Observation and study on the seismic phase SmS generatedby 2011 Anqing MS4.8 earthquake

The MS4.8 Anqing earthquake on 19 Jan 2011 was obviously felt even at long distances. SmS phases are identified in seismic records of some stations by examining the filtered waveforms and comparing them with synthetic seismograms. Based on our study result, we argued that the SmS phase, usually the strongest one in epicenter distance range of 70 to 200 km, is the main reason of feeling at such long distances. Some previous studies suggest that the SmS phases are controlled by the local crustal structure. We perform some tests using synthetic seismograms. The results show that focal depth can change the critical distance of SmS. The larger focal depth makes the critical distance smaller. Also effects of focal mechanism on SmS are investigated qualitatively by comparing the amplitude ratio between SmS and S. The amplitude ratio between SmS and S becomes smaller with larger attenuation in the shallow crust, indicating SmS is sensitive to that. Overall, the SmS phase can be affected by many factors besides local crustal structures, such as dip angle, rake angle of the focal mechanism， focal depth and attenuation.