MICROZONATION OF SEISMIC PARAMETERS IN GEOLOGICAL FORMATION UNITS ALONG THE OPAK RIVER USING MICROTREMOR MEASUREMENTS

Abstract

The 2006 Yogyakarta earthquake, with a magnitude of Mw 6.3, caused significant damage primarily on the west side of the Opak River. The Opak River area encompasses formations that exhibit distinct responses to seismic wave propagation. Formations consisting of unconsolidated and young sedimentary materials tend to experience wave amplification during seismic events. This study aims to investigate the seismic wave characteristics within each formation unit using microtremor measurements. The seismic wave parameters analyzed include dominant frequency, amplification factor, shear wave velocity, weathered layer thickness, seismic vulnerability index, and lithology based on N-SPT data. The study utilized 190 microtremor data sets and 7 N-SPT data sets. The microtremor data was processed using the Horizontal to Vertical Spectral Ratio Nakamura method, while the shear wave velocity data was processed using the Imai and Tonouchi approach. The findings revealed higher amplification factors in areas dominated by unconsolidated sedimentary materials and young formations. Quaternary formations are dominated by unconsolidated sedimentary material composed of sand, silt, clay, and breccia. Formations with a tertiary age are composed of more complex lithologies such as breccia-tuff, dacite tuff, andesitic tuff, volcanic breccias, lavas, siltstone, sandstone, and conglomerate. These conditions make Quaternary formations have seismic parameter characteristics such as lower dominant frequency, higher amplification factor, thicker sediment layer thickness, and higher seismic vulnerability index compared to Tertiary formations. Quaternary formations correlate with damage to buildings after the Yogyakarta earthquake, with an average shear wave velocity character of 279-293.67 m/s, and are in a zone with a vulnerability index >20. Tertiary formations demonstrated higher seismic resistance compared to Quaternary formations, indicating their relative stability.