Estimation of Strain Rate in the Opak Fault with Postseismic Correction After the 2006 Yogyakarta Earthquake

The 2006 Yogyakarta earthquake was indicated as a result of the Opak fault which is still active today. Global Navigation Satellite System (GNSS) measurements as soon as possible after an earthquake are very important because it is an early indication of postseismic deformation. The characteristics of postseismic deformation can be modeled by logarithmic equations with the assumption that the deformations that occur due to the influence of afterslip. In this study, fifteen periodic GNSS data provided by Geodetic Laboratory Universitas Gadjah Mada (UGM) and four continuous GNSS data provided by Geospatial Information Agency of Indonesia (BIG) were used to determine the velocity and strain rate around the Opak fault. The result showed that the Yogyakarta region especially in the Opak fault area has been deformed with the variying horizontal velocity rate. The stations move to the southeast direction as an effect of the movement of the Eurasian and Indo-Australian plates. Based on the computation of the principal strain using modified least square method, strain rate value in the Opak fault area are less than 1 micro strain/yr with extensional strain is dominated. Some stations especially on continuous stations have decreased of strain values and their standard deviasion after corrected by postseismic parameters in logarithmic functions. It shows that to captured the postseismic deformation would be better to use continuous data.