Abstract：Frequent pulses, whose period from minutes to hours, were recorded with the YRY4 strainmeter at Guza station before the Wenchuan earthquake in 2008. Via highpass filtering, strong correlation between the recorded pulses and the main shock is revealed. In order to prove,an Overrun Rate Analysis (ORA) is carried out for a systematic and quantitative examination of the abnormal changes, based on three criteria with respect to the periods for before, during and after the main shock, respectively. The phenomenon does not seem to be close to the so called precursory longperiod wave preceding a macroquake but is similar to the acoustic emission prior to the fracture of rocks.
