Imaging the Subsurface Structure of Mount Agung in Bali (Indonesia) Using Volcano-Tectonic (VT) Earthquake Tomography

The eruption of Mt. Agung in 2017 provided a vital opportunity for scientific study due to the sufficient seismic observation networks in place. This study investigates the subsurface structure of Mt. Agung in Bali, one of the highest-risk volcanoes in Indonesia, using local seismic tomography. The research team conducted travel-time tomography using P- and S-wave arrival times of volcano-tectonic (VT) events to determine the three-dimensional (3D) Vp, Vs, and Vp/Vs ratio structure beneath Mt. Agung. They used 1,926 VT events, with corresponding 9,482 P- and 8,683 S-wave arrival times recorded by eight seismic stations over an observation time spanning from October 18 to December 31, 2017. The team obtained the hypocenter solution for VT events using the maximum likelihood estimation algorithm and used an optimum 1D velocity model as input for the Joint 3-D seismic tomographic inversion. Local earthquake tomography revealed five anomalous regions that are useful to describe the overall seismic activity around Mt. Agung. The researchers interpreted these anomalous regions qualitatively due to limited data resolution in this study. They successfully localized a high Vp/Vs ratio (∼1.82), low Vs (−1.9%), and high Vp (+3.8%) within a low seismicity zone at depths between 2 and 5 km below the Mt. Agung summit, which may be related to a shallow magma reservoir. There is also an anomalous region between Mt. Agung and Batur with moderate to high Vp/Vs ratios (1.76–1.79) where most of the earthquakes recorded before the 2017 eruption originated. The results of this study provide new insights into the subsurface structure of the magma plumbing system beneath Mt. Agung, which can be used to improve the quality of determining the location of the hypocenter and source modeling for future eruption forecasting. The study's findings are significant, given the dense population and intense socioeconomic activities around the volcano, as well as its status as one of the most popular tourist destinations in Indonesia. The research team's use of local seismic tomography has provided a detailed understanding of the volcano's internal structure, which can inform strategies for volcanic disaster mitigation. Mt. Agung is an active Sunda arc stratovolcano formed in the subduction zone where the Indo-Australian plate subducts below the Sunda block. The volcano's edifice consists of an almost symmetrical cone, aligned along a NW-SE direction with the nearby Abang and Batur-Pawon volcanoes. Mt. Agung's composition range is limited to basaltic andesite, and occasionally andesite, with no evidence for large Plinian-style fall or ignimbrite deposits. In contrast, Mt. Batur has a broader compositional range from basalt to rhyolite, with the composition of erupted rocks becoming more mafic over time. Several studies have been conducted to infer the structure of the volcanic features. A petrological analysis using mineral-melt thermobarometry data of the 1963 lava flows shows the existence of two major magma storage regions: the first region is located around the Moho depth at about 18–22 km, and the second one is in the shallower region at about 3–7 km depth. The most recent study of the ambient seismic noise tomography around the areas also shows a low-velocity anomaly zone between Mt. Agung and Mt. Batur at a depth of about 2 km below the surface. The authors interpreted the low-velocity zone as volcanic deposits from past eruptions in the topographic saddle between Mt. Batur and Mt. Agung and the presence of over-pressurized hydrothermal fluids where seismicity may be induced at shallow depths.