A volcanic tremor is a seismic event linked to volcanic processes,
identified by ongoing ground shaking. While there are different theories
regarding the mechanism of volcanic tremors, 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 data 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. A noticeable
increase in the amplitudes of retrograde Rayleigh waves from 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.