Estimation methods for occurrence probability of large earth-quakes in the Haiyuan fault zone in northwestern China
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摘要: 近20余年来， 国内外学者在海原断裂带上获得了大量的古地震数据， 为探讨该断裂带上大地震的重复行为提供了重要依据. 本文利用这些古地震资料， 推断海原断裂带上存在全段破裂、 中段破裂和西段破裂等3种破裂源， 其中中段和西段破裂属于次级破裂， 受全段破裂的控制. 文中提出一种随机特征滑动模型来计算海原断裂带上的强震发生概率， 该模型假定分段断层源上的地震矩累积率是恒定不变的， 而分段断层源上的震级分布则符合特征地震模型. 同时根据海原断裂带上的古地震数据及其不确定性范围， 利用蒙特卡罗方法得到两级破裂源(西段破裂、 中段破裂和全段破裂)上的最大可能年平均地震矩累积率. 计算结果表明， 海原断裂带未来百年强震(MW≥6.8)发生的可能性为0.0586.Abstract: In recent 20 years， some researchers have revealed a lot of paleoearthquake events in the Haiyuan fault zone. All available information allows more reliable analysis of the occurrence patterns of large earthquakes occurring in the Haiyuan fault zone. Based on this paleo-seismological information， three fault rupture sources can be inferred， the middle segment and the western segment are secondary rupture sources which are controlled by the full segment. A stochastic characteristic-slip model is presented to calculate the occurrence probability of large earthquakes in the Haiyuan fault zone， assuming the magnitude distribution of different rupture sources all fit the characteristic earthquake model and the average seismic moment accumulation rate of each rupture source is a constant. In addition， preferred annual mean seismic moment accumulation rates can be inferred by using the Monte Carlo method. The result shows that the occurrence probability of large earthquakes (MW≥6.8) is about 0.0586 in the next 100 years along the Haiyuan fault zone.
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图 1 海原断裂带西段地震矩累积率Ṁ0取值与合成古地震序列震级M分布的数学期望关系
Figure 1. The relation between the value of seismic moment accumulation rate，Ṁ0，and the expectant magnitude，M，of synthetic paleoearthquake events on the western segment of the Haiyuan fault zone
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图 2 海原断裂带中段地震矩累积率Ṁ0取值与合成古地震序列震级M分布的数学期望关系
Figure 2. The relation between the value of seismic moment accumulation rate，Ṁ0，and the expectant magnitude，M，of synthetic paleoearthquake events on the middle segment of the Haiyuan fault zone
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图 3 蒙特卡罗模拟得到的海原断裂带西段的古地震震级分布
Figure 3. The magnitude distribution of paleo-earthquakes on the western segment of the Haiyuan fault zone from Monte Carlo simulations
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图 4 蒙特卡罗模拟得到的海原断裂带中段的古地震震级分布
Figure 4. The magnitude distribution of paleo-earthquakes on the middle segment of the Haiyuan fault zone from Monte Carlo simulations
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图 5 海原断裂带上的地震矩特征地震释放比θ估计值分布
Figure 5. Distribution of estimates of the release ratio of seismic moment from characteristic earthquakes in the Haiyuan fault zone
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图 6 海原断裂带中段强震发生的概率密度分布
Figure 6. The probability density function of large earthquakes on the middle segment of the Haiyuan fault zone
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图 7 海原断裂带中段100年条件概率分布
Figure 7. The conditional probability for a 100-year exposure period on the middle segment of the Haiyuan fault zone
表 1 海原断裂带古地震发生时间的不确定性范围
Table 1 The uncertainty of occurrence time of paleoearthquakes on Haiyuan fault
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