Design earthquakes. The design earthquakes for hydraulic structures are the OBE and the MDE. 设计地震：
水工结构的设计地震有OBE和MDE，即运行基准地震和最大设计地震。
The actual levels of ground motions for these earthquakes depend on the type of hydraulic structure under consideration, and are specified in the seismic design guidance provided for a particular structure in conjunction with ER 1110-2-1806.
地震动参数的实际取值跟水工建筑物的类型有关，具体可参照ER 1110-2-1806。
(1) Operating basis earthquake (OBE). The OBE is an earthquake that can reasonably be expected to occur within the service life of the project, that is, with a 50 percent probability of exceedance during the service life. The associated performance requirement is that the project function with little or no damage, and without interruption of function.
运行基准地震（OBE）：OBE是指在工程的服务生命周期中可能合理预期发生的地震，即在工程生命周期中超过50%的发生概率，在该地震作用下，工程的相关性能要求几乎没有或没有破坏，工程的相关功能没有中断。
(2) Maximum design earthquake (MDE). The MDE is the maximum level of ground motion for which the structure is designed or evaluated. The associated performance requirement is that the project performs without catastrophic failure, such as uncontrolled release of a reservoir, although severe damage or economic loss may be tolerated. The MDE is set equal to the maximum credible earthquake (MCE) or to a lesser earthquake, depending on the critical nature of the structure (see ER 1110-2-1806 and paragraph 2-4b).
最大设计地震（MDE）：MDE是结构设计或评估的最大地震动水平。在该地震作用下，虽然引发严重的经济损失和破坏，但是水工结构的相关性能要求不至于遭到毁灭性的破坏，比如，水库库水不至于不受控制地下泄。依据结构的关键特性，MDE等于或小于MCE（最大可信地震），参见ER 1110-2-1806的2-4b段。
(3) The MCE is defined as the greatest earthquake that can reasonably be expected to be generated by a specific source on the basis of seismological and geological evidence.
最大可信地震（MCE）：MCE是基于地震和地质证据，某个震源合理预期可能发生的最大地震。
e. Earthquake ground motion(s). The ground motions for the design earthquakes are defined in terms of smoothed elastic response spectra and, if required, also in terms of acceleration time-histories. Standard ground motions selected from published ground motion maps can be used in
preliminary and screening studies, and for final design or evaluation in areas of low to moderate seismicity where the earthquake loading does not control the design. Site-specific ground motions, as described in Chapter 3, are required for projects with high to significant hazard potential in case of failure and located in areas of high seismicity, and in areas of moderate seismicity where the earthquake loading controls the design (ER 1110-2-1806).
地震动参数：设计地震的地震动参数可以是光滑后的弹性反应谱，如果有需要，也可以示加速度-时间历程曲线。对初步的或者方案比选阶段的研究，标准地震动参数可以从已出版的地震动参数图上面获取。在较低或中等地震强度区域的最终设计和评估，地震产生的荷载并不是控制因素。第三章所讲述的特定场地的地震动参数针对的是处于高地震地区的工程、地震是其失事的高影响因素的工程，或者虽然位于中等地震地区，但地震荷载是设计的控制因素的工程。
Design Earthquakes and Ground Motions 设计地震和地震动
a. Maximum credible earthquake (MCE). This earthquake is defined as the greatest earthquake that can reasonably be expected to be generated by a specific source on the basis of seismological and geological evidence. Since a project site may be affected by earthquakes generated by various sources, each with its own fault mechanism, maximum earthquake magnitude, and distance from the site, multiple MCE’s may be defined for the site, each with characteristic ground motion parameters and spectral shape. The MCE is determined by a DSHA. 1、最大可信地震（MCE）
MCE定义为基于地震学理论和地质证据某个震源合理预期产生的最大地震。一个工程可能同时被几个震源产生的地震所影响，每个震源有它自己的震源机制、最大地震强度，距离工程场地的距离也不同，因此，一个场地对应多个MCE，对应各自的地震动参数和反应谱。MCE由确定性地震危险性分析（DSHA）确定。
b. Maximum design earthquake (MDE). The MDE is the maximum level of ground motion for which a structure is designed or evaluated. The associated performance requirement is that the project perform without catastrophic failure, such as uncontrolled release of a reservoir, although severe damage or economic loss may be tolerated. For critical features, the MDE is the same as the MCE. For all other features, the MDE shall be selected as a lesser earthquake than the MCE which provides
economical designs meeting appropriate safety standards. The MDE can be characterized as a deterministic or probabilistic event. 2、最大设计地震（MDE）
MDE是设计或评估某结构所遭受的最大地震动水平。相应的性能要求是即使因为地震破坏产生了严重的破坏和经济损失，但结构功能不发生灾难性失效，比如水库的不受控泄水。对关键结构，MDE等于MCE，对其他结构，在考虑合适的安全标准下，也考虑经济的因素，MDE相应的小于MCE。MDE可分为确定性和概率性结果。 c. Operating basis earthquake (OBE). The OBE is an earthquake that can reasonably be expected to occur within the service life of the project, that is, with a 50-percent probability of exceedence during the service life. (This corresponds to a return period of 144 years for a project with a service life of 100 years.) The associated performance requirement is that the project function with little or no damage, and without interruption of function. The purpose of the OBE is to protect against economic losses from damage or loss of service, and therefore alternative choices of return period for the OBE may be based on economic considerations. The OBE is determined by a PSHA. 3、运行基准地震（OBE）
OBE是指在工程的使用寿命周期中可能合理预期发生的地震，即在工程生命周期中超过50%的发生概率（如果一个工程的使用寿命是100年，那么需研究的重现期是144年），在该地震作用下，工程的相关
性能要求几乎没有或没有破坏，工程的相关功能没有中断。设定OBE的目的是避免工程因为性能破坏或丢失而产生经济损失，因此OBE的重现期选择可基于经济性考虑。OBE通过概率性地震风险性评估决定。
d. Estimating OBE and MDE ground motions.
Estimates are usually made in two phases. The first estimates are used as a starting point for the study and are obtained from the National Earthquake Hazard Reduction Program (NEHRP) spectral acceleration maps (Appendix D). Site-specific studies in accordance with paragraph 5h(2) are often required for selecting the final estimates of OBE and MDE ground motions. Both DSHA and PSHA approaches are appropriate. Combining the results of deterministic and probabilistic analyses is often an effective approach for selecting MDE ground motions.
4、OBE和MDE地震动的评估
OBE和MDE地震动的评估通常划分为两个阶段。第一个阶段的评估是研究的起点，可通过国家地震减灾计划（NEHRP）提供的加速度图（附录D）获得相应的基础资料。5h(2)段中的特定场地研究常常要求选取一个最终的OBE和MDE地震动参数。DSHA和PSHA方法均为建议的方法。综合确定的和概率的分析方法常常是选择MDE的有效途径。
Typical results of a probabilistic analysis are a hazard curve and an equal
hazard spectrum which relate the level of ground motion to an annual frequency of exceedance or return period. This information can be used to complement the deterministic analysis by removing from consideration seismic sources that appear unreasonable because of low frequencies of occurrence, by justifying mean or mean-plus-standard deviation estimates of deterministic ground motion, or by ensuring consistency of MDE ground motions with some performance goal.
概率分析的典型结果是与地震动水平的年超越频率或重现期相关的风险曲线和等风险谱。这些信息可作为确定性分析的补充，通过剔除已考虑的震源发震概率很低的不合理震源，或者通过判断确定地震动参数的平均偏差或分位值偏差（mean-plus-standard deviation），或者通过确保一些性能目标和MDE地震动参数的一致性。