Analysis of swarm earthquakes around Mt. Agung Bali, Indonesia prior to November 2017 eruption using regional BMKG network

Abstract

Mt. Agung, located in Karangasem-Bali, Indonesia, had a significant increase of swarm earthquakes from September 2017 until the recent eruption in November 2017. To analyze the seismic swarm and its correlation with the magmatic movement, we worked on the regional seismic data recorded by Agency for Meteorology, Climatology and Geophysics of Indonesia (BMKG) between September 14 to October 20, 2017. P-and S-wave phases of the swarm events had been manually picked. In total, 804 events in the time period of September 14 to October 20, 2017 were successfully determined. To improve the location precision, the double-difference relocation method was performed. We identified most of the events as Volcano-Tectonic type A (VT-A) earthquakes and located between Mt. Batur and Mt. Agung. Those events form a cluster striking in NE-SW direction at a depth between 2 and 20 km. Focal mechanism solutions for selected events below Mt. Agung show a thrust and strike-slip faulting regime. Interestingly, a trend of event propagation toward the summit of Mt. Agung was observed. The frequency of VT-A event occurrences is significantly increased at the later stage of the swarms. We concluded that the increased seismic activity in Mt. Agung was due to the migration of magma from the deep chamber to the shallow reservoir.

Use of Local Seismic Network in Analysis of Volcano-Tectonic (VT) Events Preceding the 2017 Agung Volcano Eruption (Bali, Indonesia)

“…and Gunawan et al (2020), seismic activity on Agung began to increase significantly in early September, with the number of detected VT events reaching more than 700 events per day for 4 weeks. The seismic crisis stopped at the end of October (insert Figure 4).…”

“…Agung is one of the most active volcanoes in Indonesia and is located on the island of Bali. After more than 50 years of slumber, Agung Volcano erupted explosively on November 21, 2017 (PVMBG, 2017;Albino et al, 2019;Syahbana et al, 2019;Gunawan et al, 2020). The last major eruption happened in 1963; with a VEI 5, it was described as one of the largest eruptions in the twentieth century (Zen and Hadikusumo, 1964).…”

“…The 2017 eruption followed a "seismic crisis" that culminated in September 2017 when local earthquakes numbered more than 800 events per day (Albino et al, 2019;Syahbana et al, 2019;Gunawan et al, 2020). Due to the increasing seismicity, the Center for Volcanology and Geological Hazard Mitigation of Indonesia (CVGHM) raised the volcanic alert level (VAL) to Level 2 on September 14, 2017, this then went to Level 3 on September 18, 2017, as seismicity continued to accelerate rapidly; the Real-time Seismic Amplitude Measurements (RSAM) values peaked on September 22, 2017, prompting the CVGHM to elevate the VAL to Level 4 (the highest level).…”

“…So far, there is no published catalog of VT events preceding the Agung 2017 eruption obtained using the local seismic network. Previous publications employed the regional Indonesian Meteorological, Climatological, and Geophysical Agency (BMKG) catalog in analyzing the magma migration beneath Agung Volcano, e.g., Syahbana et al (2019) used catalog from BMKG as one of the inputs for their conceptual model, and later Gunawan et al (2020) relocated the BMKG catalog using the double-difference method. In this study, for the first time, we processed the recorded waveform data of the local CVGHM seismic station network and estimated the hypocenter locations of VT events preceding the 2017 eruption.…”

Characterization of the Mamasa Earthquake Source Based on Hypocenter Relocation and Gravity Derivative Data Analysis

“…Earthquake swarms are series that are formed without a specific mainshock. They are caused by volcanic activity [1,2] and fluid movement in the hydrothermal system [3] and geothermal system [4]. Furthermore, earthquake swarm activity can occur in active fault areas [5].…”