Clayey injection veins and pseudotachylyte from two boreholes penetrating the Chelungpu Fault, Taiwan: Their implications for the contrastive seismic slip behaviors during the 1999 Chi-Chi earthquake |
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| Authors: | Kenshiro Otsuki Takayuki Uduki Nobuaki Monzawa and Hidemi Tanaka |
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| Institution: | Department of Geoenvironmental Science, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan (email: );and Department of Earth and Planetary Science, Graduate School of Science, Tokyo University, Tokyo 113-0033, Japan |
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| Abstract: | Abstract The seismic slip that occurred during the 1999 Chi-Chi earthquake in Taiwan showed contrastive behaviors in different regions along the Chelungpu Fault: A large and smooth slip occurred in the north, while a relatively small slip associated with high-frequency seismic wave radiation occurred in the south. The core samples from shallow boreholes at northern (Fengyuan) and southern (Nantou) sites penetrating the seismic Chelungpu Fault were analyzed. The fault zones at the northern site are characterized by soft clayey material associated with clayey injection veins. This suggests that the fault zones were pressurized during ancient seismic slip events, and hence the elastohydrodynamic lubrication occurred effectively. In contrast, the fault rock from the southern site is old pseudotachylyte that has been shattered by repeated ancient seismic slip events. Statistical analysis of many pseudotachylyte fragments reveals that the degree of frictional melting tended to be low. In this case, the seismic slip is restrained by the mechanical barrier of a highly viscous melt layer. These contrastive fault rocks were produced by repeated ancient seismic slip events, but the two corresponding mechanisms of friction are likely to have also occurred during the 1999 Chi-Chi earthquake, thus causing the contrastive slip behaviors in the north and south. |
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| Keywords: | Chelungpu Fault Chi-Chi earthquake clayey injection vein elastohydrodynamic lubrication frictional melting pseudotachylyte |
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