Cheng-Horng L in(林正洪)and Wen-Yen Chang

1)Institute of Earth Sciences,Academia Sinica,Taipei 115 2)Science Council,Taipei 106

Since the Earth outer core is liquid,the shear waves that are generated by a large earthquake and traveling to the co re(ScS)will totally reflect with strong seismic energy.It is interesting to note that one large aftershock(MW=6.0)of the 2008 Sichuan earthquake(MW=7.9)in China just occurred when the ScS phase returned around 15.5 minutes later after the mainshock occurred.The estimated peak dynamic stress(～3.3 kPa)of the ScS amplitude was reaching to the generally lower bound for capable of triggering earthquakes.Both the temporal correlation between the aftershock occurrence and the ScS arrival as well as the large enough dynamic stress suggest that the largest aftershock might be triggered dynamically by the ScS waves.Further statistic examinations of 24 major earthquakes(MW＞7.5)since 2000 show some large aftershocks(MW＞5.0)also coincidentally occurred around the ScS phase arrived.Those observations imp ly that aftershocks might likely be triggered dynamically by the ScS phases,and thus a tempo ralwarning for potentially destructive aftershocks might be considered in the future great earthquakes.