SEISMIC MICROZONATION STUDIES CONSIDERING LOCAL SITE EFFECTS FOR YOGYAKARTA CITY, INDONESIA

Abstract

An Mw 6.3 earthquake hit Yogyakarta, Indonesia in 2006, causing massive damage in this densely populated area. As a consequence, risk reduction efforts in terms of local seismic response and peak ground acceleration (PGA) mapping is needed to improve the spatial planning and early assessment beforeanother disastrous earthquake. The determination of PGA for microzonation study was conducted by referring to the Indonesian seismic code and empirical prediction by examining attenuation relationships considering the local site effects. The site investigation was conducted at 87 locations, comprising 13 core drillings and 74 microtremor measurements. Further, the nonlinear earthquake site response analyses were conducted to calculate the local seismic response occurring in a layered soil. Referring to the seismic codewith a 2% probability of exceedance in 50 years and site characteristics, the maximum considered earthquake geometric mean PGA for Yogyakarta City is 0.50-0.57g, while the southern area has higher PGA. The empirical prediction by attenuation relationships that considers the 6.3 Mw Yogyakarta earthquake 2006 with 50-100 year return period resulted in PGA of 0.14g to 0.21g. Meanwhile, the empirical prediction based on soil predominant period resulting from microtremor measurements shows that the southern part of the cityhas higher PGA of 0.2g to 0.3g whereas the northern part has lower PGA of 0.1g to 0.2g. The result of nonlinear earthquake site response analyses shows that the PGA at the southern part and the northern part of the city have higher PGA up to 0.38g. The results show the significance of local site effects and site response analysis in determining the earthquake characteristics in comparison to the present simplified empirical approach.