Asama volcano, one of the active volcanoes in Japan, has repeatedly erupted and is one of the volcanoes for which very long-term and highly accurate observation data have been accumulated. Asama volcano is known to occur volcanic earthquakes with various characteristics, and there are studies on the classification of volcanic earthquakes and the relationship between their locations and volcanic activities. Based on the previous results, Japan Meteorological Agency (JMA) classifies the major volcanic earthquakes observed at Asama volcano into the following four types: A-type earthquakes with clear P and S phases, BH-type earthquakes in which the initial motion of the S wave is unclear and relatively high frequency components predominate, BL-type earthquakes in which the initial motion phase is unclear and relatively low frequency components predominate, and BT-type earthquakes with long duration in the coda wave portion.
A small eruption occurred at the summit crater of Asama volcano on 7 August, 2019. The eruption was the first since a very small eruption on 19 June, 2015. It is known that several common precursor phenomena, such as increase in earthquakes and crustal deformation, were observed before past eruptions at Asama volcano. However, before the eruption on 7 August, no clear precursor phenomena were observed, as has been the case with previous eruptions. In this paper, we investigate the seismic activity before and after this eruption using the Matched Filter Method to understand in more detail and objectively whether any precursor-like phenomena were observed for the eruptions in August 2019.
Using the Matched Filter Method, we found that there is a change in high-frequency seismic activity prior to the eruption. In particular, the analysis using the waveforms of BH-type earthquakes immediately after the eruption as templates revealed that the number of similar earthquakes increased a few days before the August 7 eruption. These earthquakes are considered to have occurred in a relatively shallow area based on the difference in the time of first arrival of multiple stations near the crater and the estimated depth of the epicenter. In other words, the seismic activity was active before the eruption in a very shallow area just below the summit crater. Considering the sudden decrease in BL-type earthquakes, which are thought to be caused by fluid movement, it is suggested that the volcanic conduit may have been temporarily blocked.
