Detection of a hidden Boso slow slip event immediately after the 2011 M_w 9.0 Tohoku‐Oki earthquake, Japan

Abstract

Utilizing a cross-correlation detector technique, we discovered an increase in swarm-like seismicity within the source area of the Boso slow slip events (SSEs) immediately after the 2011 Tohoku-Oki earthquake. The epicentral distribution of the detected seismicity was similar to that of previously recognized Boso SSEs. In addition, small repeating earthquakes were identified within this seismic swarm sequence. These seismic observations indicate that a hidden SSE occurred along the top surface of the subducting Philippine Sea plate immediately after the Tohoku-Oki earthquake. We propose that external stress transfer by the coseismic slip of the Tohoku-Oki earthquake and the following afterslip could have led to the occurrence of the newly detected Boso SSE. In contrast to previous work, we demonstrate that the recurrence interval of the Boso SSEs has not shortened over time but has shown a more complex evolution as a result of external stress perturbations imposed by the Tohoku-Oki earthquake.

Variability of the Space‐Time Evolution of Slow Slip Events Off the Boso Peninsula, Central Japan, From 1996 to 2014

“…Furthermore, some of these studies examined the spatiotemporal patterns of slow slip and how they vary from event to event within a single series of SSEs (Graham et al, 2016;Yoshioka et al, 2015). The SSEs were accompanied by swarms of regular earthquakes (Fukuda et al, 2014;Hirose et al, 2012Hirose et al, , 2014Kato et al, 2014;Ozawa et al, 2003Ozawa et al, , 2007Reverso et al, 2016;Sagiya, 2004a) that were well correlated with the spatiotemporal evolution of slow slip (Fukuda et al, 2014;Hirose et al, 2014), but there were no observations of nonvolcanic tremor associated with the SSEs. Since the installation of a dense GNSS network in Japan in the early 1990s (Sagiya, 2004b), five M w 6.4-6.7 SSEs, with durations of ∼10-30 days, have been identified on the subducting Philippine Sea Plate interface, offshore of the Boso Peninsula, in 1996, 2002, 2011-2014(Fukuda et al, 2014Hirose et al, 2012Hirose et al, , 2014Ozawa et al, 2003Ozawa et al, , 2007Ozawa, 2014;Sagiya, 2004a).…”

“…Since the installation of a dense GNSS network in Japan in the early 1990s (Sagiya, 2004b), five M w 6.4-6.7 SSEs, with durations of ∼10-30 days, have been identified on the subducting Philippine Sea Plate interface, offshore of the Boso Peninsula, in 1996, 2002, 2011-2014(Fukuda et al, 2014Hirose et al, 2012Hirose et al, , 2014Ozawa et al, 2003Ozawa et al, , 2007Ozawa, 2014;Sagiya, 2004a). Kato et al (2014) found swarm-like seismicity immediately after the 11 March 2011 M w 9.0 Tohoku-oki earthquake and attributed this to the occurrence of a SSE that was triggered by the stress transfer from the Tohoku-oki earthquake and the following afterslip. The SSEs were accompanied by swarms of regular earthquakes (Fukuda et al, 2014;Hirose et al, 2012Hirose et al, , 2014Kato et al, 2014;Ozawa et al, 2003Ozawa et al, , 2007Reverso et al, 2016;Sagiya, 2004a) that were well correlated with the spatiotemporal evolution of slow slip (Fukuda et al, 2014;Hirose et al, 2014), but there were no observations of nonvolcanic tremor associated with the SSEs.…”

Interval modulation of recurrent slow slip events by two types of earthquake loading

“…Now, we analyze the actual SSE recurrence intervals. Referring to the previous studies (Hirose et al 2012;Ozawa 2014;Kato et al 2014;Szeliga et al 2008;Pacific Northwest Seismic Network 2014;Heki and Kataoka 2008;Heki, personal communication), we compile the occurrence times of the SSEs at the three subduction zones. Table 1 shows them.…”

“…Ozawa (2014) proposed that the shortening of the recurrence interval of the Boso SSE might represent the latter nucleation effect. Kato et al (2014) showed the existence of a hidden SSE immediately after the 2011 Tohoku earthquake and suggested a complex evolution of the SSE recurrence intervals.…”

Fast and Slow Slip Events Emerge Due to Fault Geometrical Complexity

“…Several seismic and geodetic observations suggest that SSEs may have happened just before and in regions overlapping with earthquakes. There are also suggestions that SSEs may be triggered by earthquakes either by stress waves and/or static stress transfer (Itaba & Ando, 2011;Kato et al, 2014;Wallace et al, 2017;Zigone et al, 2012). More recently, geodetic evidence of a large SSE triggering an earthquake was pointed out in the Guerrero subduction zone (Radiguet et al, 2016).…”