A 3-D electrical resistivity model beneath the focal zone of the 2008 Iwate-Miyagi Nairiku earthquake (M 7.2)

Abstract

The 2008 Iwate-Miyagi Nairiku earthquake (M 7.2) was a shallow inland earthquake that occurred in the volcanic front of the northeastern Japan arc. To understand why the earthquake occurred beneath an active volcanic area, in which ductile crust generally impedes fault rupture, we conducted magnetotelluric surveys at 14 stations around the epicentral area 2 months after the earthquake. Based on 56 sets of magnetotelluric impedances measured by the present and previous surveys, we estimated the three-dimensional (3-D) electrical resistivity distribution. The inverted 3-D resistivity model showed a shallow conductive zone beneath the Kitakami Lowland and a few conductive patches beneath active volcanic areas. The shallow conductive zone is interpreted as Tertiary sedimentary rocks. The deeper conductive patches probably relate to volcanic activities and possibly indicate high-temperature anomalies. Aftershocks were distributed mainly in the resistive zone, interpreted as a brittle zone, and not in these conductive areas, interpreted as ductile zones. The size of the brittle zone seems large enough for a fault rupture area capable of generating an M 7-class earthquake, despite the areas distributed among the ductile zones. This interpretation implies that 3-D elastic heterogeneity, due to regional geology and volcanic activities, controls the size of the fault rupture zone. Additionally, the elastic heterogeneities could result in local stress concentration around the earthquake area and cause faulting.

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.…”

“…Such complex tectonic settings suggest complex subsurface structures that may be related to the characteristic seismicity. Because electrical resistivity is sensitive to the presence of fluids, and subsequently the elasticity of the media, it is important to investigate how resistivity structures relate to earthquake generation Fujinawa et al 2002;Goto et al 2005;Guerer and Bayrak 2007;Wannamaker et al 2009;Yoshimura et al 2009;Ichihara et al 2011Ichihara et al , 2014Ichihara et al , 2016Ogawa et al 2014;Kaya et al 2013). To investigate the relationship between earthquakes and electrical resistivity structure, we gathered and analyzed the broadband (typically 0.003-10,000 s) magnetotelluric (MT) and telluric data, which resolve the resistivity structure from the surface to the depth of the upper mantle.…”

Three-dimensional magnetotelluric imaging of crustal fluids and seismicity around Naruko volcano, NE Japan

“…Further data acquisition and compilation of previous datasets are both underway, and the distribution of crustal fluids will be imaged for a larger area (Ichihara et al 2014;Kanda and Ogawa 2014) together with seismic data (Okada et al 2012;Okada et al 2014). …”

Special issue ‘Geofluid processes in subduction zones and mantle dynamics’

“…Their results suggest the presence of a deep crustal conductor that may correspond either to partial melts and/or high-salinity fluids. Ichihara et al (2014) present a 3D electrical resistivity model beneath the focal zone of the 2008 Iwate-Miyagi Nairiku earthquake that shows a shallow conductive zone beneath the Kitakami Lowland and several conductive patches beneath active volcanic areas. Yoshida et al (2014) determined pore fluid pressure distribution in the focal region of the above earthquake and suggest that geofluids supplied from the mantle wedge have contributed to the generation of high pore pressures and to the lowering of frictional strengths of seismic faults in this region.…”