Crust Structure of an Intra-Plate Earthquake Zone Associated With Crust Strain Concentration in Eastern Hokkaido, Japan, Based on Resistivity and Gravity Data

Strain concentrated zone revealed by recent GPS study and indicated increasing of delta CFF appeared after the 2003 Tokachi-oki earthquake in eastern Hokkaido, Japan. Intra-plate earthquakes (M5.5~6.5) occurred in 1960's in this area after the former 1952 Tokachi-oki earthquake. However, distribution of the earthquake faults has not been clarified by geological and topographic study, because thick Holocene deposit (more than 600m) covers most of studied area. Therefore, we have been investigating about crustal structure to discuss process of strain concentration in this area based on magnetotelluric (MT) and gravity data. MT data were acquired in 320-0.00034 Hz frequency range at 30 sites allocated in grid-like pattern with 5 km intervals to construct 3D resistivity structure. Low noise data were obtained at most of sites and apparent resistivity and impedance phase were computed by series of data processing including the remote reference process. We have delineated the detail Bouguer anomaly map using 115 of newly observed gravity data and about 600 of existing data. Based on the resistivity structure, the studied area is categorized as the northern area that have complex structure and the southern area that have simple layered structure. Most of past earthquakes were distributed on the northern area. The low resistivity zone, 10 times lower than the other part, is located in the east part of the northern area where many volcanoes are distributed. Aftershocks of the past earthquakes were distributed at a high resistivity side in the west part of northern area. This high resistivity area is may concluded by surface geological heterogeneity formed by tertiary volcanic and sediment rocks. However, gravity data indicate that this geological heterogeneity is distributed more than some km in depth. These results indicate that intra-plate earthquake occurred at the heterogeneity zone around rigid and soft rock contact. This suggests that the heterogeneity caused by contrast between volcanic and non-volcanic crust is originating strain concentration in this area.