PALEOSEISMOLOGOCAL STUDY OF THE CHELUNGPU FAULT, CENTRAL TAIWAN

The Chelungpu fault, located in central Taiwan activated in the 1999 Chichi earthquake, resulting in the significant uplift up to 9 m. Detailed studies of this fault have been carried out since then. We review the paleoseismologocal studies of this fault, in particular, we discuss 1) the relationship of the location between the surface rupture and pre-existing active Chelungpu fault, and 2) repeated faulting in late Quaternary, based on the cumulative offset of the fault scarp, and 3) nature and frequency of paleoseismic events, which were mainly revealed by the trenching study. We summarize the results as follows: 1)Most of the location of the surface rupture exactly coincides with the geomorphologically identified pre-existing active fault traces. This indicates that the exact mapping of the location of active faults is of great importance in diagnosing the future ruptures of other active faults. Also specified is a jumping found along the surface rupture from one trace to the other, however, when there are two or three subparallel active faults. 2)Progressive deformation is confirmed at many sites, showing the repeated activities of the same fault trace. Not only the main fault, but also some of the subsidiary back thrusts also are repeatedly active. 3)By now fifteen trenches were excavated across the Chelungpu fault since the Chichi earthquake. All the trench logs clearly demonstrate the low-angle reverse faults of Holocene deposits, often with the overriding of the hanging wall above the footwall. It is common that two or three faults exist within very narrow zone in a width of several meters. Each activity took place either of them. The recurrence time and the timing of the latest faulting are variable. At the northern part, the latest event occurred at ca. 2000 yrBP, and 2-300 years on the central part with more frequent interval, and three events during the last ca. 3000 years in the southern part. Together with the study of slip rate, the segmentation of the Chelungpu fault into three or four segments can be discussed.