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Spatial Variations in Fault-Zone Structure along the Nojima Fault, Central Japan, as Inferred from Borehole Observations of Fault-Zone Trapped Waves

Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, Tsukuba Central 7, 1-1-1 Higashi, Tsukuba, Ibaraki 3058567, Japan y-kuwahara{at}aist.go.jp

Hisao Ito

Center for Deep Earth Exploration, Japan Agency for Marine–Earth Science and Technology, 3173-25 Showacho, Kanazawa, Yokohama, Kanagawa 2360001, Japan hisaoito{at}jamstec.go.jp

Kin’ya Nishigami

Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Kyoto 6110011, Japan nishigam{at}eqh.dpri.kyoto-u.ac.jp

^* Present address: Seismic Product Line, Schlumberger, 2-2-1 Fuchinobe, Sagamihara, Kanagawa 2290006, Japan; tmizuno@slb.com.

We investigated the fault-zone structures (fault-zone width, shear-wave velocity, and Qs) of the Nojima fault, Japan, using the Love-wave-type fault-zone trapped waves (LTWs) recorded at two borehole stations—TOS2 and HRB—located along the fault. TOS2 (depth: 1673 m) is located at an end of the fault, while HRB (depth: 600 or 720 m) is located in the middle section of the fault where the greatest surface displacement of 2 m was recorded during the 1995 Kobe earthquake. The distance between the two stations is about 4 km. We found 22 records that exhibit typical LTW. We assumed the fault-zone structure to be a 2D uniform low-velocity wave guide and estimated the averaged fault-zone structure from the hypocenter to receiver by modeling the LTWs. Because we can infer the low-velocity fault zone to a depth of 8 km by observing the duration of LTWs, we obtain the average fault-zone structure from the borehole stations to a depth of approximately 8 km. The width, shear-wave velocity, and Qs of the fault zone beneath HRB are 100 m, 2.9 km/sec, and 60, respectively. On the other hand, the average width of the fault zone beneath TOS2 is 220 m, which is larger than that beneath HRB. We also conducted 3D finite-difference modeling of LTWs to confirm the spatial variations in the fault-zone structure to a depth of 8 km. The numerical simulation suggests that the Nojima fault has uniform elastic and attenuation properties along the fault zone to a depth of 8 km, while the width of the fault zone increases with the distance from the HRB to the south.