5:15 PM - 7:15 PM
[SVC32-P09] Increase in the seismicty of deep low-frequency earthquakes at the Mt. Iwate coincident with volcanic activity change in 2024.
Keywords:Deep low frequency earthquakes, Matched filter method
The Mount Iwate is one of active volcanoes located in the Ou Backborn Range. The Mt. Iwate has been in a state of extremely high volcanic activity with the expansion of mountain body since February 2024 and increase in the number of shallow volcanic earthquakes since around July 2024. At the same time of volcanic unrest on the surface, the Japan Metrogecal Agency has detected many deep low-frequency earthquakes (DLFEs) which occur at a depth of about 30 km near the Moho discontinuity. DLFEs are small events but have a predominant low-frequency component of 2-8 Hz. Since most of them are observed around active volcanoes, they are considered to be related to the deep magma supply system (e.g., Nakamichi et al. 2003). In addition, DLFEs can be an important clue for understanding the magma supply process from deep to shallow, as they sometimes become active several months before the eruption (e.g., Shapiro et al. 2017). In this study, we examine seismicity of DLFEs and discuss its relevance to ongoing surface events.
In order to capture the detailed seismicity, we performed detection from continuous waveforms based on the Matched Filter method (e.g., Gibons and Ringdal, 2006). For the catalog of template events, we selected 175 events relocated in Kurihara and Obara (2021). The waveforms were band-pass filtered at 2-8 Hz, and the 5 second window from 1 s before the theoretical S-wave arrival time was used as the template waveforms. Cross-correlation coefficients between the 33 channels of 11 Hi-net stations around the Mt. Iwate and the template were calculated for the period from January 2019 to January 2025. We defined events as the time when their mean exceeded 9 times the median absolute deviation.
This analysis detected about 7,000 events of DLFEs over a 5-year period. DLFEs in the Mt. Iwate occur at three different regions: depths of 10 km just beneath the summit, those of 30 km in the northern and southern parts of the Mt. Iwate. Among three clusters, the number of DLFEs in the northern parts have extremely increased since Augst 2024. Such a rapid increase in seismicity has not been experienced in the past 20 years, and therefore this seismicity is extremely unusual. The number of DLFEs also increased in the shallow cluster just beneath the summit about three months after the northern cluster. This result suggests the possibility that magma supply from deep to shallow areas has been continued. We plan to quantitatively interpret the relationship between seismicity and the background magma supply by estimating source mechanisms of deep low-frequency earthquakes in the future.
In order to capture the detailed seismicity, we performed detection from continuous waveforms based on the Matched Filter method (e.g., Gibons and Ringdal, 2006). For the catalog of template events, we selected 175 events relocated in Kurihara and Obara (2021). The waveforms were band-pass filtered at 2-8 Hz, and the 5 second window from 1 s before the theoretical S-wave arrival time was used as the template waveforms. Cross-correlation coefficients between the 33 channels of 11 Hi-net stations around the Mt. Iwate and the template were calculated for the period from January 2019 to January 2025. We defined events as the time when their mean exceeded 9 times the median absolute deviation.
This analysis detected about 7,000 events of DLFEs over a 5-year period. DLFEs in the Mt. Iwate occur at three different regions: depths of 10 km just beneath the summit, those of 30 km in the northern and southern parts of the Mt. Iwate. Among three clusters, the number of DLFEs in the northern parts have extremely increased since Augst 2024. Such a rapid increase in seismicity has not been experienced in the past 20 years, and therefore this seismicity is extremely unusual. The number of DLFEs also increased in the shallow cluster just beneath the summit about three months after the northern cluster. This result suggests the possibility that magma supply from deep to shallow areas has been continued. We plan to quantitatively interpret the relationship between seismicity and the background magma supply by estimating source mechanisms of deep low-frequency earthquakes in the future.