Jeen-Hwa Wang

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.

On the frequency distribution of interoccurrence times of earthquakes

The distributions of discrete frequency, N, versus interoccurrence time, t (in days), of M ≥ 7 earthquakes in the Taiwan region during the 1900–1994 period, M ≥ 6 earthquakes in the north-south seismic belt of China during the 1900–1990 period, and M ≥ 5.5 earthquakes in Southern California, U.S.A., during the 1914–1995 period are studied through two statistical models (gamma function and exponential function). Results show that both the exponential function and gamma function can describe the distributions. However, the former is more appropriate than the latter. This indicates that the three time series of earthquakes have a significant component of Poisson processes, even though the tectonic conditions, the fault distributions and the size of the three seismic regions are different.

The Taiwan Telemetered Seismographic Network

Abstract In 1972, the Taiwan Telemetered Seismographic Network (TTSN) was installed to monitor earthquakes occurring in an area from 21 to 26°N latitude and from 119 to 123°E longitude. The number of stations increased from six in 1972 to 24 in 1987. The stations are equipped with velocity-type seismometers with a natural frequency of 1 Hz. Since 1983, horizontal-component sensors have been added at several stations. Signals recorded by the field stations are transmitted to Taipei Recording Center through telephone lines or radio, and then recorded on FM magnetic tapes, paper, and nine-track magnetic tapes through A/D converters. Hypocenters of earthquakes are routinely determined by a HYPOC2 computer program. The size of an earthquake is estimated from the signal duration. Moreover, a simulated Wood-Anderson seismographic station is in operation at Taipei for the determination of local magnitude. A seismic work system (SWS) is being established for automatic data acquisition and data processing. To avoid clipping of signal amplitude, an Optimum Telemetry System is being linked to the TTSN.

Multifractal measures of earthquakes in west Taiwan

The generalized fractal dimension for epicentral distribution of earthquakes in west Taiwan is measured. The entire area is first divided into two zones, i.e., north and south zones, after which the two zones are further separated into three subzones for the former and two for the latter. The logCq(r) versus logr function, whereCq(r) is the generalized correlation integral andr is the distance between two epicenters, shows that a linear relation between logCq and logr exists in the range ofr smaller thanrc. The value ofrc is 25 km for the north zone, 40 km for the south and 12 km for the three north subzones. The valuesrc=25 and 40 km are almost the smallest ones of the width of epicentral distributions of the north and south zones, respectively. The value ofrc=12 km for the three north subzones is approximately the smallest size of the cluster of epicenters. For the plots of two south subzones, the pattern of data points does not bend in the range ofr in consideration, and, thus, there is not such a critical radius. TheDq−q relations forq=0, 1, 2,..., 15 are constructed for the two zones and five subzones. Results show significant multifractality and a spatial variation in multifractality for epicentral distributions of earthquakes in west Taiwan.

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.

Frequency-Dependent Site Amplifications with f 0.01 Hz Evaluated from Velocity and Density Models in Central Taiwan

The frequency-dependent site amplifications at 87 free-field strong- motion station sites in central Taiwan are evaluated from the velocity and density structures constructed from borehole data measured at shallow depths and the veloc- ity models inferred from earthquake data at great depths. Results based on the quarter- wavelength approximation method proposed by Boore and Joyner (1997) show that the site amplifications increase with frequency and are larger in the Western Plain with thick Holocene alluvium than in the Western Foothill with Pleistocene and Miocene formations. Considering wave attenuation, site amplification first increases and then decreases with increasing frequency. The turning frequency, ft, associated with the maximum amplification varies site by site.

A Dynamic Study of Two One-State-Variable, Rate-Dependent, and State-Dependent Friction Laws

I compare two one-state-variable, rate-dependent, and state-dependent friction laws, that is, the slip law and slowness law, through the analyses of stability conditions based on a dynamic one-degree-of-freedom spring-slider model, subjected to such two kinds of friction. Results show that at low velocities, the stability criteria for the two laws are equal and similar to the steady-state criteria deduced by Ruina (1983). On the other hand, at higher velocities, the stability criterion for the slip law depends not only on the parameters of the friction law, but also on the sliding velocity and state variable of the system, whereas the stability criterion for the slowness law is the same as that at low velocities.

Effect of Thermal Pressurization on Radiation Efficiency

Abstract The radiation efficiency, η R , is an important parameter showing the source property. It is strongly affected by the variation in shear stress with slip. Thermal pressurization is considered to be a significant mechanism in controlling such a variation, thus influencing η R . In this study, the formula of η R as a function of slip, δ , on the basis of two end-member models of thermal pressurization, that is, the adiabatic-undrained-deformation (AUD) model and slip-on-a-plane (SOP) model proposed by Rice (2006), is derived. The controlling parameters of the AUD and SOP models are, respectively, δ c and L * , which are dependent on thermal, mechanical, and hydraulic parameters of fault rocks. Modeled results suggest that thermal pressurization controls the variation in shear stress with slip and thus influences the radiation efficiency. Results show that η R increases with δ . The increasing rate of η R with δ is high at small δ and low at large δ . This indicates that η R varies very much with δ for small earthquakes and only slightly depends on δ for large events. For the two end-member models, η R increases with decreasing δ c (or L * ). When δ c = L * , η R is higher for the AUD model than for the SOP model. The thermal pressurization model is also applied to investigate the shear stress–slip function in a 5 ×5 km square covering a drilled site on the fault plane of the 1999 Chi-Chi, Taiwan, earthquake inferred from seismograms. Results show that the AOD model is more appropriate to describe the inferred shear stress–slip function than the SOP model, and the proposed model is a modified one from the AUD model by including a small amount of loss of frictional heat from the slip zone during faulting.