The Seismology Of'Repeating Earthquakes'

'Reneatina earthnuakes',identified by cross-correlation of seismic waveforms are found to be much more abundant in nature than conventionally expected. Using the data of GSN, Schaff and Richards (2004) found that in China about10%of earthquakes were repeating events. In recent years, with the development of digital seismic networks, waveform correlation and 'repeating earthquake'have caused much attention in measuring crustal media variation; estimating the location precision and fault slip rates at depth or earthquake recurrence.Taking the definition of waveform correlation by Schaff and Richards (2004a), we cross-correlated the seismic waveforms in Liaoning and its surrounding regions using records from Liaoning Regional Seismograph Network (LRSN). There are totally2,024events selected with clear P phase pickings at no less than3stations for the period June1999to December2006within the spatial range38～44°N,119～126°E. The2,024events can make up570,374pairs with bulleting reported separation distance less than20km. Considering the frequency response and the level of background noise, the data are filtered before the cross-correlation. For broadband recordings the pass band is taken as0.5～5Hz, and for short-period recordings the pass band is1.0～5Hz. Cross-correlating the waveforms of these preliminary pairs there are497events recognized as 'repeating events', consist of787repeating pairs, being24.6%of the whole events, with three or more stations having the cross-correlation coefficients larger than0.8. Based on the data, we also identified multiplets. Two 'multiplets' located near the seismogenic fault of the1999Xiuyan MS5.4earthquake and the February4,1975, Haicheng Ms7.3earthquake, respectively, were investigated. For the 'multiplet' occurred before and after the1999Xiuyan earthquake, apparent pre-shock accelerating-like slip behavior, clear immediate-post-seismic change, and relaxation-like post-seismic change can be observed.There were23'repeating pairs' identified by Schaff and Richards (2004a), by teleseismic waveform cross-correlation, located in Liaoning Province. These events were cross-correlated using the waveforms recorded at Liaoning Regional Seismograph Network. The result shows that 'repeating events' in the sense of teleseismic waveform cross-correlation and those in the sense of regional waveform cross-correlation have significant differences, with some overlap. However, the overall assessment of the location precision and the phase pick errors of Liaoning Regional Seismograph Network by using these two sets of 'repeating events', respectively, provide similar results, Assuming that the separation between the 'repeaters' is less than1km, uncertainty of the routine location practice of Liaoning Regional Seismograph Network can be estimated as below5km, with the average of2km, and the errors of the phase picking of Pg and Sg, respectively, in the observational bulletins, can be estimated as94%and88%, respectively, within±1s.