STYLES OF SURFACE DEFORMATION ASSOCIATED WITH THRUST EARTHQUAKES, EXAMPLES FROM THE 1999 CHI-CHI (TAIWAN) EARTHQUAKE

The 1999 Chi-Chi earthquake occurred on the western side of the Island of Taiwan near a very complex plate boundary. This event produced reverse slip of up to 6.7 meters and an extensive zone of secondary deformation. A significant amount of the building damage resulting from this earthquake occurred because of primary and secondary ground deformation. Detailed surveying at representative sites along the fault shows that surface rupture commonly produced a relatively simple 1- to 4-m-high scarp with minor hanging-wall deformation and localized (but severe) uplift, folding, and graben formation along the scarp crest. For individual scarps, the width of deformation is about 10 to 20 times the net vertical displacement. Distributed secondary faulting and folding on the hanging wall generally occurred in a zone up to 350 m from the primary fault, but in some areas extended over 1 kilometer from the rupture. We identified four types of permanent ground deformation including: primary fault rupture, secondary fault rupture, rotated fold limb, and hinge-line fold deformation. We found that primary rupture can be easily over-estimated. In the earthquake, ruptures of up to 13m were reported, but many of these large displacements were amplified by the pushing effect of the fault tip. Buildings located on the fold limb were tilted and foundation elements deformed. Buildings located above hingeline folds were severely damaged as the foundation was translated through the fold hinge.