[S-VC28] Active Volcanism

[SVC28-P18] Estimating the source of volcanic earthquakes during eruption events at Kirishima Volcanos, in 2018, using spatial distribution of earthquake amplitude.

*Daiki Goshima¹, Satoshi Matsumoto², Ayaho Mitsuoka¹, Yuhei Yuasa⁴, Yuta Kawamura¹, Mie Ichihara³ (1.Department of Earth and Planetary Sciences, Graduate School of Science, Kyushu University, 2.Institute of Seismology and Volcanology, Faculty of Sciences, Kyushu University, 3.Earthquake Research Institute, University of Tokyo, 4.Osaka Regional Headquarters, Japan Meteorological Agency)

Keywords:volcanic tremor, amplitude distribution

Kirishima volcanic area consists of about 20 volcanoes and is located around the border of Kagoshima and Miyazaki prefectures. Ichihara et al (2017) analyzed the volcanic tremors associated with volcanic activity in 2011 eruption. They obtained tremor sources just below the crater and discussed its relationship with surface phenomena. However, source of tremor that might be associated with magma migration to the crater have not been determined and their relationships are not known well. The location of magma chamber of the Kirishima Volcanos was inferred from pressure source detected by GPS observations, which is located at 5 km northwest of Shinmoe-dake and depth of 8 km (Nakao et al., 2013). On the other hand, Aizawa et al (2014) discussed the location to be 5 km northeast of Iwo-yama. In this study, we attempt to detect magma behavior associated with the 2018 eruptions of Shinmoe-dake and Iwo-yama through detection of volcanic tremor.

In order to detect tremor and determine source location, we use seismic stations around the volcano. Tremor location is determined from amplitude distribution at the seismic stations. Generally, seismic signal is affected by source, path, and site factors. Then, we estimate the site amplification characteristics at the stations using the distant earthquake occurred in Shimane Prefecture on 9 April 2018, and remove the effect from amplitude data. After that, we estimate the source location of tremor. The location was determined by spatial amplitude distribution through grid-search processing for trial source locations.

Prior to tremor detection, we performed source determination for events at known location. At the same time, we estimated optimal attenuation factor to eliminate propagation path effect. We used two natural earthquakes and two volcanic eruptions for the checking. The result of the check showed that the processing could not determine the depth of the source well but the horizontal location of the source can be estimated by it. Undetermined depth might be attributed to the effect of radiation pattern from the natural earthquake source. Therefore, we confirmed the processing can be used for the determination of tremors.