[SVC45-P11] Forward analysis of seismic waves accompanied by tilt ground motion during the eruption of Mt. Motoshirane, Japan, on January 23, 2018
Keywords:source mechanism, tilt motions
In this study, we calculated Green's functions of translational and tilt motions at three station sites, N.KSHV (Hoshimata), N.KSNV (Nikenya), and N.KSYV (Yazawahara)of V-net operated by the National Research Institute for Earth Science and Disaster Resilience. We used a 3D finite-difference method with a mesh size of 40 × 40 × 40 m . We assumed a compressional wave velocity Vp=2500 m/s, a shear wave velocity Vs=1443 m/s, and a density ρ=2500 kg/m3 as typical structures of Mt. Kusatsu-Shirane and other active volcanoes. As the source mechanism, we used a north-south opening (east-west striking) vertical crack at 1200 m above sea level below the craters, assuming a source time function that changes from inflation to deflation, and calculated the waveforms with a time constant of 250 s. We then convolved the Green's functions with the seismometer's translational and tilt responses to produce synthetic waveforms where the both types of motions are taken into account. Results showed that the shapes and time constants of the synthetic waveforms were consistent with those of observed ones low-pass filtered at 10 s, except for the NS component of N.KSHV and N.KSNV. Especially, the observation that the horizontal components have much larger amplitudes than vertical ones was reproduced by the synthetic waveforms. These results suggest that the characteristics of the observed horizontal and vertical waveforms can be explained by the superposition of translational and tilt motions.
However, the assumed source location and mechanism did not reproduce the observed amplitude ratios among the components and the polarities of several traces. We will try to explain them by investigating a better source location and mechanism using the waveform inversion method in the future.
This work was supported by JSPS KAKENHI Grant Number 19K04016.