Spatial and Temporal Distribution of Slip for the 1999 Chi-Chi, Taiwan, Earthquake

We investigated the rupture process of the 1999 Chi-Chi, Taiwan, earth- quake, 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 north- ern 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 determina- tion. 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 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.