BIDIRECTIONAL AND DIRECTIVITY EFFECT IDENTIFICATIONS OF SYNTHETIC GROUND MOTIONS AT SELECTED SITE IN YOGYAKARTA CITY, INDONESIA

The purpose of this paper is to identify the possibility of the presence of directivity effects on synthetic ground motions on selected sites in the Yogyakarta City of Indonesia. This is because the dominant source of the earthquake that occurred on May 27, 2006 was triggered by the Opak river fault which was located just over 10 km from the city of Yogyakarta. According to [1] the epicenter distance as mentioned falls between near fault to near field term, in which the directivity effects might occur on the site. For this purpose, bidirectional synthetic earthquake ground motions need to be presented. Probabilistic Seismic Hazard Analysis (PSHA) has been carried out and Uniform Hazard Spectrum (UHS) has been obtained. Twelve pairs of synthetic ground motions (SGM) were obtained after spectral matching of UHS. A pair of SGM consists of two orthogonal horizontal motions i.e east-west (EW) and north-south (NS) component which are visually presented in bidirectional form. After identification, no tendency was found for large amplitude long period pulses as a criterion of the possibility of directivity effects occurrence [2] on the 12 pairs of SGM that have been obtained. In addition, several seismic intensity measures (SIM) for 12-pairs synthetic ground motion have been calculated and then to be correlated with the concept of potential destructiveness PD proposed by [3].