Predictions of high-frequency ground-motion in Taiwan based on weak motion data

Abstract

Following a recent paper we use weak-motion waveforms to calibrate a model for the prediction of earthquake-induced ground-motion in Taiwan, in the 0.25-5.0 Hz frequency range, valid up to M w 7.6. The excitation/attenuation model is given in terms of frequency-dependent seismic wave attenuation, Q s(f), geometrical spreading, g(r), a magnitude-dependent stress parameters Δσ for the excitation terms, and a site term for each seismic station used in the study. A set of weak-motion data was gathered from about 170 aftershocks of the Chi-Chi earthquake, M w 7.6, of 1999 September 20, (17:47 UTC), recorded by 10 broad-band seismic stations. The moment magnitudes of the registered aftershocks ranged from M w 3.0 to 6.5, and the hypocentral distances from a few kilometres to about 250 km. A frequency-dependent crustal quality factor, Q(f) = 350f 0.32, was obtained, to be coupled with the geometrical spreading function Earthquake-related excitation spectra were calibrated over our empirical results by using a magnitude-dependent Brune model with a stress drop value of Δσ= 8.0 ± 1.0 MPa for the largest event of M w 6.5 in our data set and with a near surface attenuation parameter of κ= 0.05 s. Results on region-specific crustal attenuation and source scaling were used to generate stochastic simulations both for point-source and extended-fault ruptures through the computer codes: Stochastic Model SIMulation, SMSIM and Extended-Fault Model Simulation, EXSIM. The absolute peak ground accelerations (PGA), peak ground velocities (PGV) and 5 per cent-damped Spectral Accelerations (SA) at three different frequencies, 0.33 Hz, 1.0 Hz and 3.0 Hz for several magnitudes and distance ranges were predicted at large magnitudes, well beyond magnitude M w 6.5, the upper limit for the events of our weak-motion data set. The performance of the stochastic model was then tested against the strong-motion data recorded during the M w 7.6 Chi-Chi earthquake, and against several other empirical ground-motion models.