S-Wave Velocity Structure of the Taiwan Chelungpu Fault Drilling Project (TCDP) Site Using Microtremor Array Measurements

Abstract

The Taiwan Chelungpu Fault Drilling Project (TCDP) drilled a 2-km-deep hole 2.4 km east of the surface rupture of the 1999 Chi–Chi earthquake (Mw 7.6), near the town of Dakeng. Geophysical well logs at the TCDP site were run over depths ranging from 500 to 1,900 m to obtain the physical properties of the fault zones and adjacent damage zones. These data provide good reference material for examining the validity of velocity structures using microtremor array measurement; therefore, we conduct array measurements for a total of four arrays at two sites near the TCDP drilling sites. The phase velocities at frequencies of 0.2–5 Hz are calculated using the frequency-wavenumber (f–k) spectrum method. Then the S-wave velocity structures are estimated by employing surface wave inversion techniques. The S-wave velocity from the differential inversion technique gradually increases from 1.52 to 2.22 km/s at depths between 585 and 1,710 m. This result is similar to those from the velocity logs, which range from 1.4 km/s at a depth of 597 m to 2.98 km/s at a depth of 1,705 m. The stochastic inversion results are similar to those from the seismic reflection methods and the lithostratigraphy of TCDP-A borehole, comparatively. These results show that microtremor array measurement provides a good tool for estimating deep S-wave velocity structure.