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

Following a recent paperwe useweak-motionwaveforms 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 Mw 7.6. The excitation/attenuation model is given in terms of frequency-dependent seismic
wave attenuation, Qs(f ), geometrical spreading, g(r), amagnitude-dependent stress parameters
σ 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,
Mw 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 Mw 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
g (r ) =
⎧⎪
⎪⎨⎪
⎪⎩
r−1.2 1 < r < 10 km
r−0.7 10 < r < 40 km
r−1.0 40 < r < 80 km
r−0.5 r > 80 km.
Earthquake-related excitation spectra were calibrated over our empirical results by using a
magnitude-dependent Brune model with a stress drop value of σ = 8.0 ± 1.0 MPa for the
largest event of Mw 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-FaultModel Simulation, EXSIM.
The absolute peak ground accelerations (PGA), peak ground velocities (PGV) and 5 per centdamped
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
magnitudeMw 6.5, the upper limit for the events of ourweak-motion data set. The performance
of the stochastic model was then tested against the strong-motion data recorded during the Mw
7.6 Chi–Chi earthquake, and against several other empirical ground-motion models.