Spatio-temporal evolution of the postseismic slip associated with the 2005 Miyagi-Oki earthquake (M7.2) estimated from geodetic and seismological data

On August 16, 2005, a M7.2 earthquake occurred along the plate boundary off Miyagi Prefecture, northeastern Japan, where the Pacific plate is subducting beneath the overriding continental plate at a rate of about 80 mm/yr. There are at least three asperities that were ruptured during the 1978 Miyagi-Oki earthquake (M7.4) there, and one or two of them were reruptured during the 2005 earthquake. We estimated spatio-temporal evolution of the postseismic slip associated with the 2005 Miyagi-Oki earthquake using continuous land GPS and campaign ocean bottom GPS/acoustic observation data in order to investigate whether the strain accumulation process at the unruptured asperities are affected by the event in 2005 or not. Daily site coordinates were estimated using a PPP (Precise Point Positioning) strategy of GIPSY-OASISII Software based on the GPS data observed at continuous sites operated by GSI (Geospatial Information Authority of Japan) and Tohoku University. Data from January 2004 to December 2007 have been analyzed. The linear trends with annual and semi-annual variations for the period from January 1, 2004 to August 15, 2005, and co-seismic displacements due to the main shock were estimated by least square modeling and subtracted from the original onshore GPS time series. As to the ocean bottom displacement data, we calculated secular velocities at the offshore site locations from an interplate coupling model based on the linear trends of land GPS sites and subtracted them from the observed time series. We regarded that the detrended time series represented the deformation due to the afterslip of the 2005 earthquake and were inverted them to obtain spatiotemporal slip distribution on the plate boundary by using a time dependent inversion analysis. We applied an inversion method devised by Yagi and Kikuchi (2003) to estimate the evolution of the fault slip in both space and time. We also estimated spatiotemporal evolution of the aseismic slip based on the activities of small repeating earthquakes, and compared the temporal variations of the accumulated slip at various places estimated from the seismological data with those from geodetic data. Results from the geodetic inversion show large afterslip on the trench-ward side of the rupture areas of the 1978 and 2005 events. Temporal pattern of the cumulative slip estimated from the geodetic data conforms well to those obtained from the small repeating earthquake activity in the region that includes the asperities of the 1978 Miyagi-Oki earthquake. Spatial and temporal change in back-slip distribution in NE Japan is also estimated from onshore GPS data, and the results suggest that the afterslip associated with the 2005 event tends to concentrate in the regions with significant activity of the small repeating earthquakes and that the interplate coupling in the unruptured asperities was strong even in the afterslip period. Therefore, the strain in the asperities is thought to have been considerably accumulated.