by Win-Gee Huang, Jeen-Hwa Wang, Bor-Shouh Huang, Kou-Cheng Chen, ... Abstract The general features of the rupture of the 1999 Chi-Chi, Taiwan, earth-.
Bulletin of the Seismological Society of America, 91, 5, pp. 1190–1198, October 2001
Estimates of Source Parameters for the 1999 Chi-Chi, Taiwan, Earthquake Based on Brune’s Source Model by Win-Gee Huang, Jeen-Hwa Wang, Bor-Shouh Huang, Kou-Cheng Chen, Tao-Ming Chang, Ruey-Der Hwang, Hung-Chie Chiu, and Chu-Chuan Peter Tsai
Abstract The general features of the rupture of the 1999 Chi-Chi, Taiwan, earthquake (Ms 7.6) can be explained by the displacement waveforms derived from the accelerograms recorded at short distances from the fault traces. Applying Brune’s 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 strikeslip 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 ChiChi 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. Introduction Dislocation rise time (s) is defined as the time required for the final slip at a point on a fault plane to occur during an earthquake rupture process. Several studies for the rise time have been presented in theories on the source model (Brune, 1970; Savage, 1972; Sato, 1989; Heaton, 1990). Savage (1972) assumed that the rise time on a rectangular rupture surface is related to the fault width (W) and rupture velocity (vr); hence, s ⳱ W/4.6vr. Based on a compilation of various source models, Sato (1989) set forth a scaling relationship between the rise time and the magnitude (M) of an earthquake, by which s ⳱ 100.5Mⳮ1.4/80. Heaton (1990) investigated the dislocation time histories of models from waveforms of earthquakes and concluded the rise time is of the order of 10% of the overall duration (i.e., total rupture time) of the earthquake, albeit with considerable variation between different models. Despite various source models on rise time, s is usually to scale as a source dimension. From
source dimension and seismic moment, the average (global) stress drop over the entire fault plane can then be determined. The average stress drop deduced from the rise time described is always model dependent, and none is an actual stress changes during the faulting. In fact, to date there has been no theory that adequately explains the rise time of an earthquake. Nevertheless, some relationships have been developed to best fit the observations. In actual fault zones, the stress drop varies in general complexity from place to place. Locally, the stress drop can be much higher than the average stress drop of an earthquake. The deduction of large ground motions is often based on a localized region of high stress and hence a greater degree of slip during the faulting. To obtain the localized stress drop, the most straightforward way has been through observations made during ruptures in the vicinity of the fault. In this study, we utilize the near-
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Estimates of Source Parameters for the 1999 Chi-Chi, Taiwan, Earthquake Based on Brune’s Source Model
fault observations to determine local stress drop following Brune (1970). Recently, on 21 September 1999, the Chi-Chi earthquake (Ms 7.6) ruptured the ground surface along the Chelungpu fault in central Taiwan. The earthquake triggered almost all strong-motion stations operated by the Central Weather Bureau (CWB) around the epicentral area. This data set is important for at least two reasons. First, the near-fault ground displacement, as obtained from accelerograms by double integration, clearly shows a ramp-type waveform (cf. Chung and Shin, 1999), thus indicating a permanent displacement of the ground. Such fault offset is very similar to the idealized slip-time function in the vicinity of the fault (e.g., Brune 1970). Second, these recordings provide a rare opportunity to investigate the fault movements of an earthquake using near-fault measurements. Results from ground-motion observations at close-in sites (Chung and Shin, 1999) as well as from far-field seismograms (Kikuchi et al., 2000) led one to believe that the fault plane of the Chi-Chi earthquake can essentially be divided into two segments that break the surface. Based on the near-fault recordings, Chung and Shin (1999) showed that the character of ground motions exhibits a major change between the southern and northern segments of the fault. At the southern segment, the ground motions were dominated in short-period motions with large accelerations. Conversely, long-period motions with large velocity and displacement were predominant in the northern segment. Kikuchi et al. (2000) constructed a rupture model for the earthquake by inversion of teleseismic body waves. Their results present estimates of the fault width (⬃40 km), fault length (⬃100 km), and average rupture velocity (⬃2.5 km/ sec). Also, they pointed out that there were two significant moment releases during the faulting. The first, a smaller one, was close to the epicenter. The second, which was much larger, occurred 35 km north of the first. Both studies expand our insight into the macroscopic features of the fault rupture. However, in order to get a more detailed understanding of the rupture properties, a study using local strong-motion data is required. The purpose of this study, therefore, is to analyze the displacement waveforms at the stations nears the Chelungpu fault for inference of slip velocities and permanent offsets. Here, we employed Brune’s (1970) model to quantify some source parameters, such as the rise time and stress drop. Based on the results, we are able to determine the physical properties of the fault plane of the Chi-Chi earthquake.
