We give a preliminary report on results of detecting low-frequency earthquakes (LFs) occurring at Mt. Fuji, Japan, using the matched filter method (MF method: e.g., Peng & Zhao, 2009). LFs have been observed in the depth 10-25km beneath Mt. Fuji (Hamada, 1981; Ukawa et al, 2005). These LFs seem to occur at an almost constant rate at all times, but it may become remarkably active as in the fall of 2000 (Yoshida et al., 2006). Because it is considered that the activity of LFs is associated with behavior of magmatic fluid at depth (e.g., Nakamichi et al., 2003), an investigation into the relationship between LFs and volcanic activity (e.g., Harada et al., 2010) is important. Understanding of the details of LFs activity is the first step for this investigation. Here, a system using the MF method for detecting LFs at Hakone volcano, Japan (Yukutake, 2017; Yukutake et al., 2019), was modified to be applicable to the detection of LFs at Mt. Fuji. Then, this was applied to continuous seismic record at seismic stations around Mt. Fuji during the period of 2012-2020. Next, the template waveforms of LFs were prepared on the basis of the earthquake catalog maintained by the Japan Meteorological Agency (JMA). Then, the cross-correlation analysis was conducted between the template waveforms and the seismic records. Finally, a catalog of LFs, obtained by using the MF method, was created. Using this catalog, we confirmed that LFs in 2012-2020 occurred at an almost constant rate, and that this is also true for LFs included in the JMA catalog. However, our case shows that LFs occurred at a rate of about 1,250 per year, which is about 10 times higher than that shown for the JMA case (a rate of about 125 per year). It was also confirmed that the larger LFs tend to have fewer numbers and smaller LFs tend to have more numbers, again a feature found by using the JMA catalog. Our research is underway, and tackling challenges such as selection of appropriate template waveforms of LFs, correction of magnitude estimate, and extension of analysis period will improve our results, which will be reported in the presentation.

Monitoring stress state in the Earth’s crust plays a crucial role in our understanding of an earthquake’s mechanism, especially how earthquake ruptures nucleate, as well as in calculating the distribution of hazards. Crustal deformation due to the 2019 Ridgecrest earthquakes, which occurred near the town of Ridgecrest, California, that culminated in a preceding earthquake of magnitude (M) 6.4 and a subsequent M7.1 event, caused stress perturbation in nearby regions. However, implications of future seismic activity are still unclear. Here we analyze the occurrence of small earthquakes compared to larger ones—the b-values, showing how the nucleation area for both the M6.4 and M7.1 earthquakes had low b-values before these events occurred, and mid-to-high b-values thereafter. The slip distribution of the M7.1 event is also well correlated with the b-value map. Additionally, the time and local-dependent variations in b-values of the Ridgecrest earthquakes are linked with estimates of changes to Coulomb stress. The main conclusion is that the b-value mapping provide insight into the stress state in the fault zone, which is likely closely related to the nucleation and evolution of earthquakes in the sequence. The combined approach of stress-change and b-value analyses to the post-M7.1-quake sequence shows an area that is currently being stressed near the Garlock fault that hosted past large earthquakes. The b-values are not as low as those immediately before the M6.4 and M7.1 events, but contribute the most recent values in a decreasing trend of the b-value. Together with geodetic and seismological observations, monitoring the spatial and temporal distribution of b-value would contribute to seismic hazards in the Eastern California Shear Zone.