Excessive seismicity over a limited source: the August 2019 earthquake swarm near Mt. Salak in West Java (Indonesia)
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ORIGINAL ARTICLE
Excessive seismicity over a limited source: the August 2019 earthquake swarm near Mt. Salak in West Java (Indonesia) Dimas Sianipar
&
Renhard Sipayung & Emi Ulfiana
Received: 4 November 2019 / Accepted: 15 September 2020 # Springer Nature B.V. 2020
Abstract The triggering mechanism of swarm-like seismicity in the Java Island of Indonesia is generally not well understood. Understanding earthquake swarm phenomena 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such a catalog is not available due to various reasons including the existing limited seismic network and using an outdated algorithm for event detection.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 Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10950-020-09957-w) contains supplementary material, which is available to authorized users. D. Sianipar (*) Sekolah Tinggi Meteorologi, Klimatologi, dan Geofisika (STMKG), Badan Meteorologi, Klimatologi, dan Geofisika (BMKG), South Tangerang, Indonesia e-mail: [email protected] R. Sipayung Stasiun Geofisika Banjarnegara, Badan Meteorologi, Klimatologi, dan Geofisika (BMKG), Banjarnegara, Indonesia E. Ulfiana Stasiun Geofisika Denpasar, Badan Meteorologi, Klimatologi, dan Geofisika (BMKG), Denpasar, Indonesia
the sequence as well as the spatiotemporal evolution of micro-seismicity. The six largest events were hybridlike-type volcano-tectonic earthquakes with obliquethrust mechanisms. The relocation procedure shows that all of the events are located within an area of ~ 2 × 2 km2, 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/SEtrending 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. Keywords Mt. Salak . Volcano-tectonic swarm . Hypocenter relocation . Matched filter . b-value
1 Introduction The seismicity patterns of any swarm might be related to fluid intrusion into the seismogenic zone causing the temporal evolution that is not characterized by a distinct mainshock and hence cannot be described by the law of aftershock decay rate such as modified Omori’s law (Hill 1977; Parotidis et al. 2003; Hainzl 2004). Additionally, the evolution of an earthquake swarm
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