Observed Data Earthquake Location and Fault The epicenter (Fig. 1) of the Chi-Chi earthquake as determined by Chang (2000) using both strong motion and real-time monitoring data was 23⬚49.2⬘N and 120⬚51⬘E. Its focal-depth was determined to be 8 km (Chang, 2000) and
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resulted in a ⬃80 km surface break. The fault trace is shown in the curve in Figure 1. In the southern section, the trace is nearly due north. In the middle section, the trace turns slightly to the northeast, but further north, the trace returns to a more northerly direction once again. At the northern end, the trace rotates N60⬚E. Observations of surface slip revealed a peak value of about 1–2 m in the southern section and about 6–8 m in the northern section. The fault-plane solution (Chang, 2000) shows that the earthquake occurred as a thrust on a plane dipping approximately 34⬚ E with an average strike of N5⬚E and rake of 65⬚. Instrumentation Eight stations nearest (all within 2 km, 6 within 1 km) to the Chelungpu fault trace were chosen for this study (Fig. 1). Each station is equipped with a triaxial forcebalance accelerometer (Teledyne Geotech A900) that has a flat instrument response from 0 to 50 Hz. The accelerometer is capable of recording a full scale of Ⳳ 2g. The outputs were digitized and recorded with a 16-bit resolution at a rate of 200 samples/sec. Ground Motion Near the Fault Figure 2 depicts the accelerograms, within the most significant 40-sec time windows, at these eight stations. The direct P-wave-arrival alignment exists between these traces. As shown in Figure 2, the degree of complexity in the waveforms decreases northward from the epicenter and some properties can be figured out. The records at the southern three stations (TCU129, TCU076, TCU075) are relatively more complex, compared to the northern three stations (TCU052, 102, 068). Moderate complexity appears at the remaining two central stations (TCU065 and TCU067). The moderate complexity accelerograms in these two stations may be responsible for the release of energy caused by the southern and northern segments of the fault, since both stations lie near to the intersection of the two fault segments (Fig. 1). The southernmost station (TCU129) recorded the largest peak east–west horizontal acceleration of 982 gal (cm/sec2), whereas the northernmost station (TCU068) registered the largest peak vertical acceleration of 519 gal. The velocity and displacement waveforms, integrated once and twice from the accelerograms, are shown in Figures 3 and 4, respectively. Like the accelerograms, the velocity waveforms degenerate in complexity from the south to the north. At the northern three stations, each trace is essentially characterized by one single pulse with a duration of about 6–8 sec. Noteworthy are displacement waveforms (Fig. 4). Firstly, all east–west components are characterized by ramplike features, indicative of permanent ground displacement. Secondly, the horizontal displacement at two northern stations (TCU052 and TCU068) points westerly and northerly, while at the other six stations, the ground displacements are in the opposite directions, east and south. Thirdly, the two largest peak horizontal displacements, which exceed 5 m
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Figure 1.
