Recent volcanic activity in Azumayama, Japan

In Azumayama, one of active volcanoes in Japan, volcanic activity increases above its background levels every few years. However, no eruption has been recorded since 1977. From 2014 to 2015, Azumayama's activity temporary increased. Since April 2014, the inflation trend of the volcanic edifice was observed in tiltmeter and GNSS data. Then, volcanic seismicity increased since October, a volcanic tremor occurred on 12 December 2014 and the ground deformation rate accelerated. Furthermore, swarm of volcanic earthquakes was observed on 14 January 2015, and the slight upward migration of hypocenter was observed. After September 2015, the ground deformation changed to a declining trend toward the crater, and volcanic seismicity decreased and had remained at low levels since May 2016. Volcanic activity has increased since 2018. A volcanic tremor occurred on 22 July 2018; data from tiltmeters near Azumayama showed an associated rising trend while data from tiltmeters far from Azumayama showed an associated declining trend on the direction toward the crater. Then the ground deformation with a rising trend around the crater continued, and volcanic earthquakes and volcanic tremors increased. Shallow inflation and deflation of the edifice attributed to hydrothermal activity were often observed with tiltmeters and broadband seismometers near the crater. The hypocenter of volcanic tremors determined by amplitudes source location (ASL) method, the estimated tremor source from long-period particle motions and the expansion source determined from ground deformation observation are located at the same point which is around an altitude of 1 km near the crater. This indicates that the activation of the hydrothermal activity pressurize the hot water reservoir and causes volcanic tremors. As mentioned above, the activation events of the hydrothermal activity in the shallow area which may lead to the phreatic eruption have periodically occurred in Azumayama. This presentation will highlight the comparison results of volcanic activity of Azumayama in 2014-2015 and 2018-2019.