Changes in seismicity in a volcanically active region, on the eastern side of Izu Peninsula, Japan

The Izu-Tobu region, on the eastern side of Izu Peninsula, in Japan, is volcanically and seismically active. In this region, earthquake swarms of ordinary earthquakes frequently occur at shallow depths, which is considered to be associated with magma intrusion. Beneath ordinary earthquakes, low-frequency earthquakes (LFEs) are infrequently observed. We conducted a timeseries analysis of both types of earthquakes during the time period 2005-2020, using a variant of the Epidemic-Type Aftershock Sequence model. For this analysis, we used the Japan Meteorological Agency catalog of ordinary earthquakes and the catalog of LFEs produced using the matched filter method. The observed result, which was common to both types of earthquakes, showed a significant change in seismicity, which became quiet, with the inflection point falling sometime between late 2009 and mid-2013, during which two out of three pronounced earthquake swarms occurred. We associated this seismic quiescence with changes in background rate to be low, where background rate, by removing the triggering effect of aftershocks, was interpreted as having been caused directly by the magma source, which can vary with time. We used surface displacement data obtained from the Geospatial Information Authority of Japan, and observed that the uplift due to magma intrusion was significant during the 1970s-1990s whereas it was in abatement or unobservable during the studied period (2005-2020). We also found that the seismic quiescence occurred without significant crustal movement during the studied period. Our implication from this finding is that magma source, which caused magma intrusion into the Izu-Tobu region, is in a transition phase, becoming less active, compared with the magma source during the 1970s-1990s.