1:45 PM - 2:00 PM
[SVC32-01] Characteristics of Earthquake Swarm Activity in the Aira Caldera
Keywords:Aira caldera, earthquake swarm
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. Receiver function inversion analysis also revealed that low velocity anomalies were distributed over a wide area, centered on the Aira caldera, from depths of 10 km to 35 km, with particularly significant low velocity anomalies of -10% observed at a depth of 15 km (Shibutani, 2024). Since 2003, A-type earthquakes (volcano-tectonic earthquakes) occurring in the southwest of Sakurajima and the northeast of Aira Caldera have become more active, with more frequent swarms and a wider distribution of hypocenters. In this presentation, we will clarify the characteristics of seismic activity during the magma accumulation period in Aira Caldera and its relationship to inflation of the ground deformation.
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 are mainly 1. Shallower than 3 km deep around the Wakamiko submarine volcano in the caldera, 2. 3-11 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. The focal mechanisms are 1. normal and reverse fault types, 2. strike-slip fault type with a P axis in the northeast-southwest direction, 3. the main type is the normal fault type. GNSS baseline length indicated inflation of the Aira caldera in 2015-2017, May-July 2023, and November 2023. Immediately afterwards, seismic activity increased in the areas of 1. the northeast of the caldera and 3. the southwest of Sakurajima, followed by a tendency for Sakurajima's eruption activity to become more active. The focal mechanism of earthquakes that occur in the southwest part of Sakurajima is the normal fault type, and it is thought that earthquakes occur when magma move from the magma reservoir of the Aira caldera into beneath the Sakurajima.
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 are mainly 1. Shallower than 3 km deep around the Wakamiko submarine volcano in the caldera, 2. 3-11 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. The focal mechanisms are 1. normal and reverse fault types, 2. strike-slip fault type with a P axis in the northeast-southwest direction, 3. the main type is the normal fault type. GNSS baseline length indicated inflation of the Aira caldera in 2015-2017, May-July 2023, and November 2023. Immediately afterwards, seismic activity increased in the areas of 1. the northeast of the caldera and 3. the southwest of Sakurajima, followed by a tendency for Sakurajima's eruption activity to become more active. The focal mechanism of earthquakes that occur in the southwest part of Sakurajima is the normal fault type, and it is thought that earthquakes occur when magma move from the magma reservoir of the Aira caldera into beneath the Sakurajima.