11:00 AM - 11:15 AM
[SVC26-08] Seismic Activity Change around Aira Caldera in Magma Accumulation Stage
Keywords:Aira Caldera, Volcano-tectonic Earthquakes
Aira Caldera with a diameter of about 20 km is situated at Kagoshima bay in southern Kyushu. The Sakurajima volcano was formed on the southern rim of the Aira caldera, and is the most active volcano in Japan. The last eruption of the Aira caldera occurred approximately 29,000 years ago that produced the Ito pyroclastic flow (Okuno, 2002). Subsequently, Sakurajima Volcano was formed on the southern rim of the caldera and continues to be active eruption. Since 1992, the ground around the Aira caldera has continued to inflation, based on GNSS observations and leveling surveys, the inflation source is estimated to be approximately 10 km deep below the center of the Aira caldera (for example, Iguchi, 2013; Yamamoto et al., 2013). A three-dimensional seismic velocity structure analysis using seismic tomography was performed by adding artificial seismic survey data to natural seismic observation data, and significant low S-wave velocity anomaly related to be a magma reservoir was found at a depth of 12-22 km in the central part of Aira caldera. Hidayati et al. (2006) showed that volcano-tectonic earthquakes increased during the inflation period from 1995 to 2005 of the Aira caldera. In this presentation, we will clarify the relationship between seismic activity around the Aira caldera during the magma accumulation period, seismic velocity structure, and attenuation structure.
In order to determine the hypocenters and the focal mechanisms of volcano-tectonic earthquakes around the Aira Caldera, temporary observation (maximum 17 stations) installed throughout the southern Kyushu area and permanent stations (Sakurajima Volcano Observatory, Disaster Prevention Research Institute, Kyoto University and the Hi-net data of National Research Institute for Earth Science and Disaster Resilience) are used.
The volcano-tectonic earthquakes that occur around the Aira caldera during inflation period are mainly 1. Shallower than 6 km deep around the Wakamiko submarine volcano in the caldera, 2. 5-10 km deep from the east to southeast of the Aira caldera rim, 3. 6-12 km deep in the southwestern part of Aira Caldera and Sakurajima, 4. Depth of less than 4 km just below the summit of Mt. Sakurajima. The focal mechanisms are 1. strike-slip fault or normal fault type with a P-axis in the northeast-southwest direction, 2. strike-slip fault type with a P axis in the northeast-southwest direction, 3. the main type is the normal fault type, 4. various fault types related to volcanic activities of Sakurajima volcano. The occurrence of the volcano-tectonic earthquakes increased from 2015 to the first half of 2017 during accelerated inflation of the Aira caldera from GNSS baseline length. The earthquake number increased by about 1.5 times compared to normal. In addition, when the inflation was progressing in 2021, a felt earthquake occurred in the northwest area outside the caldera. The upper end of the S-wave low-velocity anomaly region within the caldera is around 12 km deep, and the hypocenter depths of volcano-tectonic earthquakes at the Aira caldera do not occur at depths deeper than 12 km. It is considered that the volcano-tectonic earthquakes at the Aira caldera occur due to the accumulation of strain from the upper part of the magma chamber.
In order to determine the hypocenters and the focal mechanisms of volcano-tectonic earthquakes around the Aira Caldera, temporary observation (maximum 17 stations) installed throughout the southern Kyushu area and permanent stations (Sakurajima Volcano Observatory, Disaster Prevention Research Institute, Kyoto University and the Hi-net data of National Research Institute for Earth Science and Disaster Resilience) are used.
The volcano-tectonic earthquakes that occur around the Aira caldera during inflation period are mainly 1. Shallower than 6 km deep around the Wakamiko submarine volcano in the caldera, 2. 5-10 km deep from the east to southeast of the Aira caldera rim, 3. 6-12 km deep in the southwestern part of Aira Caldera and Sakurajima, 4. Depth of less than 4 km just below the summit of Mt. Sakurajima. The focal mechanisms are 1. strike-slip fault or normal fault type with a P-axis in the northeast-southwest direction, 2. strike-slip fault type with a P axis in the northeast-southwest direction, 3. the main type is the normal fault type, 4. various fault types related to volcanic activities of Sakurajima volcano. The occurrence of the volcano-tectonic earthquakes increased from 2015 to the first half of 2017 during accelerated inflation of the Aira caldera from GNSS baseline length. The earthquake number increased by about 1.5 times compared to normal. In addition, when the inflation was progressing in 2021, a felt earthquake occurred in the northwest area outside the caldera. The upper end of the S-wave low-velocity anomaly region within the caldera is around 12 km deep, and the hypocenter depths of volcano-tectonic earthquakes at the Aira caldera do not occur at depths deeper than 12 km. It is considered that the volcano-tectonic earthquakes at the Aira caldera occur due to the accumulation of strain from the upper part of the magma chamber.