IASPEI - Picture Gallery

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS008 Oral Presentation 1984
Three dimensional resistivity structures in intra-plate earthquake zones in
Hokkaido, northern Japan
Mr. Hiroshi Ichihara
Institute of Seismology and Volcanology Hokkaido University IASPEI
Toru Mogi, Ryo Honda, Yusuke Yamaya
Clarifying heterogeneous subsurface structures in intraplate earthquake areas is an important element
in understanding mechanisms of the intraplate earthquakes, i.e. how/where the stress concentrates and
how/where the fault ruptures. In this paper, we focus on two intraplate earthquake areas in Hokkaido
district, Northern Japan. Hokkaido locates in a complex tectonic setting, where the Okhotsk, Pacific and
Amurian plate are colliding with each other. One of the intraplate earthquake areas is Teshikaga region
where there had been occurring 12 large earthquakes (> M5) during 1938 and 1967 (Hirota, 1969).
Teshikaga region is located in a volcanic belt formed along the northern margin of the fore-arc sliver of
the Kurile arc. The other one is focal area of the 2004 Rumoi-nanbu earthquake (MJMA 6.1), located
near the boundary between the Okhotsk and Amurian plates. In order to image heterogeneity of the
crustal structure, we performed wide-band MT surveys around these intraplate earthquake areas. As a
preliminary study, several 2-D resistivity models were analyzed. In both areas, these resistivity profiles
were compared with gravity data and geological structure to discuss relationships between the
intraplate earthquakes and crustal structures. In Teshikaga region, a high resistivity (> 300 ohm-m)
body is imaged at 0-5 km in depth with a horizontal width of 10-20 km, in the focal area of most of the
1938-1969 earthquakes. Comparisons with borehole (NEDO, 1985) and density structure indicate that
the high resistivity zone seems to represent Miocene volcanic rocks. On the contrary, conductive zones,
which are regarded as Quaternary-Pliocene sediments, are distributed around the resistive body. This
suggests that the earthquakes occurred around boundary between rigid rocks and non-rigid rocks by
stress concentration to the heterogeneity. The 2-D models also clarified that the fault of the 1938
earthquake corresponds to the wall of the Kutcharo caldera. Three profiles of 2-D resistivity images
were obtained around the focal area of the 2004 Rumoi-Nanbu earthquake. All images are roughly
comprised of two layers: upper conductive layer, existing surface to 3-5 km in depth, and lower resistive
layer. Comparisons of the resistivity image with the surface geology and drilling data indicate that the
upper conductive layer and the lower resistive layer correspond to Cretaceous-Tertiary sediment rocks
and older igneous rocks, situating as its basement, respectively. On the basis of this correspondence,
we found a clear upheaval structure in one profile across center of the focal area. This structure implies
steep variation in rigidity around the focal area. The rigidity variation suggests local accumulation of
strain, which probably triggered the earthquake. The gravity anomaly (Honda et al., 2007) and the
anticline structure observed in surface geology indicate that this structure extends along strike direction
of the fault. In order to clarify above subsurface structures including along strike variation, we
constructed three dimensional resistivity structure using 44 sites and 29 sites of magnetotelluric data at
Teshikaga and Rumoi area, respectively. The resistivity modeling was operated using the 3-D forward
modeling code developed by Fomenko and Mogi (2002). Around these seismogenic zones, both models
show heterogeneous structures, where resistive body surrounded by conductive zones. The 3-D
analyses also resolve incoherencies in 2-D modeling and result in accurate modeling. This enabled us to
discuss lower crustal heterogeneity under the focal area, resistivity of which is difficult to detect because
of existence of shallower conductive layers.
Keywords: magnetotelluric, intra plateearthquake