Slip Distributions of Short‐Term Slow Slip Events in Shikoku, Southwest Japan, From 2001 to 2019 Based on Tilt Change Measurements

Abstract

In the southwest Japan subduction zone, short-term slow slip events (SSEs) accompanying nonvolcanic tremor (ETS: episodic tremor and slip) occur repeatedly with recurrence intervals of several months. It is important to know the detailed slip areas of the SSEs because short-term SSEs occur at the downdip extension of a megathrust earthquake rupture zone, and hence, the SSEs play a key role in the stress buildup processes of the megathrust earthquakes. Although most of the previous studies model a SSE fault motion with uniform slip on a rectangular fault, this may lead to a bias in estimating the slip area. In this study we estimate a spatial slip distribution for each of the short-term SSEs from tilt offset measurements. In total, the slip distributions of 61 SSEs in Shikoku from January 2001 to March 2019 are obtained. The ETS zone in Shikoku is divided into three segments in terms of slip history of the SSEs. Before 2011, the slip areas of most of the SSEs are limited in any one of the three segments. After 2012, however, the number of SSEs having a slip area that extends to multiple segments increases. The slip histories at each position in the ETS zone suggest that the short-term SSE activity is affected by the occurrence of nearby long-term SSEs. The average slip rate over 18 years suggests that an estimated slip deficit in the ETS zone in western Shikoku could be released by long-term SSEs, or coseismic slip or afterslip of the megathrust earthquakes.

Cascading Rupture of Patches of High Seismic Energy Release Controls the Growth Process of Episodic Tremor and Slip Events

“…8b). This is consistent with the MO ∝ T 0.811 scaling relationship for 61 SSEs in Shikoku, the Nankai subduction zones (Hirose and Kimura, 2020).…”

Detection of deep low-frequency earthquakes in the Nankai subduction zone over 11 years using a matched filter technique

“…Results And Discussion Figure 2 shows space-time plots of detected LFEs from April 2004 to August 2015. Each major streak, occurring on a distance scale of up to 100 km, indicates an intensive burst of LFEs migrating along the fault-strike direction, corresponding to a major ETS episode with equivalent Mw ~6.0 (Hirose and Obara, 2010;Sekine et al, 2010;Hirose and Kimura, 2020). In the western part of Shikoku island, we identify long-lasting bursts of LFEs during 2010 and 2014 (horizontal bars in Figure 2); these correspond to geodetic detections of long-term slow slip events in the Bungo Channel using GNSS data (Yoshioka et al, 2015;Ozawa, 2017).…”

“…But, note that the spatio-temporal evolution of LFEs has changed from ~2012, in Shikoku island where major ETS episodes more frequently migrated long-distance through multiple segments than before the Tohoku-Oki earthquake. This behavioral difference has indicated an enhancement of rupture connectivity (Takagi et al, 2016), and resulted in an increase in the number of short-term SSEs with larger slip extent and seismic moment after 2012 (Hirose and Kimura, 2020).…”

Detection of Deep Low-frequency Earthquakes in the Nankai Subduction Zoneover 11 Years Usingamatched Filter Technique

“…Figure 2 shows space-time plots of detected LFEs from April 2004 to August 2015. Each major streak,occurring on a distance scaleof up to 100 km,indicates an intensive burst of LFEs migrating along the fault-strike direction, corresponding to a majorETSepisode with equivalent Mw ~6.0 (Hirose and Obara, 2010;Sekine et al, 2010;Hirose and Kimura, 2020).In the western part of Shikoku island, we identify long-lasting bursts of LFEs during 2010 and 2014 (horizontal bars in Figure 2);these correspond to geodetic detections of long-term slow slip events in the Bungo Channel using GNSS data (Yoshioka et al, 2015;Ozawa, 2017). It is notable that series of LFEswarms occurred alongstrike intermittently during the long-term SSEs.…”

“…This observation is consistent withKono et al (2020),who demonstratedthat there were no temporal variations in the estimated rate of seismic moment release by tectonic tremoralong the Nankai subduction zoneas a result of the Tohoku-Oki earthquake.But, note thatthe spatio-temporal evolution of LFEshas changed from 2012, in Shikoku island where major ETS episodes more frequently migrated long-distance through multiple segments than before the Tohoku-Oki earthquake. This behavioral differencehas indicated an enhancement of rupture connectivity (Takagi et al, 2016), and resulted in anincrease in the number of short-term SSEs with larger slip extent and seismic moment after 2012 (Hirose and Kimura, 2020).…”