Spatio-temporal characteristics of b value in the northern margin of Ordos block and its earthquake prediction efficiency
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摘要: 使用中国地震台网1970—2017年地震目录，在评估目录完整性和删除余震的基础上，使用最大似然法对鄂尔多斯地块北缘进行b值时空扫描. 扫描结果显示，鄂尔多斯地块北缘的低b值区域主要分布于阿拉善和晋冀蒙地区，低b值区域的空间分布随时间呈现一定变化. 低b值区域与中强地震存在一定的对应关系，对14次震例的回溯性检验显示，其中10次地震发生在低b值区域及其边缘. 使用Molchan图表法和R值评分法对b值时空扫描结果进行预测效能检验，以低于阈值的区域为预测区域，计算不同阈值的漏报率、报准率和时空占有率。Molchan图表法检验结果显示，本文所使用的b值时空扫描方法的预测效果优于泊松分布，最优结果的概率增益达到2，显著性水平达到0.25. R值曲线显示，当阈值为0.8时，达到最高R值0.28，并达到97.5%置信水平.
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关键词:
- b值 /
- 时空扫描 /
- 预测效能 /
- Molchan图表法 /
- 鄂尔多斯地块北缘
Abstract: Using earthquake catalogue of China Earthquake Networks Center from 1970 to 2017, the b values of the north edge of Ordos block were scanned by the maximum likelihood method after evaluating completeness and deleting aftershocks. The results show that the low b value region of the north margin of Ordos block is mainly distributed in the Alxa area and the Shanxi-Hebei-Inner Mongolia area, and the spatial distribution of low b value region changes with time to a certain extent. Meanwhile, there is a certain corresponding between the low b value region and the location of moderate-strong earthquakes. The retrospective test of 14 earthquake cases shows that ten earthquakes occurred in the low b value regions and their edges, and four earthquakes occurred beyond the low b value region. And then Molchan chart method and R value scoring method were used to test the prediction efficiency of b value spatio-temporal scanning. Taking the area below the threshold as the predicted area, the missing rate, the hit rate and the spatio-temporal occupation rate of different thresholds were calculated. The result of Molchan chart test shows that the prediction efficiency of spatio-temporal scanning is better than that of Poisson distribution, the probability gain of the optimal result reached 2, and the significance level reached 0.25. The R value curve shows that the highest R value is 0.28 at the threshold of 0.8 with 97.5% confidence level. -
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图 1 鄂尔多斯地块北缘的构造背景和地震分布
Figure 1. Tectonic settings and earthquake distribution in the northern margin of Ordos block
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图 2 余震删除前后的地震频次时序统计（a）和震级频次统计（b）
Figure 2. Seismic frequency time series statistics （a） and magnitude frequency statistics （b） before and after aftershock deletion
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图 3 鄂尔多斯地块北缘的最小完整性震级计算结果
（a） Mc空间分布；（b） δMc空间分布；（c） Mc时序分布
Figure 3. Minimum completeness magnitude of the northern margin of Ordos block
（a） Mc spatial distribution；（b） δMc spatial distribution；（c） Mc time-series distribution
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图 4 参与计算的地震目录（ML≥2.0）与全部地震目录的频次对比
Figure 4. Comparison of frequencies of earthquakes used in calculation （ML≥2.0） with those of all earthquakes
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图 5 鄂尔多斯地块北缘b值 （a）和δb （b）的时空扫描结果
Figure 5. Spatio-temporal scanning results of b value （a） and δb （b） in the northern margin of Ordos block
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图 6 鄂尔多斯地块北缘14次震例的b值回溯检验
Figure 6. Retrospective test for b values of the fourteen earthquake examples in the northern margin of Ordos block
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图 7 Molchan检验
（a） 不同概率增益G下的τ - υ曲线；（b） 不同显著性水平α下的τ - υ曲线；
Figure 7. Molchan test
（a） τ - υ curve with different probabilistic gain G；（b） τ - υ curve with different significant level α
表 1 鄂尔多斯地块北缘14次震例的回溯情况
Table 1 Retrospective test for the fourteen earthquake examples in the northern margin of Ordos block
编号 年-月-日 北纬/° 东经/° MS 地点 低b值 1 1976-04-06 40.20 112.10 6.2 和林格尔 × 2 1976-09-23 39.90 106.40 6.2 巴音木仁 √ 3 1979-08-25 41.20 108.10 6.0 五原 √ 4 1981-08-13 40.58 113.41 5.6 丰镇 √ 5 1989-10-19 39.92 113.91 5.9 山西大同 √ 6 1991-01-13 40.40 105.60 5.2 阿拉善左旗 × 7 1991-03-26 40.00 113.80 5.8 山西大同 √ 8 1991-09-14 40.20 105.50 5.1 阿拉善左旗 × 9 1996-05-03 40.80 109.60 6.4 包头 √ 10 1997-10-21 41.20 107.10 5.0 乌拉特后旗 √ 11 1998-01-10 41.10 114.30 6.2 河北张北 × 12 1999-03-11 41.20 114.60 5.6 河北张北 √ 13 1999-11-01 39.80 113.90 5.6 山西浑源 √ 14 2015-04-15 39.80 106.30 5.8 阿拉善左旗 √ 
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