A vertical exposure of the 1999 surface rupture of the Chelungpu fault at Wufeng, western Taiwan: Structural and paleoseismic implications for an active trust fault

We mapped and analyzed two vertical exposures-exposed on the walls of a 3- to 5-m-deep, 70-m-long excavation and a smaller 3-m-deep, 10-m-long excavat ion-across the 1999 rupture of the Chelungpu fault. The primary exposure re vealed a broad anticlinal fold with a 2.5-m-high west-facing principal thru st scarp contained in fluvial cobbly gravel beds and overlying fine-grained overbank deposits. Sequential restoration of the principal rupture require s initial failure on the basal, east-dipping thrust plane, followed by wedg e thrusting and pop-up of an overlying symmetrical anticline between two op posing secondary thrust faults. Net vertical offset is about 2.2 m across t he principal fault zone. From line-length changes, we estimate about 3.3 m of horizontal shortening normal to fault strike. The ratio of these values yields a total slip of 4.0 m and an estimate of about 34 degrees for the di p of the fault plane below the excavation. This value is nearly the same as the 35 degrees average dip of the fault plane from the surface to the hypo center. Restoration of the exposed gravelly strata and adjacent overbank se diments deposited prior to the 1999 event around the principal rupture sugg ests the possible existence of a prior event. A buried 30-m-wide anticlinal warp beneath the uplifted crest of the 1999 event is associated with three buried reverse faults that we interpret as evidence for an earlier episode of folding and faulting in the site. The prior event is also recorded in t he smaller excavation, which is located 40 m south and is oriented parallel to the larger excavation. Radiocarbon dating of samples within the exposed section did not place tight constraints on the date of the previous event. Available data are interpreted as indicating that the previous event occur red before the deposition of the less than 200 C-14 yr B.P. overbank sands and after the deposition of the much older fluvial gravels. We interpret th e previous event as the penultimate event relative to the 1999 Chi-Chi eart hquake. We estimated the long-term slip rate of the Chelungpu fault to be 1 0-15 mm/yr during the last 1 Ma, based on previously published retrodeforma ble cross sections. This rate is, however, significantly higher than geodet ic rates of shortening across the Chelungpu thrust where two pairs of perma nent Global Positioning System stations suggest 7-10 mm/yr of shortening ac ross the fault. Given the 4 m of average slip, the long-term slip rate yiel ds an interseismic interval of between 267 and 400 yr for the Chelungpu fau lt.