Oral

CHARACTERISTIC FEATURES OF STRONG GROUND MOTION FROM THE

SEPTEMBER 20, 1999, CHI-CHI EARTHQUAKE IN CENTRAL TAIWAN

JER-MING CHIU and JOSE PUJOL, (Center for Earthquake Research and Information, The University of Memphis, Memphis, TN 38152, chiu@ceri.memphis.edu)

Although the tectonic environments in central U.S. and central Taiwan are quite different, similarities exist between the 1811-1812 New Madrid earthquake sequence and the September 20, 1999 Chi-Chi earthquake sequence. These similarities include (1) many large aftershocks (M > 6.0) occurred immediately and several months after the mainshock, (2) a segmented aftershock seismicity, (3) about 100 km of surface rupture from the predominantly thrust movement along a 30 degrees dipping active fault which is bounded at both ends by strike-slipfaults, (4) a waterfall of 6 to 10 meters emerged at the intersection of the ruptured fault and the river channel, and (5) a large region affected by liquefaction. While no data are available from the 1811-1812 New Madrid earthquake, the strong motion data from the Chi-Chi earthquake has provided a unprecedented opportunity to explore the ground deformation that could be produced by a future large earthquake in the central U.S. Three-component acceleration data from the Chi-Chi earthquake recorded by a strong motion network in Taiwan are analyzed to investigate the characteristic features of ground deformation during and after a large earthquake. A linear least-squared method is first applied to the three-component acceleration data to remove the overall DC-offset based on a baseline determined from the first 20 seconds of pre-event data. Velocity seismograms can be obtained from a direct integration of acceleration data. Results of the first integration reveal that the baseline of the strong motion sensor was offset during the arrival of strong ground motion in the near field. At far distances, an extremely low frequency sinusoidal baseline becomes dominant. From fitting the pre-event data and the baseline after the main seismic arrival, a third order polynomial is determinedto represent the baseline and to remove the baseline drift. Displacement seismograms can thus be obtained from a direct integration of the velocity seismogram after baseline correction. The resultant three-component displacements are consistent with that observed from nearby GPS stations. Permanent co-seismic displacement in the ranges of a few centimeters to a few meters can be identified from the resultant displacements. Most of the near field displacement can be easily explained by a simple dislocation model. However, site effects and possibly secondary events become significant outside of the ruptured region. There is no clear relationship between the magnitude of the observed acceleration, velocity, and displacement. The strong ground motions are mainly concentrated on the hanging wall side of the ruptured fault, within a 30 km zone to the east, which is consistent with the areas of severe damage and heavy casualties. The largest displacement occurred in a region 40 to 60 km to the north of the mainshock epicenter. Significant basin amplification can be clearly seen from the comparison of seismograms observed inside the Taipei and Ilan basins in northern Taiwan with those outside of the basins. Thus, it is very important for regional hazard assessment to understand the characteristic features of active faults and their associated ground deformation in any future major earthquake.