Time-frequency analyses for borehole strain anomaly at Guzan station before 2013 Lushan MS7.0 earthquake based on wavelet transform
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摘要: 2013年4月20日芦山MS7.0地震前，四川省姑咱台四分量YRY-4型钻孔应变仪记录到显著的高频应变畸变信号。为了避免该畸变信号的高频部分在时频演化过程中发生频率混叠现象，同时便于讨论同一研究频段内震前更早记录到的弱应变异常时段与显著应变异常时段的相关性，本文对2012年1月1日至2013年4月20日期间姑咱台的钻孔应变数据进行了时频分析，并使用时域分段的小波变换方法，在局部时频谱中提取了应变异常，分析了各应变异常的频率相关性。结果表明，研究频段内存在两个峰值频率，峰值频率附近频段内弱应变异常时段与显著应变异常时段的相关系数为0.85，统计出的无震时段与其显著应变异常时段之间的相关系数只有−0.007，且已排除远震以及外界环境带来的干扰，因此该峰值频率作为异常频率的可信度较高，由此推测四分量钻孔应变仪所记录到的两种不同程度的应变畸变信号的演化过程可能与芦山MS7.0地震相关。Abstract: Before the 20 April 2013 Lushan MS7.0 earthquake, a significant high frequency strain distortion signal has been recorded by the four-component YRY-4 borehole strainmeter at Guzan station in Sichuan Province. In order to avoid the frequency aliasing phenomenon of the high frequency part of the distortion signal in time-frequency evolution process in the same spectrum, and for the purpose of discussing the correlation between the weak strain anomaly period recorded earlier and the significant strain anomaly period in the same studied frequency band before the earthquake, this paper carried out time-frequency analysis on the borehole strain data of Guzan station during the period from 1 January 2012 to 20 April 2013 by using the continuous wavelet transform method. The results show that there are two peak frequencies in the studied frequency band, the correlation coefficient between the weak strain period and signifi-cant strain anomalous period near the peak frequency reach 0.85, and the correlation coefficient between aseismic period and significant strain anomalous period is only −0.007. Therefore, the peak frequencies is highly reliable as the anomalous frequency, and the evolution of two kinds of deformed strain signals recorded by four-component borehole strain gauge may be related to Lushan MS7.0 earthquake.
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图 1 2008年1月1日—2014年11月25日姑咱台钻孔应变四分量观测曲线
Figure 1. Observation curves of four-component borehole strain at the station Guzan during the period from 1 January 2008 to 25 November 2014
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图 2 2013年4月17日一阶差分应变数据的小波变换结果
（a） 差分数据的频段为0—8.3 mHz；（b） 差分数据为高频段0.6—8.3 mHz
Figure 2. Wavelet transform results of the first-order differential strain data on 17 April 2013
（a） Frequency band is 0−8.3 mHz；（b） High frequency band is 0.6−8.3 mHz
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图 3 2013年4月1—20日应变差分数据的局部小波变换结果
Figure 3. Local wavelet transform results of the strain differential data from April 1 to April 20，2013
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图 4 2013年4月2，6，9，11日弱异常时段与4月19日高频异常时段归一化频率的分布对比结果
虚线表示异常时段在5.9 mHz和7.2 mHz两个异常频率峰值点的对应情况
Figure 4. Comparison of the normalized frequency distribution of the weak abnormal time interval on 2，6，9 and 11 April 2013 with that of the high-frequency abnormal time interval on 19 April 2013
The dashed lines indicate the correspondence at the abnormal peak frequency 5.9 mHz and 7.2 mHz
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图 5 2012年1月1日—2012年8月31日姑咱台钻孔应变北南分量观测曲线处理结果
（a） 完整时段；（b） 平稳时段；（c） 日变化统计
Figure 5. Observation curves of the NS component of borehole strain recorded at the station Guzan from 1 January 2012 to 31 August 2012
（a） In the whole time interval; （b） In the stationary time interval；（c） Daily variation statistics
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图 6 2013年4月19日高频异常时段与3次同震时段归一化频率分布对比结果
虚线表示异常时段在5.9 mHz和7.2 mHz两个异常频率峰值点的对应情况
Figure 6. Comparison of the normalized frequency distributions of high-frequency anomalies in three coseismic time intevals with that of high-frequency anomaly interval on April 19，2013
The dashed lines indicate the correspondence at the abnormal peak frequency 5.9 mHz and 7.2 mHz
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图 7 无震时段以及5次高频异常时段归一化频率分布对比结果
虚线表示异常时段在5.9 mHz和7.2 mHz两个异常频率峰值点的对应情况
Figure 7. Comparison of the normalized frequency distribution in the aseismic time interval with that in five high-frequency abnormal time intervals
The dashed lines indicate the correspondence at the abnormal peak frequency of 5.9 mHz and 7.2 mHz
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图 8 钻孔应变观测东西（a）、北东分量（b）弱异常时段与可靠异常时段频率分布对比结果
虚线表示异常时段在5.9 mHz和7.2 mHz两个异常频率峰值点的对应情况
Figure 8. Comparison of the normalized frequency distribution of EW- （a） and NE-components （b） in weak abnormal intervals with that in the reliable abnormal time intervals
The dashed lines indicate the correspondence at the abnormal peak frequency 5.9 mHz and 7.2 mHz
表 1 异常频率下三分量应变观测相关系数统计
Table 1 Correlative coefficients for three-component strain at the abnormal peak frequencies
峰值频率/mHz 相关系数 北南分量 东西分量 北东分量 5.9 0.82 0.64 0.85 7.2 0.85 0.79 0.81 
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