8:45 AM - 9:00 AM
[J06-3-02] Comprehensive detection of low frequency tremor triggered by teleseismic surface waves in northern Kii and western Shikoku, southwest Japan
Deep low frequency tremor is sometimes triggered by surface waves from teleseismic events. Such tremor is called as triggered tremor. In this study, we investigated finer spatiotemporal characteristics of triggered tremor to reveal its activity. We applied matched filter technique to detect triggered tremor in northern Kii and western Shikoku area in southwest Japan, where triggered tremor episodes were observed many times. We used waveforms of low frequency earthquakes based on the JMA hypocenter catalog as templates of tremor. We analyzed NIED Hi-net waveform data for 67 teleseismic events with magnitude larger than 7.5 after Dec. 26, 2004.
As a result, triggered tremor episodes were observed at 9 and 15 teleseismic events in northern Kii and in western Shikoku, respectively. The activity area was limited in one spot in northern Kii. In western Shikoku, two spots were activated by several teleseismic events, and either one of the two was by other events. Along-dip migrations of triggered tremor were observed in both areas. Migration speed of triggered tremor is about 10 km/h in northern Kii and about 40 km/h in western Shikoku. In northern Kii, the directions of migrations are same in all cases. In western Shikoku, both up-dip and down-dip migrations were observed.
Although the estimated migration speed is much faster than that of episodic tremor and slip, about 10 km/day, tremor migration with similar speed to our result has been reported during non-triggered tremor in previous studies. In northern Kii, migration speed of triggered tremor is similar to rapid tremor reversal and rapid tremor forward (Houston et al., 2011). In western Shikoku, migration of triggered tremor is similar to rapid streak (Ghosh et al., 2010).
We considered models of the migrations of triggered tremor based on previously proposed models. The fluid model (Ghosh et al., 2010) and tremor asperity model (Ando et al., 2012) may be reasonable for migrations of triggered tremor.
As a result, triggered tremor episodes were observed at 9 and 15 teleseismic events in northern Kii and in western Shikoku, respectively. The activity area was limited in one spot in northern Kii. In western Shikoku, two spots were activated by several teleseismic events, and either one of the two was by other events. Along-dip migrations of triggered tremor were observed in both areas. Migration speed of triggered tremor is about 10 km/h in northern Kii and about 40 km/h in western Shikoku. In northern Kii, the directions of migrations are same in all cases. In western Shikoku, both up-dip and down-dip migrations were observed.
Although the estimated migration speed is much faster than that of episodic tremor and slip, about 10 km/day, tremor migration with similar speed to our result has been reported during non-triggered tremor in previous studies. In northern Kii, migration speed of triggered tremor is similar to rapid tremor reversal and rapid tremor forward (Houston et al., 2011). In western Shikoku, migration of triggered tremor is similar to rapid streak (Ghosh et al., 2010).
We considered models of the migrations of triggered tremor based on previously proposed models. The fluid model (Ghosh et al., 2010) and tremor asperity model (Ando et al., 2012) may be reasonable for migrations of triggered tremor.