Middle value ground acceleration map and site effect in the Merapi sedimentary basin under the 2006 Yogyakarta, Indonesia earthquake

The purpose of this paper is to develop a ground acceleration map in Yogyakarta Special Province under the 2006 Yogyakarta earthquake and look for evidence of site effects that affect to the ground response and building damage. The earthquake was more than one and half decade away, but its ground acceleration map and soil-site effects are still unclear. An estimation of the peak ground acceleration map under the Yogyakarta earthquake was conducted but limited for upper bound value and not including the influence of the site effects (Widodo in Bull N Z Soc Earthq Eng 51(2):92–104, 2018). Middle value PGA map at YSP including the site effects has been successfully developed. The development of PGA map is based on the maximum ground acceleration at two control points, i.e., at YOGI and BJI stations (see Box 2 in Fig. 1) and the result of earthquake intensity Imm field surveying (Wijaya in Isoseismal, Kerentanan dan Rasio Kerusakan Bangunan Rumah Tinggal; Studi Kasus Gempa Bumi Yogyakarta 27 Mei 2006, 2009). Two ground motion prediction equations have been used to interpolate ground acceleration between two control points and produce an equation of the relationship between ground acceleration with earthquake intensity. The relationship between earthquake intensity Imm with distance L (Wijaya 2009) is then combined with the previous equation and lead to final equation. The ground acceleration in all point data in the field is computed by using the final equation. The contours of the PGA as obtained apparently do not form a circle but tend to follow the path of Opak river fault. Moreover, the maximum ground acceleration does not occur in the epicenter area but shifts about 14 km west of the epicenter. The maximum ground acceleration in Imogiri sub-district reaches 0.45 g, while in the epicenter is only around 0.17 g. Site effects phenomena have occurred in the field both based on the result of analysis and the result of horizontal–vertical spectral ratio (HVSR) tests in the field. HVSR results from field tests by using microtremor showed that in the epicenter was obtained that the site period T = 0.078 s (rocky site), sediment thickness H = 6.6 m and percentages of housing collapsed was only 14.5%. Meanwhile, in the Imogiri sub-district (about 14 km west of the epicenter) the value of site period T = 1.52 s (soft soil), sediment thickness H = 86.6 m and the percentage of collapsed building reached 77.6%. This means that irregularities have occurred in earthquake intensity, ground acceleration and building damage, which are the characteristic of the site effect occurrence (Montalva et al. in Earthq Spectra 32(3):1469–1488, 2016).Fig. 1The Aceh, Indonesia earthquake series of 26th December 2004