Inthispaperchangesinfocalmechanisms,parametersofwavespectra,andstressdropsfortheMS=5.0foreshockandMS=6.0mainshockinFebruary2001inYajiangCounty,Sichuan,andseismicityinepicentralregionarestudied.ComparisonoffocalmechanismsfortheYajiangearthquakeswithdistributionpatternsofaftershocks,thenodalplaneI,strikinginthedirectionofNEN,oftheYajiangM=5.0eventischosenasthefaultingplane;thenodalplaneII,strikinginthedirectionofWNW,oftheM=6.0eventasthefaultingplane.Thestrikesofthetwofaultingplanesarenearlyperpendiculartoeachother.ThelevelofstressdropsintheepicentralregionbeforetheoccurrenceoftheM=6.0earthquakeincreases,whichisconsistentwithincreaseofseismicityintheepicentralregion.TheratedecayoftheYajiangearthquakesequence,changesinwavespectraforforeshocksandaftershocks,andfocalmechanismsarecomplex.