The regional map around the source area in west-central Taiwan. Solid star represents the epicenter of the Chi-Chi earthquake. The solid and shaded bold curves denote the southern and northern segments of the Chelungpu fault, respectively. Solid triangulars denote the major cities near the fault trace. Numbered solid squares show are the seismographic stations
each, occur at these same two northern stations, in contrast to the peak values of 0.4 to 2.6 m at other stations. Fourthly, the waveforms for the vertical displacements at stations TCU052 and TCU068 are quite simple compared with other stations where some shorter-period ripples are abundant. Upward-rising motion at stations TCU052 and TCU068 is consistent with the fact that both stations are on the hanging wall of the thrust. For other stations, we cannot recognize their locations relative to the fault trace from the complexity of waveforms. Fifthly, the peak vertical displacements at stations TCU052 and TCU068 are greater than 4 m, while at the other stations, the recorded peak values range only from 0.3 to 0.7 m.
Basic Theory The expression for Brune’s ground displacement u(t) caused by stress drop Dr near the earthquake source is given by the following: u(t) ⳱ (Drb/l)s(1 ⳮ eⳮt /s),
(1)
where l is the shear modulus and b is the shear-wave speed in the source volume. The time constant, s, can be approximated by the process time a/b, i.e., s ⬃ a/b, with a being the equivalent radius of the fault surface area. Equation (1) can be approximated by the following expressions: at t ⳱ s, u(s) ⳱ 0.63 (Drb/l)s,
(2)
and at t ⳱ ⬁
The D in equation (3) denotes the static offset. If the particle displacement is averaged over the process time a/b, we have ˙¯ ). The stress drop can then be an average particle speed (U ˙ ¯ expressed in terms of U ˙¯ Dr ⳱ Ul/0.63b
(4)
˙¯ is measurable with sufficient precision, we can estimate If U the stress drop Dr through equation (4). From equations (3) and (4), s can be determined by the expression ˙¯ s ⳱ 0.63D/U.
(5)
At a site close to the fault, the ramp-function-like displacement records should reflected the movement either at the hanging wall or at the footwall rather than their relative motion. Before the source parameters were estimated, we rotated the displacement waveforms from recorded orientation to the fault plane to obtain fault dip-slip, strike-slip, and normal-slip motion components. According to the rotated seismograms, we calculated the static offset D and estimated ˙¯ by finding an asymptotic line the average particle velocity U to fit the rising part of the displacement waveforms in the least-squared sense. The rise time and stress drop were then ˙¯ for each station. Based on the vecalculated from D and U locity model of Chen (1998) and the focal depth (about 8 km) for the Chi-Chi earthquake, we used b ⳱ 3 km/sec and l ⳱ 3 ⳯ 1011 dyne/cm2 for the shallow crust in our calculation.
Analysis and Results Rotated Displacements
u(⬁) ⳱ (Drb/l)s ⳱ D.
(3)
Figure 5a shows the original displacement waveforms at station TCU052. The rotated one, based on the focal
Estimates of Source Parameters for the 1999 Chi-Chi, Taiwan, Earthquake Based on Brune’s Source Model
Figure 2.
Accelerograms (vertical, east, and north components) at eight stations arranged from the north to the south (Fig. 1). Station identifications precede seismograms. Each accelerogram is normalized to its peak value (cm/sec2) as enumerated above each trace
Figure 3.
Ground velocities integrated once from accelerograms shown in Figure 2. See Figure 2 caption. The peak ground velocity (cm/sec) is shown at the beginning of each time series
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Figure 4.
Ground displacements integrated twice from accelerograms shown in Figure 2. See Figure 2 caption. The peak ground displacement (cm) is shown at the beginning of each time series
Figure 5.
