Growing Correlation Length Prior to Moderate Strong Earthquakes in the Boundaries along the Northeast Margin of Tibet Block
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摘要: 为研究大震前地震空间相关长度的变化特征, 分析了青藏块体东北缘6次中强地震前的地震活动的时空演变.在计算空间相关长度过程中, 采用长轴与孕震区主要断裂走向一致的椭圆形空间窗, 并设计了优化程序.比较椭圆形和圆形空间窗的结果, 表明对所研究的6次中强地震前都观测到了增长的地震空间相关长度.椭圆形空间窗的结果比圆形的结果更好, 相关长度增长幅度较大, 曲率参数值较小.利用3种合成地震目录对结果进行了显著性检验.3种随机数据的检验结果都否定了零假设, 表明观察到的模型不是由随机数据干扰造成的, 这一观点的置信水平被证实为99%以上.最后得出主震震级与临界区的尺度(等效半径)关系为logRe~0.46(±0.02)M, 与理论值符合得较好.Abstract: For studying the characteristics of seismic correlation length before great earthquakes, the temporal-spatial change of seismicity prior to 6 moderate strong earthquakes (ML>5. 5) in the boundaries along the northeast margin of Tibet block have been analyzed. We use ellipse, whose long axis is coincident with the strike of main fault belt in the source area, as the spatial window to analyze growing correlation length prior to main-shock and design the corresponding optimization procedure. Comparison between the results from elliptical and circular spatial windows shows that the growing correlation length prior to 6 moderate strong earthquakes studied have been observed in both two spatial windows. The results from ellipse are better than that from circular, and the growth ranges of correlation length in former are larger than those in later yet the smaller curvature parameter values of C are obtained in elliptical spatial windows. Three types of random synthetic catalog are used to test the significance of the results. The results reject the null hypothesis for three types of random synthetic catalog and show that the observed patterns are not consequences of random fluctuations. That theory is found to be of above 99% confidence levels. The scaling relation we found, log Re~0.46 M between the main-shock magnitude M and the equivalent radius Re of the critical region is in good agreement with theoretical values.
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图 1 研究区位置, 所研究的6次地震的震中分布以及研究区断层分布
Fig. 1. The location of the study area relative to Tibet block, the faults in the area and epicenter distribution of the earthquakes studied
图 2 研究区域内地震分布(1980-2003年，ML≥2.5)
Fig. 2. Earthquake distribution in the study area from 1980 to 2003, ML≥2.5
图 3 所用目录的震级—频度图（研究区1980—2003年）
Fig. 3. The magnitude-frequency plot for catalog from 1980 to 2003 in the area studied
图 4 所研究的6次地震的临界区域（椭圆形和圆形）
a.门源地震（ML＝6.4）；b.景泰地震（ML＝6.2）；c.永登地震（ML＝5.8）；d.景泰地震（ML＝5.9）；e.玉门地震（ML＝5.8）；f.山丹地震（ML＝6.1）
Fig. 4. Critical regions (ellipse and circular) for 6 earthquakes
图 5 6次地震前相关长度的时间进程
（1）椭圆临界区；（2）圆形临界区；实线表示幂次率拟合曲线；虚直线表示线性拟合直线
Fig. 5. Time evolution of correlation length for 6 earthquakes
图 6 景泰6.2级地震空间窗等效半径Re（椭圆）、半径R（圆）和椭圆长轴方位角的优化过程
Fig. 6. The optimization process of Re (for ellipse), R (for circular), and azimuth angle (for long axis of ellipse) of Jingtai earthquake (1990—10—20, ML ＝ 6.2)
图 7 在3种类型的合成地震目录中, 基于每种1000个目录的曲率参数小于或等于C的累积概率曲线
对于景泰6.2级地震, C＝0.46, 对应3种合成目录的概率分别为P＝ 16.2%, 14.5%, 9.5%.（1）, （2）和（3）分别表示3种类型的合成目录
Fig. 7. Probability distribution to find a curvature parameter smaller than or equal to in three types of random earthquake catalog based on 1000 catalogs one type
图 8 所研究的6次地震的临界区域的等效半径Re和主震震级的函数关系（拟合的直线的斜率为0.46）
Fig. 8. Equivalent radius Re of the critical region as a function of mainshock magnitude for the earthquakes studied
表 1 研究的6次地震的参数和分析结果
Table 1. The parameters of 6 arhquakes suied and the resuts analyed
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