Resistivity structures around intraplate earthquake zones in Hokkaido, Japan

Deciphering heterogeneity in subsurface structures underneath intraplate earthquake zones is an important element in understanding how stress is concentrated and how fault ruptures occur. In this paper, we focus on two intraplate earthquake zones in Hokkaido, Japan, where the Okhotsk, Pacific and Amurian plates are bounded. One is Teshikaga region where 11 large earthquakes (> M5) occurred during 1959 and 1967 (Hirota, 1969). Teshikaga region is located at the northern margin of the fore-arc sliver of the Kurile arc. The other one, located near the boundary between the Okhotsk and Amurian plates, is western part of Dohoku region where the 2004 Rumoi-nanbu earthquake (M6.1) occurred. We performed wide-band MT surveys and analyzed several 2D resistivity structures to image heterogeneity of the crustal structure beneath these two areas. These resistivity profiles were compared with gravity data and geological structureto discuss relationships between the intraplate earthquakes and crustal structures. In Teshikaga area, a high resistivity (> 300 ohm-m) zone with a horizontal width of 10-20 km is imaged down to the depth of about 5 km from surface. The high resistivity zone seems to represent Miocene volcanic rocks. This was confirmed by borehole (NEDO, 1985) and gravity data. Aftershocks of the 1968 earthquake (Hirota, 1969) occurred in and around the high resistivity zone, suggesting that the earthquake occurred at more resistive and more competent zone where stress is concentrated. In the 2004 Rumoi-nanbu earthquake area, we analyzed resistivity structure along two profiles: one crosses the focal area (North profile), while the other (South profile) does not. In general, both profiles show low resistivity (1-10 ohm-m) from surface to a depth of about 5 km, and moderate resistivity (10-100 ohm-m) at deeper zone. However, the North and South profiles differ in that the former is characterized by a complex undulation within the upper low resistivity zone whereas the latter shows relatively simple structure. Comparison with surface geology suggests that the above complex undulation represents folding and faulting structure. These results imply that the crustal heterogeneity due to complex folding and faulting is related to the occurrence of the intraplate earthquake. Acknowledgement: We thank Japan National Oil Corporation for offering borehole data and Esashi Observatory of the Geographical Survey Institute for allowing us to use MT data.