Elastic and Inelastic Strain Accumulation Along the Northern and Central Itoigawa-Shizuoka Tectonic Line, Central Japan
Abstract:Itoigawa-Shizuoka Tectonic Line (ISTL) in central Japan is one of the most active fault systems in Japan. The Japanese government evaluated a M8 class earthquake may occur at the Gofukuji fault in the central ISTL with a possibility of 14% in the next 30 years. So we analyze GPS data around the northern and the central ISTL to monitor tectonic strain accumulation and to propose a fault model for future earthquake in this area.
Along the northern and central ISTL, there exist active faults such as the Kamishiro fault (KF), the East Mathumoto Basin fault (EMBF) and the Gofukuji fault (GF). KF and EMBF are east-dipping reverse faults, and GF is a left-lateral strike slip fault. We analyzed GPS data of 34 campaign sites during 2002-2010 and 55 continuous sites during 1998-2013 to obtain 3-dimentional velocities in the ITRF 2008 reference frame. Around GF, the velocity field represents a typical inter-seismic pattern around a strike slip fault. By applying an elastic dislocation model, we estimate a fault slip rate as 5-7 mm/yr with a locking depth of over 5 km. These parameters are consistent with the seismogenic zone depth and the geological slip rate of GF. On the other hand, for KF and EMBF, we model the deformation pattern with faults in an elastic layer overlying a viscoelastic substratum to represent steady contraction. The modeling result shows KF dips at 30-40 degree and its locking depth is only 2 km, implying that the whole fault is creeping. EMBF dips at 40-50 degree with a locking depth of 2 km. The results indicate that there is ongoing stress accumulation around GF, but KF and EMBF accommodates contraction inelastically. GF at the central ISTL is considered to store strain energy more than 1.000 years and a future major earthquake should occur to release shear stress along the central part. The rupture may continue to the south, but more observation and modeling effect is necessary.