Study On Coseismic Slip Gradient And Rupture Termination Of Strike-slip Active Fault Zone

The active fault traces of strike-slip fault on which earthquakes occur are generally not continuous,and are commonly composed of segments that are separated by discontinuities,such as step-over.Those discontinuities might slow or stop rupture prapagation.Statistica of historical earthquakes and numerical simulations suggest that the width of the step-over and the coseismic slip gradient are the two main factors controlling the earthquake rupture.The 1920 Haiyuan Mw 8.3 earthquake broke through multiple step-overs along the fault,the western endpoint of which was ended near the 4 km-wide Jingtai step-over and failed to jump to the Laohushan fault,while the eastern endpoint os which was ended in the eastern piedmont of Luipanshan fault with strong thrust shortening property.What factors control the termination of the surface rupture of the 1920 Haiyuan earthquake? What are the characteristics of the 1920 Haiyuan earthquake when the surface rupture broke through the other step-overs? If there is any regularity of the spatial and temporal distribution of surface slip at the western endpoint of the surface rupture of the 1920 Haiyuan earthquake?As more high-resolution topographic data is applied to the study of offset measurement,it is significant to discuss the accuracy and applicability of offset measurement based on high-resolution topographic data with different resolution or optical image data.Researchers differentiate the offset data of different events by Cumulative Offset Probability Density(COPD)after obtaining the slip data along strike-slip fault,and associated it with the characteristic slip and earthquakes theoretical model.However,there are still some uncertain factors to be investigated.Compared the Li DAR DEM data with 1 m resolution of the Haiyuan fault zone,DEM of 0.2 m resolution obtained by the high-resolution aerial image and Sf M method could describe the micro-geomorphology more clearly and accurately that provide a unprecedented high-resolution basic data to research micro-geomorphic offset and analysis the fault geometry characteristic.Interpretation of offset based on high-resolution topographic data and offset measurement using La Di Caoz software provide more abundant and accurate offset data to research the termination of the western endpoint of the Haiyuan fault.Comparing offset error distribution from 3D topographic data and optical image data,it is suggested that the error distribution would be characterized more objectively based on high-resolution 3D topographic data with the possibility that Image data could not identify the gully well.Comparing COPD of multiple sets of coseismic offset and cumulative offset data sets,it is suggested that the accuracy of differentiating offset data based on COPD model is affected by offset error,data volume,offset variation along fault,and whether considering data in fault branches or not.Analyzation of the global coseismic slip normalized frequency distribution suggests that the distribution of coseismic slip of each type of earthquake could not only be used to analyze the general law of global coseismic offset distribution but also could be used as a reference for single coseismic slip distribution.Analyzation of COPD near the western endpoint of Haiyuan fault zone provides reliable data to study offset recurrence pattern of each segment but also highlights the limitation of COPD model for the offset distribution description in the entire segment with significant offset gradient.Taking the local maximum offset along the fault zone as the research object,it is suggested that the horizontal offset of the Haiyuan earthquake in 1920 is in the form of “wave” decreasing from the middle to the end of the earthquake,with the “peak” appears at the end of each secondary rupture near the epicenter and the “valley” appears at the end away from the epicenter.The step-over along the fault is the main controlling factor for decreasing as “wave”.The maximum co-seismic slip gradient obtained in this study is 82 cm/km,indicating that the rupture of the western end of the rupture in 1920 is very rapid with a strong possibility to jump the step-over which means there is large seismic hazard in the western endpoint of haiyuan fault.However,the fact is that the rupture of the 1920 Haiyuan earthquake was suddenly stopped at the western endpoint that the 4km-wide stepover might be the important controlling factor.The influence of step-over on dynamic process of slow or stop rupture propagation may include:Increase of structural complexity in a fault zone as rupture approach a stepover may transfer deformation away from the main fault strand and dissipates rupture energy in the form of stretch,compress,folding and other processes.Another possibility is that the behavior of a rupture approaching a stepover is governed by stress interactions from previous earthquakes in the system.A third possibility is that large-scale changes in fault strike could leave a fault segment suboptimally oriented for rupture and this bends in faults may account for diminished slip.The fourth potential explanation for the observed behavior may be related to contrasts in elastic moduli in the rock masses juxtaposed at the fault.