A Blind Thrust Fault Ruptured During the 10 October 2018 Situbondo, East Java, Indonesia, Earthquake Estimated Using GNSS Data

The 2018 Situbondo earthquake occurred along an unidentified fault source in East Java, Indonesia. Historical records from 1821 to 2018 show that no earthquakes before 2018 were recorded in this region. In this study, we use Global Navigation Satellite System (GNSS) data to estimate the coseismic displacements of stations in the study region. Significant coseismic displacements were estimated for stations located south of the epicenter. Using focal mechanism as preliminary information for the fault geometry, we conduct coseismic slip inversion on two possible fault sources, i.e., a northward dipping fault and a southward dipping fault. The seismic moment of the southward dipping fault, which is 2.7 × 10¹⁸ N·m (equivalent to Mw 6.2), is identical to the seismic moment from the seismic analysis. Also, displacement model of the southward dipping fault model better fit data than the northward dipping fault. We also find that high slip is concentrated at depths greater than 9 km. Our assessment of the fault suggests that it could rupture every 700 to 1700 years. Since the dilatation rate to the west from the epicenter is much higher, our result emphasizes the urgent need for further investigation of the Kendeng thrust, which may affect the megacity of Surabaya.