Comparison of the (a) original and (b) rotated ground displacements at station TCU052. All traces are normalized to the same scale. Arrows demarcate a time window for the rising part of the dip- or strike-slip components. The bold line segments represent the best fit of the envelope of the rotated displacements, which gives the average particle velocity
Estimates of Source Parameters for the 1999 Chi-Chi, Taiwan, Earthquake Based on Brune’s Source Model
mechanism determined by Chang (2000), is depicted in Figure 5b. The result reveals the ramplike displacement component along the fault dip or strike exceeds 6.8 m, while a pulselike normal component has a peak displacement of 2.2 m. The partition of displacement components suggests that the rotated displacement waveforms can reasonably explain an idealized slip motion along the fault length and width. Figure 6a depicts the original vertical displacement waveforms for all stations. Shown in Figure 6b is the dipslip displacement of the rotated waveforms. Comparing these two sets of displacement waveforms, we found that all the rotated waveforms are indicative of a permanent ground displacement. Furthermore, the relative motion between the two sides of the fault can be recognized in Figure 6b. The waveforms at stations TCU052 and TCU068 show positive displacement, but those at the other stations always exhibit negative displacement. This indicates that stations TCU052 and TCU068 are located on the up-thrown hanging-wall side, and the others stations are located on the down-thrown footwall side. At these two stations, the permanent ground displacement exceeds 8 m. In contrast, other stations experienced considerably less permanent displacements (0.87 to 1.5 m). In addition, Figure 6b exhibits two distinctive groups of displacement waveforms. At the five southern stations (i.e., TCU129, TCU076, TCU075, TCU065, and TCU067), the ground displacements rise immediately and propagate substantially to the north. The duration (T) for the ground displacement arrests within about 4.0 sec at these five stations. The fault slip seems to delay about 3 sec between stations TCU067 and TCU052 before continuing farther north. At the northern three stations (i.e., TCU052,
Figure 6.
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TCU0102, and TCU068), the ground displacements rise relatively slow and arrest after enduring for 10 sec. Determination of Static Offset, Slip Velocity, and Rise Time Based on the data presented in Figure 5b, an example ˙¯ was estimated. A is given to illustrate how the value of U data segment was demarcated (between arrows) from the onset of the rising part at a time of about 12 sec to a time of about 16 sec when further displacement began to fade away. Using a fit of least squares, a solid line was drawn to approximate a ramp envelope. Its slope gives the slip veloc¯˙ d ⳱ 127 cm/sec for dip slip and U ¯˙ s ⳱ 110 cm/sec for ity: U strike slip, both with a linear correlation coefficient of 0.97. ¯˙ d and U ¯˙ s may be considered as average At this step, both U values over the demarcated time segment because there are some variations in the details. In reality, the slip history can be fairly complex from place to place on the fault plane. ˙¯ to D. The The rise time is estimated as the ratio of U last step is to determine the final displacement when the earthquake rupture almost arrests. Here, the final displacement is taken from the mean value of the late arrivals (time ⬎ 22 sec), which stay at a relatively static level. The estimates of static offset for the two aforementioned components are Dd ⳱ 798 cm for dip slip and Ds ⳱ 636 cm for strike slip. The corresponding rise times are sd ⳱ 4.0 sec and ss ⳱ 3.6 sec, respectively. The negligible discrepancy (0.4 sec) between the rise times for the two components supports our continuing usage of Brune’s slip model and our processes. Following the procedures outlined previously, the dip-
Comparison of the displacement waveforms of (a) vertical and (b) dipslip components at all eight stations. See Figure 2 caption.
