A VARIATION IN THE RECURRENCE INTERVAL OF THE BOSO PENINSULA SLOW SLIP EVENTS, CENTRAL JAPAN

Around the Boso Peninsula, Japan, slow slip events (SSEs) accompanying earthquake swarms recurs with repeating intervals between two to seven years, associated with the subduction of the Philippine Sea Plate (PHS) beneath the Kanto metropolitan area from the Sagami trough. These SSEs share an almost identical source area for each episode and the area is adjacent to the source area of the Kanto megathrust earthquakes. Because both of the SSEs and the Kanto earthquakes are slip events on the same PHS plate interface, studying the recurrence behavior of the SSEs provides important information not only for the nature of the SSEs itself, but also about the slip property on the plate interface.

The latest event occurred in January 2014, the shortest interval of 27 months after the previous event in October 2011 (Ozawa, 2014) while the average recurrence interval is 61 months for over 30 years since 1983 (Hirose et al., 2012). Although the recurrence intervals could be modulated by static stress changes caused by nearby large earthquakes, such as the 2011 Tohoku earthquake and its afterslip (Hirose et al., 2012), there is no such large earthquake or other phenomenon that can hasten the latest SSE, and thus other factors are required for the shortening of the time interval.

In order to clarify factors that contribute to the fluctuation in recurrence intervals of the SSEs, we estimated the source slip processes for the recent SSEs whose crustal deformations have been observed with tiltmeters at NIED Hi-net stations and with GNSS at GSI GEONET. The 2007 and 2011 SSEs have similar size in seismic moment, while the 2014 SSE has a smaller seismic moment than the two previous SSEs. The maximum stress drop for the 2014 SSE is estimated as about 1/2 of those for the 2007 and 2011 SSEs. Although we can not specify mechanisms that hastened the occurrence of the latest SSE, this might suggest that the strength of the source area might have reduced and the slip event occurred at a lower stress level.