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.

The continuous estimation of changes in seismic velocity and seismic scattering property by passive interferometry using seismic ambient noise is a promising tool for monitoring volcanoes. To improve the usefulness of this method, it is necessary not only to detect subsurface structural changes but also to quantitatively compare the estimated changes in seismic wave velocity and seismic wave scattering property with other observations such as ground deformation. We applied passive interferometry to continuous seismic records from Suwanosejima volcano, Japan, recorded between April 2017 and December 2021. We detected repeated significant waveform decorrelations in seismic ambient noise cross-correlation functions, indicating seismic scattering property changes in the shallow areas of the volcano. These decorrelations were observed from 2 week to a few days before the increase in the number of explosions, suggesting that seismic scattering properties changed significantly during that period. We found that the timing of the decorrelation in seismic ambient noise cross-correlation functions and tilt changes related to magma accumulation and injection beneath Suwanosejima were well synchronized. The high correlation between the amounts of decorrelation and tilt change during the magma accumulation period suggests that a large volume of accumulated magma caused great changes in the scattering property. These results provide a significant first step toward a quantitative comparison of the amount of changes in the scattering property with the amount of magma accumulation beneath volcanoes.