[S-VC28] Active Volcanism

[SVC28-17] Waveform inversion of the ultra-long-period seismic event associated with a ground tilt motion during the eruption of Mt.Kusatsu-Shirane, Japan, on January 23, 2018

*Ryohei Takahashi¹, Yuta Maeda¹, Toshiki Watanabe¹ (1.Graduate School of Environmental Studies, Nagoya University)

Keywords:ultra-long-period event, waveform inversion, Mt.Kusatsu-Shirane

We conducted waveform inversion of an ultra-long period (ULP; ~240 s) event associated with the phreatic eruption of Mt. Kusatsu-Shirane on 23 January 2018. We used broadband seismic and tilt records from three stations surrounding the eruption site. The horizontal components of the broadband seismic records were severely contaminated by tilt motions. We applied a waveform inversion algorithm that can take both translational and tilt motions into account. To reduce the number of free parameters, we assumed a tensile crack source and conducted grid searches for the centroid location and orientation of the crack. Among the six traces of the horizontal broadband seismic records, one trace (N.KSHV.BN) showed the maximum amplitude at a time different from the others. It is uncertain if this time difference is caused by true volcanic processes. Therefore, we used the following four datasets: (1) all the broadband seismic data (nine traces), (2) broadband seismic data excluding N.KSHV.BN (eight traces), (3) all the broadband seismic and tilt records (15 traces), and (4) all the broadband seismic and tilt records except for N.KSHV.BN. Waveform inversion results from each dataset consisted of two or three local minimum centroid locations with similar residuals between observed and synthetic waveforms, giving a total of 11 candidate sources. However, either the location, orientation, or size of most of these candidate sources was inconsistent with tilt records around active craters, locations of hydrothermal fluids expected from a resistivity structure, volcanically active regions, source depths, or surface crater locations. The most likely source is an N-S opening vertical crack near the eruptive craters. The source time function showed rapid inflation of 105 m3 of the crack, followed by a slow deflation starting at 9-12 s before the eruption onset. An interpretation is that a volcanic fluid migrated from depth to the surface through the estimated crack during the eruption.

This work was supported by JSPS KAKENHI Grant Number JP19K04016. We used data from National Research Institute for Earth Science and Disaster Resilience.