Coseismic Fault Model of the M6.2 Northern Miyagi Earthquake, Northern Japan, Estimated from Multiple Geodetic Data

A shallow M6.2 earthquake occurred in Miyagi prefecture, northern Japan on July 26, 2003. The focal mechanism roughly shows that the earthquake released strain accumulation in the northeastern Japan arc under the stress regime of east-west compression due to the interplate locking on the subducting plate boundary between the Pacific and the North American plates. The coseismic displacement was observed not only by the GEONET (continuous GPS) but also by campaign GPS, leveling, and InSAR. We have a plenty of the deformation data by multiple geodetic measurements, which is unusually abundant as the M6-class earthquakes. It is noted that the leveling route in the epicentral area was surveyed on the day just before the earthquake by chance, and that it was resurveyed just after the earthquake. Though correlation of SAR images by RADARSAT is poor except urban area, SAR interferograms provide high resolution of surface displacements which reach 24 cm. We construct coseismic rectangular fault model by inversion of all available geodetic data. The number of observation points used in the inversion are 20, 17, 49 and thousands for continuous GPS, campaign GPS, leveling, and InSAR, respectively. We found that two thrust-type faults whose strike differs by 60^o are necessary to reproduce the observed deformation. Because rake angle of two faults is different by only 15^o, azimuth of slip vectors is significantly different between two faults. It suggests heterogeneity of stress field in the hypocentral region, which cannot be explained by the interplate coupling on the subducting plate boundary. Two faults having different orientation may be supported by apparent disagreement of focal mechanisms estimated from initial phase of P wave and CMT waveform inversion. Seismic moment for both faults is 1.8*E¹⁸ Nm (M_w 6.1), assuming rigidity of 30 GPa.