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and strike-slip displacement waveforms were used to esti¯˙ . The U ¯˙ for each station was estimated with a mate D and U correlation coefficient better than 0.91. This would increase the accuracy of the calculation for Dr and s, since they are ˙¯ . Table 1 lists the estimated values of D , proportional to U d ˙ ¯ ¯˙ s and ss at stations Ds, Ud, and sd at each station. Also, the U TCU052 and TCU068 on the hanging-wall side are included. Because of the complexity in the rising part of the displace˙¯ ment waveforms at other stations, we did not estimate U s ˙ ¯ and ss for all stations. At present, we only use Dd and Ud to calculate the stress drop at each station. Source Parameters of the Chi-Chi Earthquake On the footwall side, the static offset Dd ranges from 87 to 150 cm, whereas a Dd with amplitude greater than 8 m was found on the hanging-wall side. Similar disparity appears also for the strike-slip component: Ds ranges from 4 to 72 cm on the footwall side but 559 to 636 cm on the hanging-wall side. Also, the Dd /Ds ratios give a steeper slip angle of 70⬚–88⬚ at the five southern stations than that of 50⬚–58⬚ at the three northern stations. This distinction in slip angle implies that the southern segment mainly acts as a thrust fault mechanism, while a thrust fault with strike-slip mechanism is for the northern segment. The changing slip is an important factor for any fault modeling. ˙¯ determined from the The average particle velocity U d footwall side varies from 12 to 49 cm/sec, which is much less than the 127 cm/sec and 247 cm/sec determined at stations TCU052 and TCU068 on the hanging-wall side, respectively. Since the stress drop Dr is directly proportional ˙¯ , large stress drop occurs on the hanging wall (202 bars to U d at station TCU052 and 392 bars at station TCU068); and relatively small stress drops (19 to 78 bars) occur on the footwall side. The near-fault observations indicated that the rise times vary by a factor of 3.3 (1.4–4.6 sec) in this study. In general, the rise times are nearly identical for the five southern stations (average, 1.8 sec; range, 1.4–1.9 sec), but are greater and more varied for the northern three stations (average, 3.6
sec; range, 2.3–4.6 sec). The increased rise time is particularly evident for stations TCU052 (⬃4.0 sec) and TCU102 (⬃4.6 sec). Based on Brune’s model, the rise times yield source radii of about 5.4 km and 10.8 km in the southern and northern sections of the fault, respectively. Anderson and Richard (1975) estimated the ground motion from different dislocation models and demonstrated that the rise time and rupture velocity (i.e., rupture time) can be traded off to produce very similar waveforms. This similarity in waveforms means it will be difficult to separate the effect of rise time and rupture velocity unless their relationship is assumed. Because the rise times differ between two segments of the fault, the difference in duration (T) that the dislocation takes to reach its final state may mostly be caused by the rupture time difference. From the rise time, the rupture time (Tr) can be separated by examining T. The estimated Tr and T for each station is given in Table 1. On average, Tr is 1.9 sec in the southern segment and 6.0 sec in the northern segment. This observed difference suggests that rupture along the northern segment required a rupture velocity lower than that along the southern segment. The estimated rupture times and source radii indicate that the fault ruptured from the focus (near station TCU129) toward the north at an average velocity of 2.84 km/sec until it reach station TCU067, and slowed down to about 1.8 km/sec along the northern section.
Discussion In the present analysis, the observations were made along the Chelungpu fault, which follows approximately along the front of the Western Foothills that define the eastern border of the coastal plain area (Fig. 1). The area around the fault zone is covered by the Pleistocene–Recent alluvium. Since our stations are close to the fault traces (within 2 km), the transmission properties are simple for shorter and more homogeneous paths. Hence, the near-surface alluvium layers would probably have affected the latter part of the seismograms but not the rising part or the amplitude of the
Table 1 Source Parameters Determined by Offset and Particle Velocity from the Observations Using the Brune’s Model Station
Dd (cm)
Ds (cm)
¢˙ d(cm/sec) U
sd (sec)
T (sec)
Tr (sec)
Dr (bars)
c
TCU068 TCU102 TCU052 TCU067 TCU065 TCU075 TCU076 TCU129
887 87 798 134 150 97 89 102
559 72 636 13 54 4 22 30
247 (157) 12 127 (110) 44 49 44 34 34
2.3 (2.2) 4.6 3.9 (3.6) 1.8 1.9 1.4 1.6 1.9
9.0 10.5 9.5 4.0 4.0 3.0 3.3 3.6
6.7 5.9 5.6 2.2 2.1 1.6 1.7 1.7
392 19 202 70 78 70 54 54
0.98 0.97 0.97 0.96 0.97 0.98 0.91 0.95
¢˙ d, average particle velocity of dip-slip component; sd, rise time Dd, static offset of dip-slip component; Ds, static offset of strike-slip component; U determined from dip-slip component; T, duration for the ground displacement to reach its final offset; Tr, rupture time that separated by sd and T; Dr, stress drop based on Brune’s model; c, correlation coefficient. Number in parentheses denotes the average slip velocity and rise time estimated for the strike-slip component, respectively.
