Stress focusing in a proposed heel-shaped model reasonably explains the occurrence of the largest onland earthquake (MW 7.6) in Taiwan's history. The epicenter of the mainshock was located in a low-seismicity block, where, historically, large earthquakes have not been documented for the past century. Spatial variations in crustal deformation reported in Global Positioning System surveys suggest that the crustal strength in the low-seismicity block was stronger than in the surrounding area. Since the low-seismicity block, which roughly coincides with the northern part of the Peikang High, is defined by a convex curve, stress partitioning or reorientation might well have taken place along the convex boundary; it then likely stored a lot of strain energy before eventually failing. The low-seismicity block with strong crust characteristics was finally overcome by stress focusing, resulting in the generation of the 921 Chi-Chi earthquake near the focus of the low-seismicity block, where a cluster of earthquakes had occasionally occurred during the past decade. Significant surface ruptures with vertical offset ranging from 2 to 8 m, primarily along the Chelungpu fault, were restricted to within the low-seismicity block. In short, when a low-seismicity block with small velocity fields of crustal deformation is identified in an active foreland belt, there should be a greater risk of a larger earthquake to release energy that accumulates in the strong crust. This finding provides some useful insight with which to evaluate potential earthquakes in Taiwan and other convergent zones.

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