Seismic Travel Time Tomography Beneath Agung Volcano, Indonesia from October to December 2017: A Preliminary Results

Abstract:

Mt. Agung had a paroxysmal eruption with VEI 5 causing around 1,700 death casualties in 1963, but until now information about the underlying magma plumbing system is still very little known due to lack of monitoring instruments. The most recent eruption in November 2017 was the first period in the history of the Mt. Agung eruption to have a sufficient seismic observation network to conduct travel-time tomographic inversion to obtain 3D seismic velocity structures (Vp and Vp/Vs ratio). We used ~1,800 volcanic tectonic (VT) events with ~8,000 P- and 8,000 S-wave arrival times recorded by 8 seismic stations in the observation time span from October to December 2017. The locations of VT events were located using a maximum likelihood estimation algorithm. Only local earthquakes that have spatial uncertainty less than 5 km were used in the next process. An optimum 1D seismic velocity model derived from VELEST is used as an initial velocity model for tomographic inversion. Our preliminary results show that there were some prominent event clusters located NW of Mt. Agung, which is between Mt. Agung and Mt. Batur, with earthquake focal depths shallower than 20 km below sea level. The hypocenter distribution of VT events shows several features that may be related to shallow magma reservoirs. We also found the finger-like features with moderate Vp and relatively high Vp/Vs that may indicate the possible magma conduit.