Three‐dimensional resistivity structure in Ishikari Lowland, Hokkaido, northeastern Japan—Implications to strain concentration mechanism

Abstract

The Ishikari Lowland on the island of Hokkaido in northeastern Japan is situated at the end of a westward‐moving foreland fold‐and‐thrust belt from the Hidaka collision zone, where the northeastern Japan and Kurile arcs meet. This activity forms a tectonic zone under an east‐west compression field in this region. A magnetotelluric resistivity survey was performed to investigate the mechanism for the strain concentration in this region. A three‐dimensional (3‐D) resistivity inversion showed a conductive thick sedimentary layer and an underlying resistive basement. Remarkable conductors were found in the resistive basement beneath the Ishikari‐teichi‐toen fault zone (ITFZ) and the Shikotsu caldera. The conductors beneath the ITFZ were interpreted as aqueous fluids that accumulated in the damaged zone in connection with the formation of pull‐apart faults and horst. In contrast, the conductor beneath the Shikotsu caldera corresponds to a magmatic fluid path from the upper mantle. These features suggest that the ductile deformation in the upper crust contribute to the strain concentration in this region. The soft thick sediments allow ductile deformations to occur. Furthermore, local fluid‐rich zones in the basement cause the crustal strength to be heterogeneous. These thick sediments and local fluids in the basement both contribute to the strain concentration in this region.

Three‐Dimensional Electrical Resistivity Modeling to Elucidate the Crustal Magma Supply System Beneath Aso Caldera, Japan

“…The other three sequences of sensitivity tests consisted of similar changes to the resistivities of the C2 anomaly at 1.5–24 km depth, the C3 anomaly at 11.5–39 km depth, and the C4 anomaly at 1–8 km depth (Figure ). We evaluated the significance of the difference between the misfit of the sensitivity test models and the misfit of our original model (1.8188) based on an F test (e.g., Yamaya et al, ). The RMS misfits of the test models with a significant difference possessed an F value of >1.8447 based on the F test with the 95% confidence level and 100 × 12 × 16–1 = 19,199 degrees of freedom (Table ).…”

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

“…The 2018 Hokkaido Eastern Iburi earthquake (M w 6.6) occurred near the western edge of the HCZ and -2001 and 2005, respectively. Green diamonds are MT stations used in resistivity modeling by Yamaya et al (2017). Note that MT data from stations 420 and 450-500 were used in the 3-D inversions of both this study and that of Yamaya et al (2017).…”

“…Green diamonds are MT stations used in resistivity modeling by Yamaya et al (2017). Note that MT data from stations 420 and 450-500 were used in the 3-D inversions of both this study and that of Yamaya et al (2017). The gray line marks the seismic refraction/wide-angle reflection profile of Iwasaki et al (2004).…”

Three-dimensional S-wave attenuation structure in and around source area of the 2018 Hokkaido Eastern Iburi Earthquake, Japan

“…According to their constructed Vp model, the low-Vp, < 4.5 km/s, layers are lying near the ground surface in the west side of the HMR. These layers were interpreted as the thick sedimentary layers, and associating features are identified in other geophysical measurements, such as S-wave velocity (e.g., Nishida et al 2008), P-wave attenuation (Kita et al 2014), and electric resistivity (Yamaya et al 2017). Kinoshita and Ohike (2002) reported that the sediments would affect to reductions of Qs from the studies around the Kanto district, Japan.…”