Win-Gee Huang

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.

Phase Velocity Variation at Periods of 0.5–3 Seconds in the Taipei Basin of Taiwan from Correlation of Ambient Seismic Noise

Improving seismic hazard mitigation of the densely populated metropol- itan area of and around the capital of Taiwan requires detailed knowledge of the 3D crustal structure of Taipei basin. The high levels of ambient noise and the low levels of regional seismicity of this region complicate investigations of crustal structure with traditional seismic exploration or earthquake tomography methods. We investigate the shallow crust in the metropolitan region using surface wave array tomography with time domain empirical Greens function (TDEGF) inferred from correlation of ambient seismic noise. Analysis of the TDEGF amplitudes suggests that the dominant sources of ambient seismic noise are the coastlines and shallow continental shelf of the Taiwan Strait, northwest of the study region. Our study demonstrates that ambient seismic noise tomography is feasible at periods of 0.5-3 s, which is much shorter than the 10-30 s used in most other studies, and which opens new opportunities for high resolution studies of near-surface heterogeneity. The lateral variation in Rayleigh wave phase velocity correlates well with surface geology and suggests that faults play an important role in the regional tectonic setting. High phase velocities mark the Tatun volcanic area, the Kuanyin Mountain dominated by Quaternary igneous rock, and the Miocene Western Foothills south of the Taipei fault. Low phase velocities characterize regions are along western and southeastern edges of the Taipei basin and the Pleis- tocene Linkou tableland. Main faults in the region are either marked by low phase velocities or define transitions between regions of high- and low-velocity anomalies.

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.

Anisotropic Rayleigh-wave phase velocities beneath northern Vietnam

We explore the Rayleigh-wave phase-velocity structure beneath northern Vietnam over a broad period range of 5 to 250 s. We use the two-stations technique to derive the dispersion curves from the waveforms of 798 teleseismic events recoded by a set of 23 broadband seismic stations deployed in northern Vietnam. These dispersion curves are then inverted for both isotropic and azimuthally anisotropic Rayleigh-wave phase-velocity maps in the frequency range of 10 to 50 s. Main findings include a crustal expression of the Red River Shear Zone and the Song Ma Fault. Northern Vietnam displays a northeast/southwest dichotomy in the lithosphere with fast velocities beneath the South China Block and slow velocities beneath the Simao Block and between the Red River Fault and the Song Da Fault. The anisotropy in the region is relatively simple, with a high amplitude and fast directions parallel to the Red River Shear Zone in the western part. In the eastern part, the amplitudes are generally smaller and the fast axis displays more variations with periods.

Multiple Diving Waves and Steep Velocity Gradients in the Western Taiwan Coastal Plain: An Investigation Based on the TAIGER Experiment

Seismic data collected during explosion experiments performed as part of the TAiwan Integrated GEodynamics Research (TAIGER) project provide an excellent opportunity to obtain high‐resolution images of the structure of the crust and upper mantle beneath Taiwan. The most significant feature observed at near‐source stations located on the western coastal plain in Taiwan is high‐energy later arrivals. These high‐amplitude multiples almost completely mask the lower‐amplitude signals (seismic refraction and wide‐angle reflection) from the deep crust. The later arrivals are identified as free‐surface‐reflected multiples. The nature and generation of these high‐energy, multiple diving waves are demonstrated using synthetic examples. Their generation requires the presence of a steep velocity gradient in the shallow crust. A detailed analysis of the observation data provided information on the velocity gradients in this region. An accurate layer‐velocity model, including the boundary orientation and its depth, and velocity gradient, was constructed based on a 1D waveform simulation and 2D seismic raytracing modeling for travel times. The present results indicate that the thick sediment in the survey area dips shallowly to the east, has a surface P ‐wave velocity of ![Graphic][1] , and an average velocity gradient of about 0.72/s from the surface to 3.0‐km depth. The thick sediment of the 2D model shows lateral variations in velocity gradient, increasing from west to east. This velocity model may provide useful information for future data processing to reduce multiple diving waves with the aim of enhancing the deep‐surface refraction/reflection signal. The velocity gradient calculated for the thick sediment of the western coastal plain may require a revision of the regional seismic velocity model developed for southwestern Taiwan, to improve the accuracy of regional hypocenter determinations, and to predict the strong ground motions produced by large earthquakes beneath this region. [1]: /embed/inline-graphic-1.gif

Seismic moments of Taiwan's earthquakes evaluated from a regional broadband array

We have taken the seismic moment (Mo) values of 79 earthquakes occurring in the Taiwan region that have been published in the Global centroid-moment tensor (CMT) and regional Broadband Array in Taiwan for Seismology (BATS) catalogues for the period 1996–2005 and compared the values determined from the global and regional networks, respectively. MoG and MoB are used to denote the Mo values published in the Global CMT and regional BATS catalogues, respectively. Our results show that MoB linearly correlates with MoG and that MoB is, on average, approximately equal to 0.37MoG. This difference may be caused by the use of shorter period seismic waves in BATS for estimating MoB. The moment magnitude evaluated from regional BATS seismograms is about 0.3 less than that estimated from global data.

Vibrations of the TAIPEI 101 skyscraper caused by the 2011 Tohoku earthquake, Japan

The strong-motion recordings generated by the 11 March 2011, Tohoku, Japan earthquake recorded by the building seismic array in the TAIPEI 101, the second tallest building in the world, are more complicated than those at the free-field stations. Fundamental and higher-mode vibrations can be clearly seen on the spectra. The fundamental-mode frequency is about 0.15 Hz, which is the natural frequency of the skyscraper. Spectral ratios of ground accelerations at the building to those at a nearby borehole station increase from unity at the fifth floor underground to factors of 110 and 146, respectively, on the 74th and 90th floors above ground for the fundamental-mode vibrations. The frequency content of accelerograms, recorded on the fifth floor below ground from the great 2011 Tohoku-Oki earthquake, appears mainly in a frequency band of 0.015–0.1 Hz, leading to a fact that the ground motions did not cause high excitation in the TAIPEI 101 skyscraper.

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.