Changes in seismicity in a volcanically active region of the Izu Peninsula, Japan
The eastern Izu Peninsula in Japan is volcanically and seismically active. Ordinary earthquakes frequently occurred at shallow depths in 2006 and 2009, when they clustered as swarms. Beneath ordinary earthquakes, low-frequency earthquakes (LFEs) were infrequently observed. To better understand the characteristics of those LFEs, we produced a LFE catalog for 2005-2020, using the matched-filter method. Timeseries analyses based on the Epidemic-Type Aftershock Sequence model showed quiescence, i.e., a change in occurrence rate, which became quiet. For comparison, the same analysis was conducted using the Japan Meteorological Agency catalog of ordinary earthquakes, and similar results were shown. The change points for both types of earthquakes fell during and after each of the swarms, revealing an earlier start of quiescence for shallow ordinary earthquakes than for deep LFEs. Surface displacement data obtained from the Geospatial Information Authority of Japan showed that the uplift was in abatement or unobservable during the study period. Our seismicity and geodetic results are interpretatively hypothesized as being associated with magmatic activity, as follows. When the absence of a supply of magma from a depth toward the crustal magma reservoir caused no significant uplift, magma intruded from this reservoir into the shallow crust, resulting in the 2006 and 2009 swarms. Due to this intrusion, stresses decreased in and around shallow parts of the reservoir and then quiescence for ordinary earthquakes began during or after each swarm. Due to this decrease, stresses in and around deep parts of it late decreased, delaying the start of quiescence of deep LFEs.