EVIDENCE FOR PREHISTORIC COSEISMIC FOLDING ALONG THE TSAOTUN SEGMENT OF THE CHELUNGPU FAULT NEAR NANTOU, TAIWAN: EARTHQUAKE HAZARDS ALONG ACTIVE FOLD SCARPS

Taiwan’s devastating 1999 M_w7.6 earthquake ruptured previously unrecognized traces of the Chelungpu thrust fault. Paleoseismic studies along the southern segment of the Chelungpu fault, at the Shi-Jia site near Nantou city, suggest folding is the predominant form of deformation. Stratigraphic relations across the 1999 fold scarp suggest prior coseismic folding. Trench wall exposures show that vertical offset during the penultimate rupture was similar to observed 1999 deformation. AMS radiocarbon ages indicate that the penultimate earthquake occurred before 715 ±75 yrs BP. Therefore, in the last 700 years, two great earthquakes of similar displacement have occurred on the southern Tsaotun segment of the Chelungpu fault. Four continuous borings intersect the fault and constrain its location and dip, and provide additional stratigraphic evidence of two earlier earthquakes. Geometric relations using fault dip and vertical displacement yield a horizontal shortening rate of 7.5 to 10.8 mm/yr. Inverse models using Trishear methods yield similar rates of horizontal shortening. These rates agree with present day geodetic shortening rates observed across the Chelungpu fault.