Estimates of Source Parameters for the 1999 Chi-Chi, Taiwan, Earthquake Based on Brune’s Source Model
early arrivals. Therefore, the sedimentary overburden should not have impacted the major conclusions reached in this study. Static Offset and Average Slip Velocity The static offset Dd was large (8–9 m) on the hangingwall side, but much less (0.9–1.5 m) on the footwall side. We believe that this offset distribution is reasonable not only because it is compatible with the changes in ground deformations, but also because it is in agreement with the results obtained by Ma and Lee (2000), whose finite-fault inversion model shows a maximum surface slip of about 10 m at the northern section of the fault with relatively small slips in the southern section. Besides, we found that the average slip ˙¯ , greater than 1.3 m/sec in the hanging-wall side velocity, U d is at least 3.4 times greater than that on the footwall side. The smaller static offset and the slower average slip velocity on the footwall side imply that the footwall remained almost stationary during the faulting. In addition, the variation in Dd /Ds ratios signals a changing slip during rupture propagation. The average slip angle declines from 78⬚ in the southern part to 53⬚ in the north. The change in slip angle implies that the faulting changed from a thrust fault to a strike-slip fault as the rupture propagated northward. The two-segmented fault geometry was supported by evidence from teleseismic waveform inversion (Lee and Ma, 2000) and from near-field records (Wu and Ma, 2000). Both studies concluded that one single focal mechanism could not adequately represent the observed and synthetic seismograms at all stations. In order to achieve a good agreement with the waveforms, they allowed the slip angle to change at 40 km northward from the epicenter (i.e., close to station TCU052). Chen et al. (2000) found a variable slip through fault-trace mapping. They reported that the fault motion acted primarily as a thrust in the focal region but was accompanied by strike-slip motion away from the hypocenter. Rise Time and Rupture Velocity Apart from the focal mechanism, the rise times (Table 1) also characterize and distinguish two segments of the fault. In the southern segment, the static deformation began with a sharp rise time and grew relatively slowly in the northern segment. The long rise times of 4.0 and 4.6 sec at stations TCU052 and TCU102 agree with the rise time of greater than 4 sec used by Huang et al. (2000) in their synthetic seismograms. In addition to the long rise time, our results also indicate a decreasing rupture velocity of 1.8 km/ sec in the northern segment. This value of 1.8 km/sec seems consistent with the rupture velocity of 2 km/sec in the northern section of the fault proposed by Huang et al. (2000). As mentioned earlier, the southern segment was bombarded with more short-period radiation as compared with the northern segment (Figs. 3, 4). One possible reason for such distinction is that each segment has its own rise time and rupture velocity. For a shorter rise time and a higher rupture
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velocity, the ground accelerations and velocities are marked with high-frequency components, while a longer rise time and a slower rupture velocity yield low-frequency waveforms. Characters of the Two-Segmented Fault In terms of source parameters, both fault segments show similarities and differences. The southern segment runs from station TCU129 (closest to the epicenter) to station TCU067 (in the middle part of the fault) and is about 30 km long. The northern segment, about 25 km long, starts somewhat at station TCU067 (about 40 km from the epicenter) and ends at station TCU068 (at the northern end of the fault). As the rupture initiated and propagated to the north, the fault experienced predominantly a thrust motion, raking at 78⬚. Slip in this segment can be described as a uniform dislocation with an amplitude of 1 m and an average rise time of 1.8 sec. The local low slip velocity (several decimeters per sec) demonstrates that the region around the southern segment is within the low-stress area (several tens of bars). Along the northern segment, the strike-slip component