Time-frequency analyses for borehole strain anomaly at Guzan station before 2013 Lushan M_S7.0 earthquake based on wavelet transform

Before the 20 April 2013 Lushan M_S7.0 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 M_S7.0 earthquake.