Electrical resistivity modeling around the Hidaka collision zone, northern Japan: regional structural background of the 2018 Hokkaido Eastern Iburi earthquake (Mw 6.6)

Abstract

The Hidaka collision zone, the collision boundary between the NE Japan and Kurile arcs, is known to be an ideal region to study the evolution of island arcs. The hypocenter of the 2018 Hokkaido Eastern Iburi earthquake (M w 6.6) in the western part of the Hidaka collision zone was unusually deep for an inland earthquake, and the reverse fault that caused the earthquake has an uncharacteristically steep dip. In this study, we used three-dimensional inversion to reanalyze broadband magnetotelluric data acquired in the collision zone. The inverted resistivity model showed a significant area of high resistivity around the center of the collision boundary. We also identified a conductive zone beneath an area of serpentinite mélange in a zone of high P-T metamorphic rocks west of the high-resistivity zone. The conductive zone possibly reflects areas rich in pore fluids related to the formation and elevation of the serpentinites. Sensitivity tests indicated the need for additional magnetotelluric survey data to delineate the resistivity distribution around the epicentral area of the 2018 earthquake although the resistivity model showed a conductive zone in this area.

Imaging of An Uplifted Serpentinite Complex In The Kamuikotan Zone, Northern Japan, From Magnetotelluric Soundings

“…Hence, we interpret the C-1 anomaly to represent Cretaceous and Paleogene-Neogene conductive sedimentary rocks. Similar associations between resistivity anomalies and outcrops have been recognized in other parts of middle-western Hokkaido (Ichihara et al 2008(Ichihara et al , 2016Yamaya et al 2017;Ichihara et al 2019).…”

“…3). A similar conductive anomaly has been identi ed under a serpentinite body in the southern part of the Kamuikotan zone (Ogawa et al 1994;Ichihara et al 2019). In a general sense, these relationships can be considered to imply an association between serpentinite bodies and underlying conductive anomalies.…”