SEISMIC MICROZONATION STUDIES CONSIDERING LOCAL SITE EFFECTS FOR YOGYAKARTA CITY, INDONESIA
Keywords:
Peak ground acceleration, seismic code, attenuation relationship, microtremor, site responseAbstract
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 before
another 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 code
with 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 city
has 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.