Dimas Sianipar, Renhard Sipayung, Emi Ulfiana

The triggering mechanism of swarm-like seismicity in the Java Island of Indonesia is generally not well understood. Understanding earthquake swarm phenomena and monitoring in various tectonic settings can be improved by the detection of micro-earthquakes; however, such a catalog is not available due to various reasons including the existing limited seismic network and using an outdated algorithm for events detection. In this study, we analyze the seismic waveforms and explore the detection capability of small earthquakes during the August 2019 earthquake sequence near Mt. Salak (West Java) by using the known matched filter technique (MFT) and relocated events as templates. We analyze continuous waveforms from seven broadband seismic stations in a 150 km radius around the source center and for a ~1 month of data. Our derived complete catalog enables us to analyze the frequency-magnitude distribution of the sequence as well as the spatiotemporal evolution of micro-seismicity. The six largest events were hybrid-like-type volcano-tectonic earthquakes with oblique-thrust mechanisms. The relocation procedure shows that all of the events are located within a small area of ~2x2 km^2, probably bounded at 9-12 km of depth. The pattern of seismicity distribution was not clear, however, focal mechanisms might indicate N/NW or E/SE-trending orientation with a steep plane. We detect 280 additional micro-earthquakes to the improved catalog. The b-value of the sequence is close to 1.1, typical for many volcano-tectonic events. We show that the swarm might be initiated by the fluid intrusion into the seismogenic zone while the stress changes from the largest event affected the evolution of swarm.