Depth‐dependent activity of non‐volcanic tremor in southwest Japan

Abstract

With a new location method, we obtained a high‐resolution map of non‐volcanic tremor along the subducting Philippine Sea plate in southwest Japan and clarified the depth‐dependent behavior of the tremor activity. A bimodal distribution of tremor along‐dip is apparent in regions where short‐term slow slip events (SSEs) and very‐low‐frequency earthquakes are frequently detected. The separation of bimodal peaks is 5 to 10 km in depth. The updip tremor activity occurs episodically, coincident with major bursts that often accompany geodetically detectable SSEs. The downdip tremor activity is, however, rather stable with frequent minor bursts. This depth‐dependent tremor activity likely reflects variation of interplate slip properties, specifically weakening with increasing depth and temperature. In westernmost Shikoku, anomalous tremor activity was detected in only the updip section in late 2003. This is interpreted as being triggered by a long‐term SSE in the shallower edge of the tremor zone.

Regional and global variations in the temporal clustering of tectonic tremor activity

“…In Shikoku, several peaks are identified at periods around 6.6 to 6.8 (approximately 50 to 80 days) and 6.95 (approximately 100 days), corresponding to the recurrence intervals on the downdip and updip parts of the subducting slab, respectively. The recurrence interval of the updip part of tremor activity in Shikoku (approximately 100 days) and it bimodal distribution is roughly consistent with Obara et al (2010). A short-term peak of 50 to 80 days is less prominent for Kii-Tokai, and this is also consistent with the observations of Obara et al (2010).…”

“…The recurrence interval of the updip part of tremor activity in Shikoku (approximately 100 days) and it bimodal distribution is roughly consistent with Obara et al (2010). A short-term peak of 50 to 80 days is less prominent for Kii-Tokai, and this is also consistent with the observations of Obara et al (2010). Peaks at around 4 to 6 months for Kii-Tokai likely reflect variable recurrence intervals along the strike of the subducting slab, corresponding to the along-strike segmentation of short-term slow-slip events (Sekine et al 2010).…”

“…A similar depth-dependent transition has been reported for tremors along the San Andreas Fault at Parkfield, California (Shelly and Johnson 2011). Such spatial heterogeneities in tremor activity are considered to reflect mainly the frictional regime at the deep plate interface (e.g., Obara et al 2010). These previous studies described the large-scale, depth-dependent variations of tremor activities, either qualitatively or using some specific criteria.…”

“…Previous studies have reported that the episodic pattern of tremor activity changes spatially throughout a tectonic region. For example, in southwestern Japan, Obara et al (2010) detected two different modes of tremor activity in terms of recurrence period, depending on tremor depth. In the Cascadia subduction zone, Wech and Creager (2011) showed that tremor activity gradually changes from burst-like (episodic) to continuous with increasing depth.…”

Synchronous low frequency earthquakes and implications for deep San Andreas Fault slip

“…Yet knowledge of the upper crust is not in itself sufficient to characterize seismic hazard, as lower crustal slip beneath the seismogenic zone can drive the seismicity in the upper crust (Tse and Rice, 1986;Scholz, 2002) and lead to the nucleation of large earthquakes (Nadeau and Guilhem, 2009;Shelly, 2009;Segall and Bradley, 2012). Low Frequency Earthquakes (LFEs), which accompany the slow slip events observed on the deep extension of fault interfaces in Cascadia (Rogers and Dragert, 2003;Bostock et al, 2012), Japan (Obara et al, 2004;Shelly et al, 2006), and Mexico (Rivet et al, 2011;Frank et al, 2013), have also been observed in the Parkfield region of California (Nadeau and Dolenc, 2005;Shelly et al, 2009), and present the opportunity to illuminate the elusive elastic and slip behaviors of the lower crust (Rubinstein et al, 2010).…”