Time-frequency characteristics of four-component borehole strain at Guzan station before and after 2013 Lushan M_S7.0 earthquake

The four-component borehole strainmeter at Guzan station in Sichuan Province recorded abnormal strain signals with high frequency before the 20 April 2013 Lushan M_S7.0 earthquake, which were similar to the signals before the Wenchuan M_S8.0 earthquake. The strain data were processed in the way of S transform in order to analyze the evolutionary process of each component of abnormal signals in time-frequency domain, and study the validity of the anomalies and its relation with Lushan earthquake. The result shows that two clusters of anomalies with high energy appeared during the research period, one in October of 2012 lasting for four months, and the other several days before the earthquake. The validity of the anomalies is high, considering that the disturbance of teleseismograms and engineering construction during this period are not the main cause for the anomalies. The anomalies are composed of all kinds of signals with various periods from several minutes to hours, of which the short-period signals appeared later and decayed sooner. The whole energy of the anomalies after the earthquake began to decay gradually, and the signal components began to reduce, too. The strain observation had returned to normal by the end of August 2013. The whole evolutionary process of strain anomalies in time-frequency domain has very close relations to the Lushan M_S7.0 earthquake.