Abstract
AbstractThe Rumoi-Nanbu earthquake (M6.1) 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.
Highlights
Strain accumulation zones with a large convergence of 20 mm/year, such as the Niigata-Kobe Tectonic Zone (NKTZ) (Sagiya et al, 2000), have been reported by the Japanese nationwide GPS array (GEONET)
For the lower frequency band (0.01–0.00034 Hz), dominant strikes were determined at N45◦W-S45◦E or N45◦E-S45◦W along all the lines
The presence of seawater near but still outside of the profiles may affect the MT data, the inverted models are still reliable. One reason for this is that the model is validated by the inversion test with a hypothetical model described in Ogawa (2002) and Takakura (2004)
Summary
Strain accumulation zones with a large convergence of 20 mm/year, such as the Niigata-Kobe Tectonic Zone (NKTZ) (Sagiya et al, 2000), have been reported by the Japanese nationwide GPS array (GEONET). For the lower frequency band (0.01–0.00034 Hz), dominant strikes were determined at N45◦W-S45◦E or N45◦E-S45◦W along all the lines. Data in the lower frequency band (0.01–0.00034 Hz) clearly revealed a strike oriented in the N50◦W-S50◦E direction and a high skew angle (3◦ < β < 25◦) in most of the MT sites; this implied a strong three-dimensionality. The strike direction for line A was set to N20◦E-S20◦W, subject to the result of the G-B decomposition This direction was inconsistent with that of the induction vector and differed from that of the estimated strikes of the other lines, implying that a three-dimensionality in the structure was strong around line A as compared to the other profiles. The resistivity beneath site B4 at a depth of 3 km was 20 Ohm-m, while the resistivity of the surrounding blocks at an equivalent depth was 5–10 Ohm-m, indicat-
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