We conducted temporary GPS observations shortly after the occurrence of the 2004 Mid-Niigata Prefecture Earthquake (MJMA 6.8) at 5 sites around the southern part of its source region. These temporary sites are located between the existing continuous sites of GEONET so that we could resolve the crustal deformation pattern in more detail. We analyzed the observed data with those from surrounding GEONET as well as IGS GPS stations. The obtained daily coordinate changes revealed postseismic crustal deformation, which was characterized by the E-W shortening of the source region. Maximum postseismic displacement of about 20mm was found at Ojiya GEONET station and OGNI temporary station. In addition, the maximum uplift of about 20mm was detected at the two sites. The postseismic deformation faded out about 1 month after the main shock. It is considered that the postseismic deformation was caused by aseismic after-slip of shallow (depth<5km) fault. The fault is of a reverse type, but it is difficult to identify its dipping direction. Aseismic nature of the after-slip can be attributed to the characteristic crustal structure with thick soft sediments lying over the main shock source region. More studies are needed to resolve the generating mechanism of concentrated compressive strain in the source region of the Mid-Niigata Prefecture Earthquake.
