Field investigations show that the northern segments of the Chelungpu surface rupture zone form a right-stepping echelon with an antidilatational jog where the ground surface was flexure-folded during the 1999 Chi-Chi, Taiwan, earthquake. A circular well located on the jog was deformed into an ellipsoid shape, with the horizontal long axis oriented N70°-80°W, perpendicular to the strike of the main surface rupture zone. Deformation increases downward from the top to the bottom in the well. Based on the geometric characteristics of the surface rupture zone and deformed well, we interpret the well as having been deformed by a local NNE-SSW compressive stress in an antidilatational jog area, which was caused by left-lateral faulting in the NNE-SSW-trending right-stepping Chelungpu surface rupture zone. This study presents an example that the recognition and identification of jog between two rupture segments has the potential to provide new insight into understanding the controls and mechanisms of rupture initiation and termination during a large earthquake.

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