1999 Chi-Chi earthquake: A case study on the role of thrust-ramp structures for generating earthquakes

The 21 September 1999 Chi-Chi earthquake (M-w 7.6) occurred on east-dipping shallow thrust faults that produced a high-relief surface rupture. Extraor dinary surface breaks appeared that could be clearly traced for about 100 k m. across many counties. These thrust faults, the Chelungpu and Shihkang, a re part of an active fold-and-thrust belt related to ongoing recent are-con tinent collision. Measurement of slip vectors along the earthquake rupture indicates that the orientation of the maximum shear stress changed from a w estward direction (N70-90 degreesW) on the Chelungpu fault to a northwestwa rd direction (N30-40 degreesW) on the Shihkang fault. The stress field unde rwent a clockwise rotation of about 40 degrees during the Chi-Chi earthquak e. Near-rupture vertical displacements in the hanging wall of the Shihkang fault have more cumulative displacement than on the Chelungpu fault, which is consistent with Global Positioning System (GPS) measurements. Maximum ve rtical offset on the rupture was found to be about 10 m by the surficial ru pture and GPS measurements. In addition, analysis of crustal deformation by GPS measurements on the hanging wall defines a coseismic uplift related to a fault ramp structure. Our synthesis of geological and geodetic analyses shows the importance of r amp structures associated with thrust faults for generating large earthquak es and provides a general framework for understanding earthquake in fold-an d-thrust belts. Large surficial coseismic uplift and strong asperities appe ar to be a function of fault ramp geometry. Our analysis also indicates tha t, in general, ramp structures in fold-and-thrust belts may have a high pot ential in generating large earthquakes.