6:15 PM - 7:30 PM
[SSS31-P04] Strain rate field in Kyushu district estimated from GPS velocity data
Keywords:crustal deformation, strain rate, high strain rate region, inelastic deformation, Kyushu district, Philippine Sea plate
We estimated site velocities using GEONET F3 solution, daily coordinates of continuous GNSS sites, derived from the Geospatial Information Authority of Japan (Nakagawa et al., 2009). According to a conventional method of processing for GPS time series, we removed annual and semiannual variations, and offsets caused by earthquakes and GPS antenna replacement. Next, using the method proposed by Shen et al. (1996), we calculated strain rates by forming crossover area within a 50 km diameter every one GPS observation site, assuming uniform deformation in each crossover area. Finally, we determined a rigid motion and strain rates in the whole area simultaneously by the method of least squares, and then, we obtained strain rate distribution interpolated every 20 km. We set the same period as shown in Takayama and Yoshida (2007), when few unsteady events occurred. From estimated strain rate field, following characteristics were derived.
(1) A high strain rate region with the maximum shear strain rate of about 120 nanostrain/yr was recognized along N32o, whose width was about 50 km.
(2) The crustal shortening of about 30-210 nanostrain/yr in the northern part and the central part of Kyushu district indicated the direction of ESE-WNW on the Pacific side. On the back-arc side, the magnitude of those shortening decreased and those directions were rotating counterclockwise.
(3) The crustal shortening in the southern part of Kyushu district indicated the direction of ENE-WSW and the highest shortening rate was 130 nanostrain/yr on the back-arc side.
The result (1) suggests that the fault locking ranges from 0 to 5 km in depth, assuming the fault slip (5 mm/yr) of infinitely long fault in the EW direction under high strain rate region. As for results (2) and (3), it is necessary to consider the effects of plate subduction and back-arc spreading into the analysis. For the future, we will calculate strain or strain rate released by earthquakes (moment release) and conduct the quantification of inelastic behavior ongoing in Kyushu district by comparing strain budget.
This study was carried out under a contract with METI (Ministry of Economy, Trade and Industry) as part of its R&D supporting program for developing geological disposal technology.