V028-0003
Source mechanism of the seismic crisis preceding the 2017 Mt. Agung explosive eruption

Friday, 11 December 2020
Poster
David P. Sahara1, Puput P Rahsetyo2, Andri D Nugraha1, Sri Widiyantoro3, Zulfakriza Zulfakriza3, I Gusti Bagus Eddy Sucipta4, Irwan Meilano5, Ardianto Ardianto6 and Devy Kamil Syahbana7, (1)Institut Teknologi Bandung, Global Geophysics Research Group, Bandung, Indonesia, (2)Bandung Institute of Technology, Geophysical Engineering, Kota Bandung, Indonesia, (3)Bandung Institute of Technology, Global Geophysics Research Group, Bandung, Indonesia, (4)Bandung Institute of Technology, Petrology, Volcanology, and Geochemistry Research Group, Bandung, Indonesia, (5)Institute of Technology Bandung (ITB), Bandung, Indonesia, (6)Bandung Institute of Technology, Laboratory of Volcanology and Geothermal, Bandung, Indonesia, (7)Center of Volcanology and Geological Hazard Mitigation, Bandung, Indonesia
Abstract:
After more than 50 years of slumber, Agung volcano erupted explosively on November 21, 2017. The eruption followed a seismic crisis that began more than two months prior when nearby earthquakes increase significantly.

We worked on the regional seismic data recorded by Meteorological, Climatological and Geophysical Agency (BMKG) between 14 September to 9 October 2017. There were 9 BMKG stations surrounding Mt. Agung available. 807 events with 5251 P wave and 3057 S wave phase had been successfully identified. Waveform cross-correlation and double-difference technique were applied to reduce the error in phase picking as well as to improve the accuracy of the event’s location. We found that the seismic crisis was dominated by distal volcano-tectonic earthquake swarm located at 3-10 km north-west from the summit, between Mt. Batur Caldera and Mt. Agung. The spatial and temporal distribution of events correlated well with the magma plumbing system modeled in the previous studies.

The moment tensor was also estimated using full waveform inversion technique for two earthquakes with magnitude greater than ML 4. Moment tensor results from the two events show a complex interaction between tectonic and volcanic effect in this swarm. Those events had a thrusting faulting regime which consistent with the regional structure and a significant component of the non-double couple. Interestingly, prior to both major events, the VT events are mainly located below 7 km MSL and start to form a shallow seismic cluster at a depth between 2-7 km afterward. A further study is required to infer the mechanism sequence of Mt. Agung volcano seismicity leads to its eruption. We hypothesized that the intruded magma may have heated fluid in the vicinity of pre-existing fault, increased its pore pressure, and together with tectonic stresses triggered an earthquake swarm that failed in the context of regional structures.

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