Distribution of seismic coupling on the subducting plate boundary in northeastern Japan inferred from GPS observations

Many Global Positioning System (GPS) stations of the Geographical Survey In stitute and Tohoku University in northeastern Japan observed clear coseismi c and postseismic deformations associated with the M7.5 1994 Far Off Sanrik u earthquake, Applying the geodetic inversion algorithm given by Yabuki and Matsu'ura (Geophys. J. Int., 1992, 109, 363-376) to the observed CPS data, we estimate in this paper the spatial distributions of the coseismic and p ostseismic slip on the boundary between the subducting Pacific plate and th e overriding continental plate. It is noted that the coseismic slip estimat ed in this paper denotes the slip for a period between the day before and t he day after the main shock. The inversion for coseismic slip shows that th e main shock was thrust faulting typical for interplate earthquakes, which is consistent with the results of other seismological studies. The short-te rm postseismic deformation (STD) observed for a period of ca. 10 days betwe en the main shock and its largest aftershock shows that the initially high deformation-rate was lowered rather rapidly in comparison with the subseque nt long-term deformations (LTD) lasting longer than 1 year, which decreased at an almost constant low rate. The observed data of STD can be explained by postseismic slip distributed approximately on the same region as the fau lting area of the main shock, where the moment release during this period i s ca. 16% of that estimated for the coseismic slip. The region of the slidi ng causing LTD is found to have significantly expanded southward and downwa rd (or westward) on the plate boundary. The moment released by the postseis mic sliding for nearly 1 year amounts approximately to 80% of that released by the coseismic slip in addition to the contribution from uniform steady sliding due to the relative plate motion. Furthermore, we find from the inv ersion analysis a locked region to the south of the long-term postseismic s liding region on the plate boundary. The amount of nominal back-slip estima ted is so large that the locked region of the plate boundary is considered to have been tightly coupled for the year of 1995. This study demonstrates that the GPS observation only for ca. 1 year can distinguish the decoupled region due to the postseismic sliding around the source area of the main sh ock from the coupled one of the plate boundary. (C) 2000 Elsevier Science B .V. All rights reserved.