Paleoseismic evidence for coseismic growth-fold in the 1999 Chichi earthquake and earlier earthquakes, central Taiwan

The 1999 Chichi earthquake ruptured along previously unrecognized traces of the Chelungpu fault, because the traces were covered with thick-bedded fluvial, alluvial, and colluvial deposits. The earthquake created a 95-km-long surface rupture and associated fault-related fold scarps. This study focused on the fault-related fold at two locations, where the fold scarp is characterized with trench excavation and shallow cored boring results. The structural characteristics revealed by the two trench sites are consistent with a trishear fault-propagation fold growth above a blind thrust. Several characteristics of the fold observed in the Holocene deposits show smoothly rounded fold-hinges, unconformities, onlapped structure, and downward steepening of forelimb strata. Results from these structures suggest that the fold grows by progressive limb rotation of growth strata in sequential coseismic growth episodes. The growth strata show several unconformable contacts as indicated by paleosoil horizons developed on event horizons. Based on the syntectonic sedimentary structure, three events are revealed at the Siangong-Temple site and two paleoearthquake events on the Shijia site. Integration with the two trenches and the previous studies suggests the three paleoearthquake events occurred 300–430, 710–800, and 1710–1900 yr B.P. These data on the two trench sites indicate that the average slip rate is 4.2 and 4.5 mm/yr, respectively.