We investigated the rupture process of the 1999 Chi-Chi, Taiwan, earthquake, using high-quality near-source strong-motion records, broadband teleseismic displacement waveforms, and well-distributed Global Positioning System (GPS) data. The near-source strong-motion displacement waveforms recorded significant static offsets of up to 8 m. The teleseismic displacement records show a significant pulse with duration of about 18 to 20 sec. Taking into account the surface displacements observed along the Chelungpu fault, we considered two fault geometries: a single planar fault and a two-segment fault with a northeast-striking section near the northern end. Using the finite-fault model with variable slip vectors, we derived two models of the temporal and spatial slip distribution of the earthquake. The GPS data provided good surface displacement constraints for the slip-distribution determination. The spatial slip distribution is generally consistent with field observations. The results for the simple fault model show a large asperity located in the region about 25 to 55 km north of the hypocenter with maximum slip of about 15 m. When we use the two-segment model, the asperity further extends to the region where the fault bends toward the northeast with a maximum slip of up to 20 m. A large amount of right-lateral slip beneath station TCU068 is necessary to explain its observed large west movement. It implies a local converging slip at the corner where the fault bends to the northeast. The slip amplitude near the hypocenter is about 3 to 6 m. The seismic moments determined from the various data sets are within the range of 2 to 4 ‫ן‬ 10 27 dyne cm. Most of the slip concentrated at shallow depths (less than 10 km). The total rupture duration is about 28 sec, and the rupture velocity is 75% to 80% of the shear-wave velocity. The slip vector shows a clockwise rotation during the fault rupture. The static stress drop of the large asperity region is comparable with the dynamic stress drop, as observed directly from the slip velocity at the station near the large slip region.

Determining the seismic fracture energy during an earthquake and understanding the associated creation and development of a fault zone requires a combination of both seismological and geological field data. The actual thickness of the zone that slips during the rupture of a large earthquake is not known and is a key seismological parameter in understanding energy dissipation, rupture processes and seismic efficiency. The 1999 magnitude-7.7 earthquake in Chi-Chi, Taiwan, produced large slip (8 to 10 metres) at or near the surface, which is accessible to borehole drilling and provides a rare opportunity to sample a fault that had large slip in a recent earthquake. Here we present the retrieved cores from the Taiwan Chelungpu-fault Drilling Project and identify the main slip zone associated with the Chi-Chi earthquake. The surface fracture energy estimated from grain sizes in the gouge zone of the fault sample was directly compared to the seismic fracture energy determined from near-field seismic data. From the comparison, the contribution of gouge surface energy to the earthquake breakdown work is quantified to be 6 per cent.

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[1] We investigate the rupture process of the 1999 Chi-Chi, Taiwan, earthquake using extensive near-source observations, including three-component velocity waveforms at 36 strong motion stations and 119 GPS measurements. A three-plane fault geometry derived from our previous inversion using only static data [Ji et al., 2001] is applied. The slip amplitude, rake angle, rupture initiation time, and risetime function are inverted simultaneously with a recently developed finite fault inverse method that combines a wavelet transform approach with a simulated annealing algorithm [Ji et al., 2002b]. The inversion results are validated by the forward prediction of an independent data set, the teleseismic P and SH ground velocities, with notable agreement. The results show that the total seismic moment release of this earthquake is 2.7 Â 10 20 N m and that most of the slip occurred in a triangular-shaped asperity involving two fault segments, which is consistent with our previous static inversion. The rupture front propagates with an average rupture velocity of $2.0 km s À1 , and the average slip duration (risetime) is 7.2 s. Several interesting observations related to the temporal evolution of the Chi-Chi earthquake are also investigated, including (1) the strong effect of the sinuous fault plane of the Chelungpu fault on spatial and temporal variations in slip history, (2) the intersection of fault 1 and fault 2 not being a strong impediment to the rupture propagation, and (3) the observation that the peak slip velocity near the surface is, in general, higher than on the deeper portion of the fault plane, as predicted by dynamic modeling.

[1] We have made observations of a heat signature that is associated with the frictional heat generated at the time of faulting for a large earthquake. Temperature measurements in a borehole that intersects the Chelungpu fault at a depth of about 1100 m, show a small increase near the fault even six years after the earthquake. The temperature signature has a symmetric shape with a width of about 40 m and is centered on the fault that slipped about 5 m during the 1999 Chi-Chi, Taiwan earthquake. The small amplitude of 0.06°C for the observed temperature anomaly indicates a very low level of friction that generated heat at the time of the earthquake.

In the early morning (01:47 local time) of September 21, 1999, the largest earthquake of the century in Taiwan (Mw=7.6, ML=7.3) struck the central island near the small town of Chi‐Chi. The hypocenter was located by the Central Weather Bureau Seismological Center at 23.87°N, 120.75°E, with a depth of about 7 km. There were extensive surface ruptures for about 85 km along the Chelungpu fault with vertical thrust and left lateral strike‐slip offsets. The maximum displacement of about 9.8 meters is among the largest fault movements ever measured for modern earthquakes. There was severe destruction in the towns of Chungliao, Nantou,Taichung, FengYuan, and Tungshi, with over 2300 fatalities and 8700 injuries.

Postseismic displacements of as much as 14 cm were recorded by GPS measurements in the 3 months following the MW 7.6 1999 Chi‐Chi, Taiwan earthquake. Data from 35 continuous and 90 campaign‐surveyed stations, which show continued east over west thrusting, are analyzed to estimate the postseismic slip distribution and fault geometry. Assuming the shallow fault dips 24° E, as determined by numerous studies of the mainshock, we invert for the deeper fault structure. Our results show that the fault dip shallows with depth below the hypocenter, merging into a nearly horizontal decollement at a depth of 8–12 km. The afterslip distribution shows a maximum slip of 25 cm in the hypocentral region at 7–12 km depth as well as significant slip on the lower decollement. Afterslip is notably absent in the region of maximum coseismic slip, consistent with the afterslip being driven by the mainshock stress change.

[1] The correlation between static Coulomb stress increases and aftershocks has thus far provided the strongest evidence that stress changes promote seismicity, a correlation that the Chi-Chi earthquake well exhibits. Several studies have deepened the argument by resolving stress changes on aftershock focal mechanisms, which removes the assumption that the aftershocks are optimally oriented for failure. Here one compares the percentage of planes on which failure is promoted after the main shock relative to the percentage beforehand. For Chi-Chi we find a 28% increase for thrust and an 18% increase for strike-slip mechanisms, commensurate with increases reported for other large main shocks. However, perhaps the chief criticism of static stress triggering is the difficulty in observing predicted seismicity rate decreases in the stress shadows, or sites of Coulomb stress decrease. Detection of sustained drops in seismicity rate demands a long catalog with a low magnitude of completeness and a high seismicity rate, conditions that are met at Chi-Chi. We find four lobes with statistically significant seismicity rate declines of 40-90% for 50 months, and they coincide with the stress shadows calculated for strikeslip faults, the dominant faulting mechanism. The rate drops are evident in uniform cell calculations, 100-month time series, and by visual inspection of the M ! 3 seismicity. An additional reason why detection of such declines has proven so rare emerges from this study: there is a widespread increase in seismicity rate during the first 3 months after Chi-Chi, and perhaps many other main shocks, that might be associated with a different mechanism.