The 1999 Chi-Chi earthquake (Mw=7.6; 1999/9/20, 17:47:15.9 GMT; at 23.853N, 120.816E, depth of 7.5 km) has inflicted severe damages to the region, but at the same time, generated the best strong motion dataset available (Lee et al., 1999). Horizontal Peak Ground Acceleration (HPGA) and Horizontal Peak Ground Velocity (HPGV) increased northward (Ma et al., 1999). HPGA showed dramatic increase (> 300 mgal) from footwall to hangingwall. The maximum HPGV (330 cm/sec: highest ever recorded by instrument) coincides with the maximum surface offset while maximum HPGA is in the vicinity of hypocenter. The 0.02-0.5 Hz bandpass filtered data show remarkable coherency in HPGV directionality that disappears dramatically for f >0.5 Hz.
A damped, linear least-squares inversion was used to compute the rupture processes. We used similar methodology derived from Hartzell and Heaton, (1983), which allows for variation in rake, rise time, and rupture velocity on a 112 km by 45.5 km fault plane that is composed of 3.5 km by 3.5 km sub-faults. Seismograms from 21 stations were inverted and we allowed each subfault to slip within 10 overlapping triangle time windows, which with a duration of 3 s and an inter-triangle delay of 1.5 s. We used FK integration method (Saikia, 1994) to calculate the Green's functions based on a 1D model used to calculate moment tensor for the local events (Kao and Chen, 2000).
After testing for fault dimension, rupture velocity, station configuration and different time window configuration, we found 2 major asperities (Figure 15.1). Asperity A is mainly dip-slip, extending from the hypocenter to the northern end of the surface rupture. The dislocation time is 4 s, yielding a slip velocity of 150 cm/s. Asperity B is mainly oblique and showed temporal rake rotation, suggesting a low initial shear-stress level. The dislocation rise time is 8 s and the maximum particle velocity can reach 370 cm/s. The variance reduction (VR) is 85 percents. A total moment of 5.3 e+27 dyne-cm was released over a period of 30 seconds that peaked 18 sec after the origin time. Detailed sensitivity showed Asperities C and D can contribute only 2 more percents of variance reduction, suggesting that they are not well-constrained due to the poor station coverage in the mountainous areas. The slip in Asperities A and B is within a triangular area that can be correlated with GPS, topographic, tomographic, and gravity data.
We proposed that this event showed characteristics of a tectonic extrusion that is similar to the collision of India plate with Eurasia plate. A incoming basement high (Lu et al., 1998) might act as an indentor that comes into the west-vergent convergent zone, causing the over-riding materials to either thrust west along an out-of-sequence fault (as in the triangle area) or get squeezed out around the indentor (as along the strike-slip band). If this is the case, this study is one of the first that captured extrusion type of deformation "co-seismically." Regions with similar fault configuration in Taiwan might have higher seismic risks.
Kao, H and W. Chen, The Chi-Chi Earthquake Sequence of September 20, 1999 in Taiwan Seismotectonics of an Active, Out-of-Sequence Thrust, Science in print.
Lee, W. H.K., T.C. Shin, K.W.Kuo, and K.C. Chen, CWB Free-Field Strong-Motion Data from the 921 Chi-Chi Earthquake: Volume 1. Digital Acceleration Files on CD-ROM, Pre-Publication Version (December 6, 1999), Seismology Center, Central Weather Bureau, Taipei, Taiwan. 1999.
Lu, C.Y. , F.S. Jeng, K.J. Chang, and W.T. Jian, Impact of basement high on the structure and kinematics of the western Taiwan thrust wedge; insights from sandbox models, Terrestrial, Atmospheric and Oceanic Sciences (TAO), 9, 533-550, 1998.
Ma, K., and J. Mori, Rupture Process of the 1999 Chi-Chi, Taiwan Earthquake From Direct Observations and Joint Inversion of Strong Motion, GPS and Teleseismic Data, EOS TRAN. AM. GEOPHYS. U. 81, WP104, 2000.
Saikia, C. K., Modified frequency-wave-number algorithm for regional seismograms using Filon's quadrature-modeling of L(g) waves in eastern North America, Geophys. J. Int., 118, 142-158, 1994.