2014 Boso Slow Slip Event: the Source Slip Process based on Tilt and GNSS Measurements

Abstract:

Around the Boso Peninsula, Japan, slow slip events (SSEs) accompanying earthquake swarms recur with repeating intervals between two to seven years, associated with the subduction of the Philippine Sea Plate (PHS) from the Sagami trough beneath the Kanto metropolitan area. These SSEs share an almost identical source area for each episode, which is adjacent to the source area of the Kanto megathrust earthquakes. Because both of the SSEs and the earthquakes are slip events on the same PHS plate interface, understandings on factors that control 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, i.e., it is 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 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 interval.

In order to clarify factors that control the recurrence intervals, we estimated the source slip processes for the recent SSEs whose crustal deformations have been observed with tiltmeters and GNSS. The 2007 and 2011 SSEs have similar size in seismic moment (Hirose et al., 2014), 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.