shaking intensity distribution in Padang. In addition to the questionnaire survey, we performed single observations of microtremors at 110 sites in Padang. The results enabled us to estimate the site-dependent amplification characteristics of earthquake ground-motion. We also conducted a 12-site microtremor array investigation to gain a representative determination of the soil condition of subsurface structures in Padang. From the dispersion curve of array observations, the central business district of Padang corresponds to relatively soft soil condition with V-s30 less than 400 m/s, the predominant periods due to horizontal vertical ratios (HVSRs) are in the range of 2.0 to 4.0 s, and the seismic intensity obtained is upper 5 (5+) in the JMA(i) scale. By making these observations, we can obtain a relationship between soil types, predominant periods and seismic intensities.