Akita-Komagatake volcano in northern Honshu, Japan, has an elliptical caldera 3 km×1.5 km in diameter (southern caldera) on its southern slope. Although it has been speculated that the AK13 and AK12 tephra units (15‒12 ka) are related to the formation of the caldera, the details of these processes have not been clarified. We divided the caldera-forming activity into three stages on the basis of geological and petrological evidence developed during this study. Stage 1 activity began with a vulcanian eruption that produced Obonai pyroclastic flow, followed by a plinian eruption that produced an eruption column. These eruptions effused basaltic andesite and andesitic scoria to form southern portion of the caldera. This activity, which produced AK13 tephra, involved an eruption of volcanic explosivity index (VEI) 4 and occurred at approximately 15,100 calibrated radiocarbon years before present (cal. BP). Stage 2 involved the formation of a small pyroclastic cone, Minamidake cone, at the southern margin of the caldera. Distal tephra and lava flows derived from the cone have not been recognized. Stage 3 involved phreatomagmatic eruptions of dacitic magma that produced Kunimi pyroclastic flow, which covered the floor of southern portion of the caldera and the Minamidake cone. The activity then shifted to a plinian eruption, during which time juvenile materials changed from dacitic pumice to mixtures of basaltic andesite scoria, pumice and banded pumice. This activity terminated with repeated phreatomagmatic and Vulcanian eruptions. This stage, which produced AK12 tephra, involved an eruption of VEI＝4 and occurred at approximately 13,400 cal. BP. We conclude that the southern caldera is a composite caldera that was formed by two sequential explosive eruptions in the latest Pleistocene. Juvenile materials from Stages 1 and 3 form distinct linear trends in many silica variation diagrams, suggesting that each stage was derived from a different magma system.

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.