The Longnan Ms 5.5 Earthquake On September 12,2008:a Very Shallow Event Probably Triggered By The Wenchuan Earthquake

Based on the broadband seismographic network and InSAR direct observation data,the source parameters of the September 12,2008 Longnan Gansu Ms5.5 earthquake,which located at the northeastern end of the Wenchuan earthquake aftershock area,were determined by three methods.And the results obtained by the three methods are consistent with each other.The Coulomb stress theory was used to investigate the impact of the Wenchuan earthquake on the stress change on the Longnan earthquake fault plane,confirming that the Longnan earthquake was probably triggered by the Wenchuan earthquake.Major results of this thesis include:(1)The broadband seismograph waveform data was inverted with the gCAP method so as to obtain the focal mechanism and depth of the Longnan earthquake.The nodal plane solution is NP1:150°/45°/81°,NP2:342°/45°/98°.The non-double force component is 53%,and the centroid depth is about 1.6 km.(2)With the Rayleigh wave amplitude spectrum,the depth of the Longnan earthquake was further determined and the result is consistent with the one obtained by gCAP.It confirms the Longnan earthquake is a shallow earthquake.(3)The surface deformation observed by InSAR was used to further confirm the focal depth.The result is also in agreement with results obtained from the gCAP and Rayleigh wave amplitude spectrum methods.(4)Based on the source parameters of the Longnan earthquake,the Coulomb stress of the Wenchuan earthquake on the Longnan earthquake fault was calculated.It is found that the Coulomb stress change on the fault of the Longnan earthquake reached 30 kPa,which exceeds the usually adoptedCoulomb stress trigger threshold.This implies that the Wenchuan earthquake might have a triggering effect on the Longnan earthquake and is the result of the Wenchuan earthquake.