A Study Of Tidal Triggering Of Wenchuan Aftershock Activity

The phenomenon of integral and periodical elastic deformation of the earthcaused by tidal force is called earth tide. Solid tidal stress is a stress which is causedby Earth tide in the interior earth and featured with periodical change. The averagemagnitude is about10~3Pa, although it is much smaller than the stress drop associatedwith earthquakes,the cumulative rate of solid tidal stress change is largger than that oftectonic stress by two orders of magnitude(Heaton et al，1975；1982). Simultaneously,tidal stress has a feature that repeatedly affect the same position in the interior of theearth.This characteristic of oscillating, rather than amplitude,may play a moreimportant role in tidal triggering of earthquake. Based on the above statement,to theproblem that whether the earthquake could be triggerd or modulated by the tidal stress,domestic and foreign scholars’ research interests has not been decreasedThe fracture processes of the Wenchuan earthquake is complex,the number ofaftershocks is very large, the spatial distribution is very wide, the segmentationcharacteristics is obvious and with a uneven spatial and temporal distribution. In theresearch of the characteristics of Wenchuan aftershock sequence,JIANGHai-kun(2008) found that most of the strong aftershocks with M_S≥5.4are significantlytriggered by tidal force, full moon,first quarter and laster quarter are the advantagetime for the earthquake. On the basis of previous study, we take advantage ofWenchuan sequence data, more detailed reveal the fact that Wenchuan aftershockssequence is triggered by the Earth tide.According to the theory of the tidal deformation and elastic mechanics, quotelove numbers L_n、 H_n、 K_n as the scale factor of the spherical functionexpansion of tidal potential for the vertical displacement,horizontal displacement andadditional potential. Write a program for Schuster’s test,data smoothing, phase anglestatistical test. Make use of Wenchuan aftershocks datas from May12,2008toDecember31,2008. A statistical analysis on Wenchuan aftershock activity triggeredby Earth tide is systematically studied based on Schuster’s test by time sharing andregion dividing.Including earthquakes probably triggered by tidal force, tidal stress and tidal coulomb failure stress.The results show:1).On the whole,north part of the Wenchuan aftershock area is more obviouslytriggered by Earth tide than the south part; Result in monthly scale is clearer than thatin day scale; result from tidal force is more obvious than that from tidal stress andtidal coulomb failure stress.2). The p-value of the Schuster’s smooth test for the end of July to early Augustin2008at the north of the Wenchuan is very small, less than the threshold for tidaltrigger of earthquake0.05.So group of strong aftershocks occurred during this timemay be triggered by Earth tide.The similar conclusion is drawn by Schuster’s smoothtest of the tidal force, tidal stress and tidal coulomb failure stress.3).In addition, Wenchuan aftershock activity is obviously triggered by fortnighttide,in the north,the aftershocks happen more frequently in first quarter and lastquarter of the moon，and in the south, the aftershocks happen more frequently in firstquarter and last quarter of the moon and full moon.