Study On Focal Fault Parameter Of The 1830 Cixian Earthquake In Hebei

The Cixian magnitude 7^1/2 earthquake in 1830 was the largest earthquake ever recorded in the border area of Shanxi,Hebei,Shandong and Henan,causing a large number of collapsed or destroyed buildings and casualties.Historical documents have extremely rich and detailed records of this earthquake,and a more detailed isoseismal map can be drew by the experts.The seismogenic fault is inferred to be the Cixian-Daming fault based on the surface trace given the active structural survey.From the perspective of seismology,we study the geometric parameters of the focal fault and movement modes of the Cixian earthquake.The phases reports of 2028 ML0⁴.2 earthquakes occurred in the Cixian earthquake focal area and adjacent areas from January 2009 to December 2019 are downloaded from the "National Earthquake Catalogue System".After quality control and selected seismic phase,we use the double-difference tomography method to get the precise hypocenter of 1264 small earthquakes in the source area of Cixian earthquake and adjacent areas.After relocation,the residual root mean square of P-wave travel time is significantly reduced,from 0.76 s to 0.22 s.After relocation,the epicenters planar distribution changed little,and it was more concentrated in a striped distribution;in the vertical direction,the hypocenter depth after relocation was basically above 20 km,showing a normal distribution,forming a dominant horizon at a depth of 8-16 km.Based on the multiple data from the focal area of Cixian earthquake,we selected 336 precise relocations,the simulated annealing algorithm and the Gauss-Newton algorithm combined with the density of small earthquakes were used to perform the source fault fitting method to invert the geometric parameters of the focal fault Cixian earthquake in 1830.The results are as follows: strike of the focal fault 287° and dip angle 76°.Under the control of regional tectonic stress field,the slip angle of the focal fault of Cixian earthquake is-24.6°.The slip properties are mainly left-lateral strike-slip with a small amount of normal fault components.The Nanshancun-Chakou active fault in the western section of the Cixian-Daming fault is mainly distributed in the bedrock mountain area of the Taihang Mountains.The active mode is mainly left-handed positive dip slip.The latest surface rupture occurred in this fault zone during the Cixian earthquake in 1830.In the fault zone,the surface rupture direction is northeast east,and it is distributed diagonally in the right order,and its overall distribution direction is north-west-west.The survey results of active faults and surface rupture zones in the field are basically the same as the geometric structure and movement mode of the focal fault of the Cixian earthquake obtained by inversion of the hypocenters of current small and medium earthquakes.