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Lithofacies analysis, magnetostratigraphy, and seismic profiles of Pliocene-Pleistocene foreland basin deposits of Taiwan provide a framework to evaluate the stratigraphic development of a collisional marine foreland basin. We have recognized several scales of stratigraphic packages and unconformities in deposits of the Taiwan foreland basin. Small-scale (20 to 150 m thick) stratigraphic sequences contain upward-shallowing, marine lithofacies successions that are bracketed by thin coquina sandstones. We interpret the small-scale stratigraphic packages as ''parasequences'' in the traditional sequence stratigraphy model, the thin coquina sandstones representing marine-flooding intervals. The average duration of individual small-scale packages was in the range of 37.5 k.y., on the basis of our magnetostratigraphy. These sequences are interpreted as the product of eustatic sealevel change possibly related to the orbital time series of obliquity. Intermediate-scale stratigraphic sequences are 150 to 1000 m thick and are bounded

[1] Discrete scarps that are created or reactivated during large earthquakes are a locus of concentrated hazard. A number of the coseismic scarps activated in the 1999 Chi-Chi earthquake are actually fold scarps, which display several types of ground deformation characterized by localized folding and are distinct from classic fault scarps, which form by a fault cutting the surface. This paper documents and analyzes fold scarps that formed or reactivated in the 1999 Chi-Chi Taiwan earthquake. Our results show the Chi-Chi fold scarps can be generally divided into two types: (1) those associated with folding ahead of the tip of a blind thrust fault at shallow depths and (2) those associated with folding by kink band migration above fault bends at substantial depths ranging from $0.8 to 5 km). The previously published trishear model can be applied to model the former type, while a new curved hinge kink band migration model is provided to describe the behavior of the latter type. A key feature of fold scarps of the second type is that hinge zones are typically wide (25-100 m) relative to the displacement in a single earthquake (1-10 m), which exerts a significant control on fold scarp morphology and evolution. Because the coseismic strains of both types of fold scarps display relatively wide deformation zones (10-100 m) relative to fault scarps, wider set-back zones might be appropriate from a public policy point of view to alleviate the risk to structural damage and collapse resulting from permanent ground deformation.

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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 excavation-across the 1999 rupture of the Chelungpu fault. The primary exposure revealed a broad anticlinal fold with a 2.5-m-high west-facing principal thrust scarp contained in fluvial cobbly gravel beds and overlying fine-grained overbank deposits. Sequential restoration of the principal rupture requires initial failure on the basal, east-dipping thrust plane, followed by wedge thrusting and pop-up of an overlying symmetrical anticline between two opposing secondary thrust faults. Net vertical offset is about 2.2 m across the 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 34Њ for the dip of the fault plane below the excavation. This value is nearly the same as the 35Њ average dip of the fault plane from the surface to the hypocenter. Restoration of the exposed gravelly strata and adjacent overbank sediments deposited prior to the 1999 event around the principal rupture suggests 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 the 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 occurred before the deposition of the less than 200 14 C yr B.P. overbank sands and after the deposition of the much older fluvial gravels. We interpret the previous event as the penultimate event relative to the 1999 ChiChi earthquake. We estimated the long-term slip rate of the Chelungpu fault to be 10-15 mm/yr during the last 1 Ma, based on previously published retrodeformable cross sections. This rate is, however, significantly higher than geodetic rates of shortening across the Chelungpu thrust where two pairs of permanent Global Positioning System stations suggest 7-10 mm/yr of shortening across the fault. Given the 4 m of average slip, the long-term slip rate yields an interseismic interval of between 267 and 400 yr for the Chelungpu fault.