An Interpretation of the 1999 Chi-Chi Earthquake in Taiwan Based on the Thin-Skinned Thrust Model

The 1999 Chi-Chi earthquake (ML =7 .3) which occurred in central Taiwan marks the island’s largest inland seismic event of the twentieth century. The Chi-Chi earthquake had three notable characteristics: 1) its source depth was very shallow (8 km), with aftershocks distributed in a semi-circular belt surrounding the Peikang basement high; 2) it triggered a 90 km long, north-south trending, active fault (the Chelungpu fault), mostly of the thrust type. Its hanging wall side suffered much larger distortions than the foot wall side; and 3) the fault trace bent toward the northeast at its northern tip 50 km away from the source, where the largest surface rupture occurred (9.8 m). We suggest that the thin-skinned thrust model would be the most suitable to explain the behavior of the Chi-Chi earthquake. The overall decollement surface might not be too deep (20 km?), which would redistribute the tectonic pressure to a weak near-surface layer, this could be the Chinshui Shale (10 km?) which activated the brittle layer-coupled fault even before the accumulation of earthquake stress in the near area was ‘mature’ or ‘adequate’. By using the aftershock data and other observations, we attempt to reconcile the different aspects of the Chi-Chi earthquake based on the thin-skinned model. Many inferences emerge as remarkably reasonable, that match the observations quite well. However, a deep seismic reflection study or deep well drilling is definitely required to better evaluate the model.

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