Padang is the capital city of West Sumatra Province located on the west coast of Sumatra Island, Indonesia. The seismicity around Padang is quite active, and more than 1000 casualties occurred due to the M 7.6 earthquake of 30th October 2009. Besides, the existence of a seismic gap in the off-shore of Padang, where the plate boundary between the Eurasian and the Indo-Australian plates, has been pointed out and occurrence of an M 8.0 class earthquake is expected. In 2008, Engineers Without Borders Japan (EWB-JAPAN), a nonprofit organization (NPO) set up three earthquake ground motion observation sites in Padang and one in Bukittinggi approximately 80 km northward from Padang. Subsequently, one more site was set up in the campus of Padang State University. Currently, two sites in Padang are non-operational. So far, 86 earthquake ground motions have been recorded in total. In the present paper, 36 records whose horizontal PGA exceed 5 cm/s/s were selected, and Fourier analyses of them were conducted. The dominant frequencies of analyzed earthquake ground motion records coincided approximately the peak frequencies of the microtremor horizontal-to-vertical spectrum ratio (HVSR) obtained by the past study. Furthermore, the time-frequency analysis of two records obtained at two sites on the deep sedimentary ground was carried out. The results showed that surface waves of 2 Hz were dominant in the fat area along the coast of Padang. For more than ten years have elapsed since the installation of the observation sites, urgent maintenance works for the observation equipment are strongly required.

1 Introduction

Padang, the capital city of Western Sumatra Province, Republic of Indonesia, is located on the west coast of the Sumatra islands where the seismic activity is very high (Figure 1). The earthquake of M 7.5 occurred at the offshore Padang on 30th September 2009 and resulted in more than 1,000 casualties. It is expected that an M8 class earthquake will take place at the plate boundary of the offshore Padang shortly and will cause severe damage in Padang (Natawidjaja et al., 2006).

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