Systematic examination of 'precursor waves' of strong earthquakes by using the wavelet method

Based on the borehole strainmeter recordings immediately before the strong earthquakes (<i<M</i<<sub<S</sub<≥7.5) during the period from 2001 to 2005 observed at the Taian observation station, Shandong Province, the authors make a systematic and objective examination of the precursor waves of the quakes. The effects of Earth tides with periods larger than 128 min are eliminated through high-pass filtering; and atmospheric-pressure inferences are removed by linear regression. The 2−128 min signals are then separated into six frequency bands by employing the wavelet method. Results indicate that the wavelet method is capable of picking out information of weak variations in the signals. According to the characteristics of the 'precursor waves' obtained from wavelet transformation, the method of overrun ratio analysis is put forward for examination. All the detailed components of the wavelets have been analyzed. For the time series of the overrun rate in all these components, statisitical calculation has been made for the slopes of fit curves, and mean values and standard deviations were obtained and positive-negative slope ratios were analyzed. The three statistical data show that 'precursor waves' are not widely recorded by borehole strainmeter within 15 days before remote strong earthquakes.