Fault zone Q values derived from Taiwan Chelungpu Fault borehole seismometers (TCDPBHS)

摘要

The attenuation factor, Q, at a fault zone is an important parameter for understanding the physical properties. In this study, we investigated the Q value of the Chelungpu Fault, the main rupture of the Mw 7.6 Chi-Chi earthquake, using the 7-level TCDP borehole seismometer array (TCDPBHS). The TCDPBHS was deployed at depths from 945 to 1270m throughout the 1999 ruptured slip zone at 1111m. Three borehole seismometers (BHS1-BHS3) were placed in the hanging wall, and the remaining three (BHS5-BHS7) were placed in the foot wall, with BHS4 near the slip zone. The configuration allowed us to estimate the Q-structure of the recent ruptured fault zone. In this study, we estimated Q values between BHS1 and BHS4, Qs_1 (Qp_1) at the fault zone and between BHS4 to 2km in depth, Qs_4 (Qp_4) beneath the fault zone. We utilized two independent methods, the spectral ratio and spectral fitting analyses, for calculating the Q value of Qs_1 (Qp_1) in order to provide a reliability check. After analyzing 26 micro-events for Qs and 17 micro-events for Qp, we obtained consistent Q values from the two independent methods. The values of Qs_1 and Qp_1 were 21-22 and 27-35, respectively. The investigation for the value of Qs_4 was close to 45, and Qp_4 was 85. These Qp and Qs values are quiet consistent with observations obtained for the San Andreas Fault at the corresponding depth. A low Qs_1 value for the recent Chelungpu Fault zone suggests that this fault zone has been highly fractured. Qs values within the Chelungpu Fault, similar to those within the San Andreas Fault, suggest that the Q structure within the fault zone is sedimentary rock independent. However, the possible existence of fluids, fractures, and cracks dominates the attenuation feature in the fault zone.