Tao-Ming Chang

An Observation of Rupture Pulses of the 20 September 1999 Chi-Chi, Taiwan, Earthquake from Near-Field Seismograms

The ground-velocity recordings of the 20 September 1999, Chi-Chi, Taiwan earthquake recorded at stations near the ruptured fault trace show a simple, large-amplitude, and long-period pulse following the S wave, which is closely associated with the surface faulting and the rupture process of thrust faulting. The conspicuous pulse on the ground-velocity seismogram following the S -wave arrival, called the S 1 phase, is interpreted as the superposition of the rupture pulses that nucleate at an asperity near and underneath the station and propagate up-dip and laterally along the fault plane toward the surface stations. The arrival times of the S 1 phase and the onsets of the permanent displacement at stations near and along the ruptured fault trace increase with hypocentral distance, suggesting that the rupture of the Chi-Chi earthquake might have initiated at the hypocenter of the mainshock and propagated both upward and laterally from south to north. On the basis of the travel-time differences between the S 1 phase and the direct S wave at the stations near and along the ruptured fault trace, the rupture velocities varied from 2.28 to 2.69 km/sec, with an average rupture velocity of about 2.49 km/sec. The rupture velocities decreased from south to north.

Estimates of Stress Drop of the Chi-Chi, Taiwan, Earthquake of 20 September 1999 from Near-Field Seismograms

The apparent stress and stress drop of the Chi-Chi, Taiwan, earthquake are estimated from near-field seismograms. The estimated apparent stress and stress drop for the southern part of the fault are about 100 bars lower than those for the northern part. The estimated ratio E s/ M also suggests that there is a higher dynamic stress drop in the northern part than in the southern one. This indicates the transformation of a higher percentage of strain energy into the seismic-wave energy in the northern part than in the southern part. Based on a parameter proposed by Ramon Zuniga (1993), we propose that the stress model of frictional overshoot can interpret the rupture of the Chelungpu fault, on which the Chi-Chi, Taiwan, earthquake occurred. Manuscript received 31 July 2000.

Estimates of source parameters for the 1999 Chi-Chi, Taiwan, earthquake based on Brune's source model

The general features of the rupture of the 1999 Chi-Chi, Taiwan, earthquake ( M s 7.6) can be explained by the displacement waveforms derived from the accelerograms recorded at short distances from the fault traces. Applying Brunes model, we have determined important source parameters, such as rise time, stress drop, offset, and particle velocity. Generally, the earthquake is characterized as having had two distinct fault segments. The southern segment, dominated by thrust motion, started from the focus on a fault plane raking at 78° and extended about 30 km to the north. The northern segment, dominated by thrust with significant strike-slip motion, began next to the end of the southern segment on a fault plane raking at 53° and extended northward for 25 km. Slips in the southern segment were followed by a small dislocation (∼1 m), while those in the northern segment were followed by a much larger dislocation (∼9 m). The average slip velocity was distributed at 34-49 cm/sec, along the southern segment, and an unusual slip velocity exceeding 2 m/sec was observed along the northern segment. Furthermore, the southern segment experienced a rise time of 1.8 sec and a stress drop of 65 bars, in contrast to a rise time longer than 4 sec and a stress drop larger than 300 bars registered to the north. Our results also indicate that, along the southern segment, the rupture propagated northward at an average velocity of 2.84 km/sec, but along the northern segment, the rate declined to less than 2 km/sec. The difference in the source parameters between these two segments suggests that the rupturing associated with the Chi-Chi earthquake may have encountered a resistive patch and changed course in the middle part of the fault. After crushing that resistance, the long rise time and high stress drop probably caused substantially slower motion and larger slip along the northern segment. Manuscript received 10 November 2000.

Characteristics of strong ground motion across a thrust fault tip from the September 21, 1999, Chi‐Chi, Taiwan earthquake

Near fault tip strong motion records from the northern part of the major earthquake (Mw = 7.6), namely the Chi-Chi earthquake on September 21, 1999 in central Taiwan demonstrated systematic differences on the hanging wall and footwall, and simulated by the finite element method. The extraordinary ground motion differences on either side of the northern fault tip can be explained by a 2-dimension kinematic source model with fault rupture breaking to surface. In this study, the earthquake faulting was considered as bilateral from the center of a low angle thrust fault which is 30 km in length with a dip angle of 31°. Based on waveform modeling, the source rupture velocity, rise-time and dislocation of 2.0 km/sec, 5 sec and 6 meters, respectively are suggested. The results of this study show that on the northern part of the Chi-Chi earthquake fault there was lower rupture velocity and longer rise-time of the fault slip than that previously reported. Furthermore, the effects of surface breaking from the fault movement contributed large ground deformations near the fault tip and, consequently, induced extensive damage.

Two‐dimensional numerical modeling for near source strong ground motions of the Chelungpu fault during the 1999 Chi‐Chi Taiwan earthquake

Abstract Two‐dimensional numerical modeling based on the finite element method is employed to simulate near fault tip strong motion records of the Chi‐Chi Earthquake on September 21, 1999, in order to examine the kinematics of earthquake faulting and near field wave propagation properties. The modified ‘split‐node’ technique of Melosh and Raefsky is employed to simulate the equivalent body forces system for a double couple without moment. Different source parameters are examined to synthesize ground motions to simulate the observed near source strong motion records. Based on waveform modeling, a low angle thrust fault 30 km in length with a dip angle of 30°?is suggested for the ruptures near its northern end. The rupture is interpreted as bilateral from the center of the fault. The determined source rupture velocity and risetime are 2.0 km/sec and 5 sec, respectively, while the dislocation is about 6 meters. The estimated peak fault slip velocity is about 3.5 km/sec, showing a largest value than previously reported. Results of this study show that there was lower rupture velocity and longer rise‐time of the fault slip on the northern part of the Chi‐Chi Earthquake fault. Numerical modeling of this study indicates that the directivity of source rupture does not show significant low frequency ground motion near the earthquake fault tip. However, the effects of surface breaking of the fault movement have contributed large ground deformations near the fault tip, consequently, inducing large damage.