HR: 0800h
AN: T21A-0361    [Abstracts]
TI: Precursory slow crustal deformation before short-term slow slip event in January 2006, recorded at Shingu borehole station southern Kii Peninsula
AU: * Fukuda, M
EM: fkdmst@seis.nagoya-u.ac.jp
AF: Nagoya University, Earth and Environmental dynamics course, Earth and Environmental Sciences, Graduate School of Environmental Studies, Nagoya University, Furo-cho Chikusa-ku, Nagoya, 464- 8601, Japan
AU: Sagiya, T
AF: Nagoya University, Earth and Environmental dynamics course, Earth and Environmental Sciences, Graduate School of Environmental Studies, Nagoya University, Furo-cho Chikusa-ku, Nagoya, 464- 8601, Japan
AB: In January 2006, a deep low frequency tremor activity and an associated short-term slow slip event occurred in the eastern Kii Peninsula and this coupled activity migrated to the northeast at a rate of 10km/day. We are monitoring crustal deformation at Shingu borehole station in the southeastern Kii peninsula. The Shingu borehole site is located about 100km landward from the Nankai Trough axis, and close to the epicenter of the 1944 Tonankai Earthquake. The borehole is 500 m deep and is equipped with an integrated multi-component borehole monitoring system developed by Ishii et al. (2002), consisting of 6 strain sensors (4 in horizontal, 2 in vertical), 2 pendulum tilt sensors, a magnetic direction finder, and a quartz thermometer. Each signal is originally recorded with a sampling frequency of 50 Hz. We decimated the original data into hourly data, which we decomposed into tidal response, barometric response, smoothed trend and random noise component by applying BAYTAP-G software [Tamura et al., 1991]. In the trend component from November 2005 to March 2006, we did not found deformation signal at the time of the Jan. 2006 tremor event. However, we found three significant slow strain changes from the processed records. Two of them coincide with the occurrence of the tremor activities in the southern Kii Peninsula, and are characterized by N-S contraction (0.019-0.031 ppm) and E-W extension (0.025-0.038 ppm). These are the first evidence of the short-term slow slip event in this area. The third change is characterized by NW-SE extension (0.026 ppm), N-S contraction (0.012 ppm), E-W extension (0.022 ppm), and southwestward tilting (0.23 micro rad). It occurred from December 29, 2005 to January 2, 2006, just before the tremor and slip event in January 2006, but was not accompanied by any tremor activity. We conducted a series of inversion analysis to infer the source of this possible slow slip event. We assumed that the slow slip event was caused by a reverse fault slip on the plate boundary, and found that the fault model with the following parameters successfully reproduces the observation. Here are the estimated parameters; (34.08 (latitude), 136.23 (longitude), 214 degree (strike angle), 22.1 degree (dip angle), 35 km (depth), 4.1 cm (slip), 90.64 degree (slip angle), 34.1 km (length), 24.4 km (width)). The estimated moment magnitude was 5.9. The result strongly suggests that the strain change was caused by a slow slip on the plate boundary at the southwestern extension of the tremor activity in January 2006. Since the January 2006 tremor event migrated to the northeast, the slow transitions seem to be the direct precursory of the tremor activity. This will probably be the first observation of the short-term slow slip event without associated tremor activity along the Nankai Trough. The estimated fault model fills a gap between clusters of tremor activities, indicating lateral heterogeneity along the down-dip end of the seismogenic zone. Also, our result suggests that the slow slip is essential process and tremors are just accompanying.
DE: 8150 Plate boundary: general (3040)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting