ChiChi Earthquake Taiwan 21 September 1999
This earthquake offers several important clues for the study of Himalayan earthquakes because it demonstrated the subtle and not-so-subtle effects of surface rupture of a thrust earthquake in both urban and rural settings. The morphology of the scarp may explain why no surface ruptures were reported for Himalayan earthquakes in the past two centuries. The toe of the Chi-Chi thrust effectively covered up the evidence of its slip, removing a strip of real estate up to 9 m wide literally from the face of the Earth. The thrust is manifest as an abrupt scarp, 1-3 m high, but its disruption of surface materials was often quite gentle: trees were tilted, staircases crumpled, paddy fields rolled into gentle curves. Only in river beds could a clean fracture be examined, and these soon began to degrade in heavy rains due to the formation of waterfalls. The lesson to be learned was that a similar thrust in the Himalaya could possibly have passed undetected since its distruption of surface soils is not as dramatic as that produced by strike slip faulting. In many cases the scarp occurs in locations at the base of hills where landslides and slumps might be expected during strong shaking, and in the Himalaya might have been mis-interpreted as secondary collapse.
Although accelerations were high in many places. the accelerations at the toe of this thrust were quite low, apparently due to energy dissipation caused by the formation of millions of hairline cracks. These were visible only in the first week following the earthquake. The morphology of the ChiChi fault scarp was mapped in considerable detail near Chelengpu to document surface crumpling, rolling, wedge-toe collapse and other details of scarp formation before they were lost to repairs. Pictures of these are found in a power point presentation and are described in Bilham, R., and Ting-To Yu, The Morphology of Thrust Faulting in the 21 September 1999, Chichi, Taiwan Earthquake , J. Asian Earth Sciences 18 (3) 1-17, 2000.
For engineering aspects see EEfit report and the EERI report