Abstract

The Chi-Chi earthquake was the largest onland earthquake to occur in Taiwan in the twentieth century. It inflicted severe damage in central western Taiwan: the excited strong shaking projected impact at cities as far as 150 km away and destroyed several high-rise buildings in the Taipei basin. Having a very complex source, the Chi-Chi earthquake ruptured the 100-km-long Chelungpu fault in a series of jumping dislocations that did not follow a process commonly assumed for an orderly propagating rupture. Furthermore, the rupture developed over a surface that was by no means planar. Principally a N-S-trending thrust of shallow (30°) dip to the east, the northern end of the ruptured Chelungpu fault turned into a more easterly trending rupture surface with an oblique-slip motion. The recently completed Taiwan Strong-Motion Instrumentation Program (TSMIP) with more than 600 modern digital instruments scored the historically largest and most significant strong-motion data recovery. In the meantime, the upgraded Taiwan Central Weather Bureau Seismic Network (CWBSN), all with digital telemetered stations each having six components in both high- and low-gain operations, electronically issued earthquake information (hypocenter, magnitude, and isoseismal map) within minutes of the mainshock to all pertinent emergency management agencies. This rapid reporting significantly improved timely emergency response and effective dispatching of rescue missions. Based on these local data, as well as on GPS, leveling, and geological ground truth observations of the surface rupture, preliminary results show that the rupture started at the southern part of the Chelungpu fault. Dislocations (or the rupture of asperities) jumped around behind the S-wave front in a rather random spatial distribution by which no rupture propagating velocity can be properly defined. Large ground acceleration, some over 1g, occurred in the southern part of the Chelungpu fault where the rupture initiated. Toward the northern section of the Chelungpu fault, a decrease in ground acceleration was accompanied with an increase in ground velocity (to as much as 300 cm/sec) and ground displacement (to as much as 8 m). Most of the large motions were confined to the hanging wall (i.e., the eastern block of the thrust), and relatively small ground motions occurred in the footwall. Thus the hanging wall contributed the most to the rupture process. During the later part of the rupture, the N-S-trending Chelungpu fault made an easterly bend and the thrust motion turned into a more oblique-slip motion at the northern end. Meanwhile, the strong dynamic rupturing process triggered two M 6 events, each one having occurred in the vicinity of a known fault: one off the southern end and the other off the northern end of the Chelungpu fault.

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