A causal relationship between the slow slip event and deep low frequency tremor indicated by strain data recorded at Shingu borehole station

In the southwest Japan, synchronized deep low frequency tremors and short-term slow slip events occur repeatedly in several regions such as Tokai, northern Kii Peninsula and western Shikoku areas, and these activities are partitioned by 'gaps' of tremors. Although concurrent occurrences of slow slips and tremors have been detected at various subducting plate boundary, their physical mechanism is still not well understood. We are monitoring crustal deformation at Shingu city on the southeastern coast of Kii Peninsula, with an integrated multi-component borehole monitoring system developed by Ishii et al. [2002]. The borehole sensor unit consists of 6 strain sensors (4 in horizontal, 2 in vertical), 2 pendulum tilt sensors, magnetic direction finder and a quartz thermometer and installed at 510m depth. Fukuda et al. [2007] reported two types of strain changes, one associated with deep low frequency tremors and the other without tremors from November 2005 to March 2006. We extend the analysis period to 41 months, from January 2004 to September 2007. We identified 11 episodic strain changes. One of them was caused by heavy rainfall but the rest of the changes are considered to be slow slips. Among all the slow slips identified, five events occurred associated with reported tremor events, but the rest 5 changes were not accompanied by tremors. These slow slip events are characterized by N-S compression (0.017 - 0.063 ppm), and E-W extension (0.013 - 0.071 ppm), NW-SE extension (0.008 - 0.097 ppm), and last 4 - 9 days. We estimate a fault model for each event by forward modeling, and find that the all the strain changes can be attributed to reverse faulting on the plate boundary beneath the Kii Peninsula. An interesting strain change occurred from 26 Dec. 2004 to 2 Jan. 2005. In this period, a tremor activity propagated southwestward on central Kii Peninsula and the level of activity remarkably drops when the activity propagated into the tremor gap zone. After that, the tremors were activated again at the southwestern side of the tremor gap. During this activity, the main strain change at the Shingu borehole was recorded between two separated tremors, implying that the tremor gap is the source of the slow slip. Observations of slow slips preceding or without tremors indicate that a slow slip and tremor are independent phenomena. These results give support to a hypothesis that the slow slip smoothly propagates on the plate boundary and is the main process of simultaneous occurrences and tremor is a triggered phenomenon under special conditions.