Resistivity structure around the focal area of the 2004 Rumoi-Nanbu earthquake (M 6.1), northern Hokkaido, Japan

Abstract

The Rumoi-Nanbu earthquake (M 6.1) occurred in northern Hokkaido, Japan, on December 14, 2004. We conducted MT surveys along three profiles in and around the focal area to delineate and decipher the structural features of the seismogenic zone. The inverted 2-D resistivity images of the three sections comprised two layers: an upper conductive layer and a lower resistive layer. The boundary of these layers lay at a depth of approximately 3-5 km. A comparison with the surface geology and drilling data revealed that the upper conductive layer and the lower resistive layer corresponded to the Cretaceous-Tertiary sedimentary rocks and older basement rocks, respectively. A clear upheaval of the layer boundary was found along the profile at the center of the focal area. In addition, borehole data indicated an obvious increase in the Young's modulus toward the lower layer. Therefore, the elastic properties with a complex geometry around the focal zone tended to vary; this probably depicts the zone of stress accumulation that triggered the earthquake.

Seismicity controlled by resistivity structure: the 2016 Kumamoto earthquakes, Kyushu Island, Japan

“…This study shows that the seismogenic zones correspond approximately to resistive zones lying adjacent to conductive zones, or to the conductive-resistive transition zone. These results are consistent with previous magnetotelluric studies conducted across the epicenters of large (>M 6) inland earthquakes (Mitsuhata et al 2001;Ogawa et al 2001;Tank et al 2003Tank et al , 2005Kasaya and Oshiman 2004;Ichihara et al 2008Ichihara et al , 2014Yoshimura et al 2008;Kaya et al 2009;Umeda et al 2011Umeda et al , 2014Chandrasekhar et al 2012) with the exception that aftershocks occur in a thick sedimentary layer (Uyeshima et al 2005). Note here that the dense magnetotelluric observations occasionally image localized subvertical conductors beneath the active faults (e.g., Unsworth et al 1997;Wannamaker et al 2002;Becken et al 2008;Ikeda et al 2013;Sass et al 2014).…”

“…We hypothesize that the conductive zone preferentially deforms, such that the static stress over Kyushu (Matsumoto et al 2015;Savage et al 2016) accumulates preferentially in proximal brittle resistive zones and subsequently causes large earthquakes. The concept of local stress accumulation has been proposed based on the results of previous magnetotelluric studies (e.g., Ogawa et al 2001;Ichihara et al 2008Ichihara et al , 2014Wannamaker et al 2009). The concept is similar to the hypothesis of Iio et al (2002) who assumed that the lower crust had a deformable weak zone.…”

The September 2011 Sikkim Himalaya earthquake Mw 6.9: is it a plane of detachment earthquake?

“…Generally, earthquake occurs in the structure having heterogeneous resistivity (high or low resistivity zone) and more competent zone where stress is concentrated (Ichihara et al 2008). The magnetotelluric study of Patro and Harinarayana (2009) shows a low resistive zone near the source zone of 2011 Sikkim earthquake and its aftershock region (figure 5).…”

An improved forward modeling method for two-dimensional electromagnetic induction problems with bathymetry

“…We adopted Ogawa and Uchida's 2-D FEM forward code as a representative of 2-D FEM forward codes using rectangular elements. Since Ogawa and Uchida's code allows useful parameters such as static shifts, it is widely used in the EM induction community (e.g., Ichihara et al, 2008). Figure 4 shows the results of the appraisal in terms of the apparent resistivity and phase in the TM mode.…”