Site Characteristics Based on Microtremors and Borehole Data in the Epicentral Area of the Central Java Earthquake of 2006

Abstract eng:
The Central Java earthquake of Ms6.3 occurred on May 27, 2006. Although the magnitude of this event was relatively small, the reported structural damage in the epicentral area was quite heavy, that is, more than 140,000 houses were totally collapsed or heavily damaged and about 5,700 people were killed. The purpose of the investigation performed by the collaborated survey team of Kyushu University in Japan and Gadjah Mada University in Indonesia is to understand the primary cause of this devastating damage. For the strong ground motions in the epicentral area we conducted microtremor measurements there. Not much geophysical exploration data was available before the earthquake and therefore we report the soil amplification characterization survey conducted in the region and consider effects of soil amplification on spatial damage distribution first. We found that the strength of ground shaking should not be so high, probably 7 to 8 in MMI seismic intensity scale and the observed local damage ratio variation from site to site should be primarily controlled by the local site conditions. We also use boring data to characterize the soil conditions at several sites inside the Yogyakarta basin. Based on the collected information we concluded that the devastating damage on the residential masonry houses should be attributed to their very low seismic resistance, even though soil amplification should be attributable for the observed spatial damage distribution.

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