catches up with the thrust component. The rise time averages 3.6 sec, which is twice that along the southern segment, and implies that the rupture size was growing significantly in the northern segment. The high slip velocities (several meters per sec) at stations TCU052 and TCU068 indicate that the northern segment was highly stressed (in the several hundreds of bars range). In their teleseismic study of the ChiChi earthquake, Kikuchi et al. (2000) pointed out that the rupture pattern might have resulted from the rupturing of two separate large-slipped areas (i.e., asperity) at shallow depth. The smaller one is close to the epicenter, and the larger one is located 35 km north. Our results are consistent with theirs. The highly stressed area around the northern segment can be viewed to have resulted from relatively higher rock strength. The abrupt change in stress drop between the two segments suggests that during the course of rupturing the rupture propagation may have encountered a patch that was strong enough to temporarily bear the increasing strain without breakage. This suggestion of a stronger patch is supported by the fact that the fault trace changes its course from due north at the southern end to northeast in the middle part of the Chelungpu fault. The course change served as a geometrical barrier that prevented the fault from rupturing further. This temporary suspension of rupture is supported by the evidence of a relatively large time delay in the rising motion between stations TCU052 and TCU067 (Fig. 6b). Because the northward rupturing stopped momentarily at the intermediate branch of the fault (around station TCU067), the stress would have been enhanced near this region until the concentrated stress exceeded the strength of the localized patch. Such high-stress resistance may slow or suspend the motion of the rupture front and delay the static deformation reaching its final offset. A longer rise time and a slower rupture velocity result in the broadening of the pulse width
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(i.e., decreasing the high-frequency content), which is evident in the velocity and acceleration waveforms recorded at stations TCU052, TCU102, and TCU068 (Figs. 3, 4). As the patch obstacle was breached, the heightened stress was released promptly. This relaxation of stress at the strong patch behaved like crushing an asperity to induce an extensive sliding on the fault plane. High slip velocity and large slip along the northern segment are consequences of such patch or asperity removal. This may be a reason why severe damages aligned with the northern fault segment.
Conclusions The source parameters of the Chi-Chi earthquake of 1999 were determined using near-fault recordings. In order to assess the behavior of relative motion between the fault blocks during the rupture, the observed displacement on the ground surface was transformed onto the fault plane. Using this approach, we have characterized the source parameters in terms of offset and particle velocity of Brune’s model. It is obvious that the rupture did not progress with the same rise time, nor did it result in a uniform rate of stress release along the fault. The gross features of the Chi-Chi earthquake are summarized for the two identified fault segments: (1) The southern segment: thrust fault type, east dipping, slip with a raking of 78⬚; fault length, 30 km; average dislocation, 1 m; average rise time, 1.8 sec; average slip velocity, 34–50 cm/sec; average stress drop, 65 bars; average rupture velocity, 2.84 km/sec. (2) The northern segment: fault type, east dipping, strike slip with a raking of 53⬚; fault length, 25 km; average dislocation, ⬎8 m; average rise time, 3.6 sec; average slip velocity on the fault plane, ⬎ 2 m/sec; stress drop, ⬎300 bars; average rupture velocity, 1.8 km/sec.
Acknowledgments The authors would like to express their sincere gratitude to the strongmotion data processing group at the Central Weather Bureau for providing the accelerograms. We gratefully acknowledge comments by an anonymous reviewer. This study was supported by Academia Sinica and the National Science Council, R.O.C., under Grant NSC89-2116-M-001-038EAF.
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