2013年甘肃岷县漳县M_S6.6地震余震序列时域衰减特征分析

本文基于补充遗漏地震事件后的地震目录和修正的大森公式,对2013年7月22日甘肃岷县漳县6.6级地震余震序列时间域衰减特征进行了分析.鉴于主震后短时间内目录遗漏的余震较多,首先利用岷县台连续地震记录波形的高频包络差,检测主震后3h内目录遗漏的地震.经检测在主震后3h内共发现目录遗漏的ML1.0以上地震139个,最大震级为ML3.6.主震后1000s内检测到遗漏地震69个,约为目录给出余震数量的6倍.而后使用补充遗漏地震的目录,基于修正的大森公式分别拟合余震频度和余震地震矩随时间的变化.结果显示拟合p值约为1.07,表明岷县漳县地震余震序列衰减速率与全球平均水平接近,而未补充遗漏地震的频度拟合会造成余震序列衰减速率的低估.利用高频包络差直接计算地震频度曲线,通过三种衰减模式对地震频度曲线拟合参数比较,未观察到岷县漳县地震主震后存在早期余震缺失现象.分析认为,加入遗漏地震可以提高余震频度拟合估计衰减速率结果的准确度和精度,若缺少遗漏地震检测结果,则使用地震矩拟合所得衰减速率结果准确度较优,但需充分考虑其精度上的误差.在分析余震序列衰减特征的实际研究工作中,需根据地震目录完整性选择适当的拟合方法.

Temporal decay characteristics of the aftershock sequence of the 2013 Minxian-Zhangxian,Gansu, MS6.6 earthquake

Study of temporal decay characteristics of aftershock sequences is important for better understanding rupture processes of main shocks, seismic risk analysis and disaster relief after strong earthquakes. In existing catalogs, the data of aftershock sequences, especially those in the short time after mainshocks, are usually missing. So improving the completeness of aftershock catalogs can make the analysis of temporal decay characteristics more credible and reliable. In this paper, we analyzed the temporal decay characteristics of the Minxian-Zhangxian M_S6.6 earthquake in Gansu province that occurred on 22 July 2013. Our analysis was based on the catalog with missing aftershocks supplemented and the modified Omori's law. First we used the high frequency envelope difference of the three components with continuous waveforms of the station MXT to detect missing earthquakes in the three hours after the mainshock. In order to suppress low frequency interference, the waveforms were filtered by a 15 Hz high pass forth-order zero phase Butterworth filter. We calculated the difference of maxima envelope and minimum envelope using three-component added waveforms, and then picked out the clear "double peaks" by hand in the high frequency envelope difference as the detected missing earthquakes. The magnitude of missing earthquakes was determined by linear transformation of envelope peak values, whose transformation parameters were calculated using the envelope peak values and magnitude of aftershocks in the catalog. Second, we used the modified Omori's law to fit seismic frequency and seismic moment of the new catalog with added earthquakes detected. Temporal decay characteristics were analyzed by comparing the decay parameter p in different fitting methods and different aftershock sequences. By the detection we found 139 missing earthquakes in the three hours after the mainshock, of which the largest is M_L3.6, and about 6 times missing earthquakes in 1000 seconds after the mainshock more than that in the original catalog. The estimation result of p value is about 1.07, which shows that the temporal decay rate of the Minxian-Zhangxian earthquake is similar to the average rate of other worldwide earthquakes. The decay rate may be under estimated if the missing earthquakes are not added to the catalog. By direct obtaining the seismicity rate curve from high frequency envelope difference and fitting the curve using three kinds of decay modes, we did not observe the phenomenon of early aftershock deficiency in the aftershock sequence. We note that the accuracy and precision of the result of aftershock sequence temporal decay analysis can be improved by adding missing earthquakes to the catalog. If there is no missing earthquake detected, it is better to use seismic moment to estimate the temporal decay parameters of the aftershock sequence and give full consideration of the precision. It is important to choose appropriate fitting methods according to catalog completeness in aftershock decay analysis.