Temporal Activity Modulation of Deep Very Low Frequency Earthquakes in Shikoku, Southwest Japan

We investigated long-term changes in the activity of deep very low frequency earthquakes (VLFEs) in western Shikoku, southwest part of the Nankai subduction zone in Japan for 13 years by the matched-filter technique. VLFE activity is expected to be a proxy of interplate slips. In the Bungo channel, where long-term slow slip events (SSEs) occurred frequently, the cumulative number of detected VLFEs increased rapidly in 2010 and 2014, which were modulated by long-term SSEs. In the neighboring inland region near the Bungo channel, the cumulative number increased steeply every 6 months. This stepwise change was accompanied by episodic tremors and slips. Deep VLFE activity in western Shikoku has been low since the latter half of 2014. This decade-scale quiescence may be attributed to the change in interplate coupling strength in the Nankai subduction zone. Plain Language Summary Various types of slow earthquakes have been detected on plate interfaces near the seismogenic zones of megathrust earthquakes. We investigated long-term changes of deep very low frequency earthquake (VLFE) activity over a period of 13 years in Shikoku region, southwest Japan. VLFEs can be categorized as slow earthquakes. VLFE activity is expected to be a proxy of interplate slip. In the Bungo channel, where long-term slow slip events (SSEs) have been known to occur frequently, the activity of deep VLFEs was influenced by long-term SSEs in 2010 and 2014. VLFE activity is sensitive to the size and area of the neighboring long-term SSEs. In the neighboring inland region, deep VLFE activity is accompanied by episodic tremors and slips every 6 months. In western Shikoku, the activity of deep VLFEs and tremors has been low since the latter half of 2014. A likely explanation for the quiescence is the temporal change of interplate coupling in the Nankai subduction zone.