Seismic Wavefield Change Preceding the Eruption of Shinmoe-Dake, Kirishima Volcano, Japan, Inferred From Polarization Analysis

Abstract

A volcanic tremor is a seismic event linked to volcanic processes, identified by ongoing ground shaking. They play a crucial role in understanding internal volcanic activities and forecasting eruptions. Detecting weak volcanic tremor signals before an eruption has remained a difficult task. This study utilized polarization analysis on continuous seismograms at Kirishima volcano, Japan, to extract information on retrograde Rayleigh wave sources. Back azimuth estimates focused on the direction of the Shinmoe-dake crater before and during the volcanic eruptions in 2011, 2017, and 2018. Rayleigh waves originating from the crater direction, particularly in the 1.3–2.5 Hz band, were observed starting around March 2017, approximately 7 months prior to the October 2017 eruption. In the 1.3–2 Hz band, the back azimuths steadily moved closer to the crater’s direction between January and March 2017. Epicentral areas from the cross-bearings approach indicate that retrograde Rayleigh wave sources are located around the crater. A noticeable increase in the amplitudes of retrograde Rayleigh waves, which propagate along the crater direction, was observed starting in August 2016. Enhancing the growth in amplitude was achieved by utilizing a filter on the seismograms. As polarization analysis requires only a single three-component seismometer, this approach allows for the timely identification of weak Rayleigh waves from the crater direction, even in volcanoes with limited seismic station coverage. The results obtained in this study contribute to the growing knowledge on volcanic tremors and their potential use in volcano monitoring and eruption forecasting.