考虑巨震的四级地震设防水平一致风险导向定义与决策分析

防止结构倒塌、保障生命安全是确定抗震设防标准的首要目标。抗震设防水准的决策应该从只考虑地震危险性的“一致危险”原则向考虑地震危险性区域差异和工程结构抗倒塌能力不确定性的“一致风险”原则过渡。新颁布的第五代地震动参数区划图已提出第四级设防水准“极罕遇地震”（或“巨震”），但是现行抗震设计规范仍然采用三水准设防原则，抗震设计从当前的三水准设防原则向四水准设防原则转变已成为当前工程界迫切需要解决的问题，但是目前各国对巨震的定义还比较混乱。该文阐述四级地震设防水平决策的风险导向原理，提出确定风险导向地震设防水平的解析方法。针对我国大陆地区的地震环境和抗震设防情况，给出目标倒塌风险以及相应于巨震、大震和中震的目标条件倒塌概率的建议值。采用这些建议值以及地震危险性参数的简化确定方法，初步计算并得到了相应于四级地震设防水准的风险导向地震动参数PGA值。通过与我国现行抗震设计规范、最新版地震动参数区划图和我国学者对各级设防水准地震定义的PGA值进行对比分析，发现该文得到的巨震PGA值大于最新版地震动参数区划图定义的巨震PGA值，而小震、中震和大震的PGA值则小于我国现行抗震设计规范规定的PGA值。

Uniform risk-targeted definitions and decision-making of four seismic design levels considering very rare earthquake

The ultimate goal of decision-making of seismic fortification level is to prevent structural collapse and to ensure life safety. The criteria of seismic design level decision-making should be shifted from the principle of uniform hazard which only considers the seismic hazard, to the principle of uniform risk which considers both the site variability of seismic hazard and the uncertainty of seismic collapse capacities of engineering structures. In the newly released 5th-generation Seismic Ground Motion Parameters Zonation Map of China, the fourth level of seismic fortification, i.e., very rare earthquake (VRE), or catastrophic earthquake, has been proposed. However, the principle of three levels of fortification is still used in the current Seismic Design Code of Buildings. Therefore, it is urgent for the current seismic design criteria to transit from the three-level fortification to the four-level fortification. Another problem is that the definitions of the very rare earthquake in different countries are confused. In this paper, the principle of risk-targeted definition of four-level seismic design levels is firstly introduced, and then, an analytical approach to determinate the risk-targeted seismic design levels is put forward. Aiming at the earthquake environments and the seismic fortification situations in mainland of China, the values of target collapse risk, as well as the target conditional collapse probabilities corresponding to very rare earthquake, rare earthquake, and design basis earthquake, are proposed. Using these proposed values and the approximated method for determining the seismic hazard coefficients, the risk-targeted PGA values corresponding to the four seismic design levels are preliminarily calculated. Comparison analysis is conducted between the current seismic design code of buildings, the newest seismic ground motion parameters zonation map, and the existing research results of some Chinese scholars. It is found that the PGA values of very rare earthquake using the proposed method are larger than the ones in the newest seismic ground motion parameters zonation map, while the PGA values of rare earthquake, design basis earthquake and frequent earthquake are smaller than those in the current seismic design code of buildings.