The Contribution Of Post-critical Moho Reflections SmS To Ground Motions

In recent years,studies have shown that the post-critical Moho reflections SmS will cause the ground motion to increase within a certain epicentral range,which will change the shape of the GMPE(Ground motion prediction equation)model within the epicentral range.In view of this phenomenon,we first review the development process of ground motion attenuation relationship,and introduce the research status of consider-ing the impact of SmS on GMPE.At present,in China,studies of strong ground motion of the 2008 great Mw 7.9 Wenchuan earthquake indicate that the horizontal compo-nent peak ground acceleration(PGA)is significantly enhanced within the fault distance range of 100 to 300 km.In order to explain this observed phenomenon,we confirm the existence of large-amplitude SmS within the critical epicentral region(about 100 to 300 km)in this area,with amplitude consistently 3 to 5 times stronger than direct S,through analysis of waveform recordings of Wenchuan aftershocks of single earth-quakes at multiple stations and multiple earthquakes at single stations.According to the ground motion attenuation relationship of aftershocks,it is found that the average peak ground velocity(PGV)and PGA values of horizontal components in the same distance range also increase significantly.In order to further confirm the existence of the large amplitude SmS in the area and its influence on the ground motion attenuation relation-ship,we use the local crustal velocity model to conduct theoretical tests.It is found that the post-critical SmS arrivals clearly enlarge the amplitude,which also explains the increased PGA values of the Wenchuan mainshock within the critical distance range.At the same time,according to the observation characteristics of basin and Plateau stations,we find that SmS may not have a significant impact on ground motion.There-fore,we also test the influencing factors of SmS strength(crust structure,source pa-rameters and frequency range).The results show that the simpler the crustal structure is more conducive to the development of SmS;the focal depth will change the epicentral distance range of post-critical SmS,the greater the focal depth,the smaller the epicen-tral distance of post-critical SmS;and the influence of focal mechanism solution is more complicated for SmS,there is no obvious law,which may be related to the radiation pat-tern of earthquake;SmS(1 to 2 Hz)is a higher frequency phase than direct S(about 1 Hz).In addition,we take an example of the Longmenshan area to quantify the influ-ence of SmS on the ground motion amplitudes,and modify an existing model to obtain a new hinged-trilinear attenuation model of this area,which provides a better fit to the recorded PGA amplitudes of the Wenchuan mainshock.In summary,we infer that the impact of SmS on ground motion cannot be ignored,especially for areas with relatively simple crustal structure.