关键词: 隐伏活动断层, 断层定位, 活动断层避让, 安丘-莒县断裂

Abstract:

With the acceleration of urbanization process, solving the earthquake and its associated disasters caused by buried active fault in urban areas has been a difficult issue in the construction of urban public security system. It is difficult to deal with the anti-seismic issues of cross-fault buildings using the existing techniques, therefore, reasonable setback distance for buried active fault in urban area is the only method for the planning and construction at the beginning. At present, theoretical research about setback for active fault is becoming more and more mature, and the mandatory national standard “Setback distance for active fault” will be enacted soon. As a result, how to work on the basis of these theories and national standards is in urgent. In recent years, the exploration of urban active faults was successively completed. However, there are no typical cases of how to make full use of the achievements of urban active fault projects in the follow-up work, and how to guide urban construction based on the project conclusions, so as to ensure urban safety and rational development of urban economy.

In this paper, taking a site along the Anqiu-Juxian Fault in the Tanlu fault zone in Xinyi city as an example, based on the results of 1︰10 000 active fault distribution map, and referring to the stipulation of national standard “Setback distance for active fault”, 12 shallow seismic survey lines with a spacing of less than 50m were laid out firstly, and the results of shallow seismic exploration show the existence of two high-dip faults in the site. Secondly, considering the shallow seismic survey results and the geologic site conditions, five rows of borehole joint profiles were selected along five of the shallow seismic survey lines. Based on the location of the faults and stratigraphy in the site revealed by the borehole joint profiles, and considering the latest research results of Quaternary stratigraphy and the conclusion of urban active faults detection, the west branch fault is constrained to be a Holocene active fault and the east branch fault is an early Quaternary fault. As a result, we precisely mapped the trace, dip and upper breakpoint of the fault in the site based on the shallow seismic exploration and joint borehole profile. The accurate positioning of the plane position of the active fault differs by about 200m from the 1:1000 strip distribution map.

According to the relevant national standards and scientific research results, active faults in the site shall be avoided. Based on the surface traces of active faults revealed by the accurate detection in the site, the active fault deformation zone was delineated, and the range of setback distance for active fault was defined outside the deformation zone. The detection results accurately determined the plane distribution of the active fault in the site, which meets the accuracy of the development and utilization of the site. Based on the accurately located active fault trace, and complying with the forthcoming national standard “Setback distance from active fault”, this study not only scientifically determines the setback distance for active fault in the site, but also releases the scarce land resources in the city. This result achieves the goal of scientifically avoiding potential dangerous urban hidden active fault and making full use of land.

The case detection process confirms that the results of urban active fault detection are still difficult to meet the fault positioning accuracy required for specific site development, and the range of active fault deformation zone within the site must be determined based on the precise positioning method for hidden active faults as stipulated in the national standard “Setback distance for active fault”. The national standard “Code for seismic design of buildings” only specifies the setback distance for active faults under different seismic intensity, but does not provide any clear definition of the accuracy of active fault positioning, so it is difficult to define the required active fault positioning degree and boundary range of the deformation zone of active fault in practice. The national standard “Setback distance for active fault” clearly defines various types of active fault detection and positioning methods, determines the scope of active fault deformation zone and the accurate setback distance for active fault in different cases. The specific case proves that before developing and utilizing specific sites along urban concealed active faults, relevant work shall be carried out according to the national standard “Setback distance for active fault” to effectively resolve the issue about the relations between urban development and urban safety, so the promulgation and implementation of national standard should speed up.

Key words: buried active fault, fault location, setback distance for active fault, Anqiu-Juxian Fault