| Summary: | The objective of this study was to analyze characteristics of seismic vulnerability index to several landforms. This study was also intended to explain the relationship between the 2006 earthquake damage distributions and local site effect within Bantul Graben. Microtremor measurement were done using a portable digital seismograph equipped a velocity sensitive sensor having 2 horizontal component (NS and EW direction) and a vertical component. Sampling interval was 1/100 second and the length of each record was 30 minutes. Computation of the spectral ratio was using The GEOPSY Software based on Horizontal to Vertical Spectral Ratio (HVSR). The results of study showed that seismic vulnerability index at Bantul Graben varied from 0.1 to 28.8. Generally, seismic vulnerability index changed following the changes of landform unit. Seismic vulnerability index have a trend to gradually smaller value from
fluvio volcanic foot plain and volcanic foot plain (1.3 - 28.8) to structural mountain (0.1 -1.8) at the east and west of the Bantul Graben. Therefore, the distribution of earthquake damage that was parallel with the Opak Fault was not caused by reactivation of the Opak Fault as commonly geoscientists predicted before. The "damage belt" occured after
the earthquake had a unique character as results of local site effect in the area.
Key words: seismic vulnerability index,
icrotremor, local site effect, HVSR.
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