Nature of the fault jog inferred from a deformed well in the northern Chelungpu surface rupture zone, related to the 1999 Chi-Chi, Taiwan, M-L 7.3 earthquake

Field investigations show that the northern segments of the Chelungpu surfa ce 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 ellip soid shape, with the horizontal long axis oriented N70 degrees -80 degreesW , 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 ge ometric characteristics of the surface rupture zone and deformed well, we i nterpret the well as having been deformed by a local NNE-SSW compressive st ress in an antidilatational jog area, which was caused by left-lateral faul ting 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 i nto understanding the controls and mechanisms of rupture initiation and ter mination during a large earthquake.