关键词: 大兴凸起东缘断裂, 地震反射剖面, SKUA-GOCAD三维模型, 最大潜在地震

Abstract:

The eastern margin fault of Daxing uplift is an important boundary fault in the southeast of Beijing Plain. The fault is located in the southeast of properthe Xiadian Fault and is distributed in the correct order en echelon with the Xiadian Fault, which controls the development of Langgu secondary depression under the extensional tectonic background. Recent shallow seismic reflection profiles and borehole data have found evidence of Holocene activity in the eastern margin of the Daxing Uplift, which has changed the conclusion in recent decades that it has not been active since the late Quaternary. Because the fault is a right order echelon with the Xiadian Fault, and it is similar to the Xiadian Fault in structure, and the Xiadian Fault had the Sanhe-Pinggu M8 earthquake in 1679, it is inferred that the fault has the risk of a large earthquake. It has essential crucial application value to the seismic hazard survey in Beijing. Also, it poses a new challenge to the upper limit of the maximum potential earthquake magnitude of the fault on the eastern margin of the Daxing Uplift.
Quaternary sediments cover the fault on the east margin of Daxing Uplift and are in a hidden state, which results in its geometric features and deep and shallow coupling relationships that cannot be visually demonstrated by two-dimensional data two-dimensional data cannot visually demonstrate. It is of great significance to establish a three-dimensional model of hidden active faults for the hazard assessment of seismic active faults. In this paper, by collecting the fine location data of small earthquakes in this area and collating several shallow seismic geophysical profiles and deep seismic reflection profiles, SKUA-GOCAD 3D geological modeling software is used to build 3D models of the eastern margin fault of Daxing Uplift and the Xiadian Fault based on the section modeling method, and the distribution of the two faults in 3D space is simulated. The geometric features and the relationship between the depth and shallow structure of the two faults are revealed, including 1)a three-dimensional fault model and stratigraphic information map; 2)a three-dimensional model diagram of fault distribution according to dip Angle; 3)Three-dimensional model diagram of fault distribution according to depth and a three-dimensional map of small earthquake distribution. The 3D map shows that there are strong structural similarities between the faults on the eastern margin of the Daxing uplift and the Xiadian faults. The contrast map shown by depth shows that both faults are deep and shallow faults, the shallow faults disappear at about 15km underground, and the deep faults extend downward to cut the lower crust and the Moho surface. The contrast diagram displayed by apparentthe dip Angle clearly reflects that the two faults have obviously different dip angles in-depth and shallow. The deep fault is almost steep, and the shallow fault shows obvious differences in different sections. The distribution range of small earthquakes is 0-25km, of which the dominant distribution range is 10-20km. Therefore, it is speculated that the east margin fault of Daxing Uplift may have the seismogenic capacity similar to the Sanhe-Pinggu M8 earthquake in 1679. However, as existing studies have shown that the activity of the Xiadan fault and its southern extension section-eastern margin of the Daxing Uplift in this region gradually weakens from north to south, the maximum potential earthquake magnitude of the east margin fault of the Daxing Uplift is inferred in this paper to be less than Sanhe-Pinggu M8 earthquake in 1679.
Finally, by using the structural analogy of the Xiadian Fault on the eastern margin of the Daxing Uplift, and based on the structural similarity of the two faults, this paper evaluates the maximum potential earthquake magnitude that may be induced by the Daxing Fault using different experiential relations of magnitude-fault rupture scale fitted by predecessors in North China. The conclusion is as follows: the distribution range of the magnitude of the earthquake is 7.3-7.4.
Based on the structural analogy with the Xiadian Fault and the empirical relationship between magnitude and rupture scale, the maximum potential earthquake magnitude induced by the eastern margin fault of Daxing uplift is estimated to be magnitude 7.5. This conclusion has important scientific guiding significance for earthquake disaster prevention and control in the capital area, and should be paid attention to and actively take prevention and avoidance measures.

Key words: The eastern margin fault of Daxing uplift, seismic reflection profile, SKUA-GOCAD 3D model, maximum potential earthquake