Finite-Source Modeling of the 1999 Taiwan (Chi-Chi) Earthquake Derived from a Dense Strong-Motion Network

The 1999 Chi-Chi earthquake (MW 7.6) (20 September 1999, 17:47:15.9 UTC) (located at 23.853 N, 120.816 E, and depth of 7.5 km) inflicted severe regional scale damage to Taiwan. The strong-motion wavefield was captured by a dense network of stations (with average station spacing of 5 km), which represents the most complete strong-motion dataset to date to use to study the kinematic source process of an earthquake. We inverted velocity waveforms recorded by 21 stations for the spatial variation in slip on a planar fault model composed of 416 subfaults, each with a dimension of 3.5 km by 3.5 km. The planar model has a strike of N5 E and a dip of 30 E, and the inversion solves for the direction and magnitude of the slip. To account for possible temporal source complexity we allowed each subfault to slip within 10 overlapping time windows, each with a duration of 3 sec. The results show that the source is composed primarily of three major asperities, the first of which is mainly dip slip, extending from the hypocenter to the northern end of the surface rupture. In this asperity, slip occurred in two pulses separated in time by 5 sec. The dislocation rise time for each pulse is short (3–4 sec), yielding an approximate average slip velocity of 80 cm/sec. The second asperity is located at shallow depth near the northern end of the rupture where very large ground velocities were observed. This asperity is on average oblique and shows a temporal rake rotation from pure dip-slip to strike-slip. The rotating rake suggests a low initial shear-stress on the northern end of the fault. Slip in this asperity is dominated by a large pulse with a dislocation rise time of 8 sec. A station near the northern end of the surface rupture recorded a peak velocity of 390 cm/sec, which we find to be due to the constructive interference of energy radiated from the first two asperities. The third asperity is located south of the epicenter. The total moment from the three asperities is 4.1 10 dyne cm, which was released over a period of 30–35 sec within an area of 900 km. Synthetics calculated from the three-asperity model explain 85% of the data and represent 98% of the total variance reduction. Our results indicate that slip is confined to the shallow region of the fault, deep slip patches are less constrained, and that the slip distribution may be representative of fault segmentation along the Chelungpu fault system.