A preliminary study on low-frequency earthquakes at Mt. Fuji, Japan,
using the matched filter method
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.