Segmentation, Geometric Features, and their Seismotectonic Implications for the Holocene Xianshuihe Fault Zone

Abstract

The Holocene Xianshuihe fault zone, which consists of five main branches with left-lateral strike-slip, can be divided into two segments of different structural styles, jointing at the pull-apart area of Huiyuan Monastery. The northwestern segment has a relatively simple structure. While the southeastern segment exhibits a complex structure composed of several branches. The segmentation of fault structure is the main cause of the different historic strong-earthquake activity, and perhaps, the spatial variation of recent fault slip-rates in different segments. The "multiple-order en echelon" pattern is one of the important geometric characteristics of the fault zone. Based on sizes of stepovers, orders of en echelon discontinuities have been relatively classified. Of which, a left-stepping discontinuity in the order A en echelon, which is apparently accompanied with topographic effect of pull-apart structure, has been taken as the dividing point of the segmentation of the fault zone. This discontinuity is also reflected in stopping surficial ruptures during historic earthquakes with approximate magnitude 7. Stepping discontinuities in en echelon faults of the order B and C also have topographic effects to a certain degree, but do not appear to have been significant in stopping large-earthquake ruptures. The earthquake ground-fissures developed within the Quaternary cover are mainly in forms of even lower order of en echelons. Bending is another important geometric characteristics of the fault zone. Along-strike bends at different degrees occur along the entire fault zone and some parts of faults. Local bends along a fault is the main geometric causes for unsymmetrical rupture-spreading and intensity-attenuating during large earthquakes, and perhaps, one of the tectonic backgrounds for recurrences of large or strong earthquakes at the same localities. Finally, three-dimensional models for the source faults of two large historic earthquakes have been analysed and discussed.