Quantitative analysis of movement along an earthquake thrust scarp : a case study of a vertical exposure of the 1999 surface rupture of the Chelungpu fault at Wufeng, Western Taiwan
Rubin, Charles M.
Date of Issue2003
A vertical exposure across the principal thrust scarp of the 1999 Mw 7.6 earthquake allows quantification of fault slip. The exposure is located on the active Chelungpu fault near Wufeng, along the range front of the fold-and-thrust belt in westernTaiwan. The 1999 surface ruptures at the Wufeng site are characterized by a west-facing 2 to 3 m high principal thrust scarp and an east-facing lesser backthrust scarp. We mapped a 15 m-long, 5 m-deep exposure across the principal thrust scarp and characterized complex deformation structures, which include a main basal thrustfault, a wedge thrust, and a pop-up anticlinal fold with two secondary opposing thrustfaults. The vertical displacement across the principal thrust scarp is measured directly from the offsets of the same sedimentary horizons between the hangingwall and the footwall. The average vertical displacement is 2.2±0.1 m, and the maximum displacement is 2.5 m, at the crest of the small pop-up fold. Horizontal displacement estimates were determined using line- and area-balancing methods. With line-length methods we estimated a horizontal displacement of 3.3±0.3 m across the principal scarp for four sedimentary horizons. For area balancing, first we selected three horizontal soil/sand deposits with a total thickness of about 0.5 m. The estimate yields an average horizontal displacement of 4.8±1.0 m. Using these individual and relatively thin stratigraphic layers yielded significant standard deviations in displacement estimates as a result of thickness variations. Second, we used the 3 m-thick overbank soil/sand and the lower part of fluvial pebble/cobble to calculate a horizontal displacement of 2.6±0.2 m with the area-balancing technique. According to the geometry of the dip angle (35–40°) of the basal thrust, the line-length measurement and the 3 m-thick package area balancing both provided reasonable results of horizontal displacement. By comparing the different deposits applied to the line- and area-balancing methods, we interpret that decoupling of deformation occurred between the lower fluvial gravels and the upper overbank sand and mud deposits. Due to lesser confining pressure at the surface, additional deformation occurred in the upper 1–2 m thick overbank deposits. This additional deformation yielded further vertical uplift of 0.3–0.5 m and horizontal displacement of 0.2–0.8 m around the core of the pop-up fold. Our work suggests that determination of slip across surface thrust ruptures varies as a function of the mechanical behavior of young late Quaternary deposits.
DRNTU::Science::Geology::Volcanoes and earthquakes
Journal of Asian earth sciences
© 2003 Elsevier Ltd. This is the author created version of a work that has been peer reviewed and accepted for publication by Journal of Asian Earth Sciences, Elsevier Ltd. It incorporates referee’s comments but changes resulting from the publishing process, such as copyediting, structural formatting, may not be reflected in this document. The published version is available at: [http://dx.doi.org/10.1016/S1367-9120(03)00122-6].