Abstract: A brief description of the reliability of shakemaps is that shaking is relatively reliable near seismic recording stations, and is poorly constrained far away from stations where ground motion predictions must be relied on. We try to improve the reliability of the shakemap of the Napa earthquake constrained by the seismic source, the decay law of ground motion parameters(GMP)and the inter-earthquake bias correction term. Based on the point-ellipse and fault projection plane source model, using the method of rapid generating shakemaps considering site effects, the shakemap of peak ground acceleration(PGA)for the Napa earthquake M_W6.0, August 24, 2014 was prepared. In this process, 4 sets of ground motion prediction equations were used, including the long-and short-axis attenuation relationships in western China, the attenuation relationship of fault projection distances in the Sichuan-Tibet area and the attenuation relationship of Boore in the NGA-West2 project. Furthermore, the distribution of the shakemap was constrained by the seismic station observations, and the bias correction of the estimates were also be made by the deviation term between seismic observations and estimates. The differences of the shakemaps of different source characteristics, different attenuation relationship models, and whether or not a bias correction was considered were compared. It was concluded that the reliability of ground motion estimation is associated with unconstrained source dimensions for large earthquakes where the distance term in the GMPE is most uncertain. The addition of ground motion observations can reduce uncertainties over data-constrained portions of the map. The differences between the estimated values of different GMPE and the actual observed values can be reduced with the bias correction term applied in the shakemap, and the regional propagation characteristics of seismic waves for specific events are more reasonably described than before.

Key words: Napa earthquake, shakemap, ground-motion prediction equation, Bias correction

摘要： 震动图的可靠性简短描述为震动在观测台站附近可靠性较高，远离台站的地方较差，必须依赖于地震动的估计值。文中从震源、地震动参数衰减规律以及地震事件间的偏差校正项对地震动估计值进行约束，进而提高震动图的可靠性。分别采用中国西部长、短轴衰减关系、川藏地区断层投影距衰减关系及NGA-West2中Boore等回归的美国加利福尼亚州地区峰值加速度衰减关系，进一步增加台站观测数据约束地震动的分布，并考虑地震事件间的偏差校正项对衰减关系估计值的修正，先后基于点椭圆和断层投影面源模型，利用考虑场地效应的快速生成震动图方法计算了2014年8月24日美国加利福尼亚州纳帕M_W6.0地震的震动图。比较分析了不同震源特征、不同衰减关系模型、有无地震事件间偏差校正项的震动图差异。研究分析认为，明确的震源破裂尺度提高了衰减关系距离项的精度，有利于降低缺少台站地区地震动估计的不确定性。地震动观测值的加入降低了震动图的数据约束部分的不确定性。地震事件间偏差校正项的应用，减小了不同衰减关系估计值与实际观测值之间的差异，较为合理地反映了特定地震地震波的区域传播特性。

关键词: Napa地震, 震动图, 衰减关系, 偏差校正