Seismicity Analysis and Velocity Structure of the Two-Phase Geothermal Field in West Java, Indonesia: Preliminary Result

Abstract

We observed 754 microearthquake data by at least 15 stations for six months as a tool to monitor and characterize the subsurface condition in two-phase geothermal field in Southern Bandung. The data were reduced to 417 events having azimuthal gap less than 180°. We determine the initial earthquake locations in this study by using Geiger method and we update the hypocenter locations by performing simultaneous inversion for hypocenters and 1D velocity structure followed by 3D tomographic inversion. We obtained the velocity structure by utilizing TomoDD to perform 3D tomographic inversion. There are four clusters of seismicity observed in this study which we suspected related to the injection, production, and tectonic activity of the study area. The southern cluster is consistent with the location of injection wells and spread from 1 km depth to 8 km depth, indicating that MEQ events were triggered by injection activity. The production area is characterized by a denser population of events and the events were distributed closely ranging from 1 to 3 km depth. We suspect that the third cluster is caused by development activity of steam dominated geothermal field omit in the east of our main field. The fourth cluster is located in the southernmost of our area and suspected caused by tectonic activity. Low Vp/Vs values near to the surface are observed in the north direction and coincide with the location of the first cluster. The location of anomaly is highly corresponded with the production area and the low value of Vp/Vs anomaly is interpreted as the steam zone.