Borehole strain anomalies before the 20 April 2013 Lushan M_S7.0 earthquake

The YRY four-component borehole strainmeter of Guzan Station, 70 km away from the epicenter of Lushan earthquake, recorded eight tensile strain steps with the range of ten times of tidal amplitude on April 16—19, which has never happened over the past 6 years’observation. The strain anomalies are related to Lushan earthquake both in time and space. The principal stress direction deduced by the abnormal strain, similar to that of in-situ stress measurement by hydrofracturing at Baoxing before the earthquake, is in accordance with 3-months-records during the five months before the Lushan earthquake. Some of the strain steps occurred when teleseismic waves arrived. As mentioned above, we presume the strain anomalies of April 16—19 probably reflect the strata creeping before the rupturing of the Lushan earthquake generating fault.