2011年安庆M_S4.8地震SmS震相观测与研究

Observation and study on the seismic phase SmS generated by 2011Anqing M_S4.8 earthquake

2011年1月19日发生的安庆MS4.8地震,在较远范围引起了明显震感.通过仔细辨认此次地震不同频段的波形记录,发现在一些台站记录到了清晰的SmS震相.采用CAP方法反演了其震源机制解,并利用理论地震图与实际观测数据对比进一步确认了SmS震相的存在.结合已有实验结果,推断在人类有感频率(1Hz至几Hz)范围内,震中距70—200km内SmS震相往往是振幅最大的震相,是引起远距离有感的主要原因.利用理论地震图研究了震源深度对SmS震相的影响.结果表明,震源深度可改变SmS震相发育的临界震中距,震源越深对应的临界震中距越小.进一步通过对比SmS与S的振幅比,定性讨论了震源机制解中倾角、滑动角和地壳浅层衰减等因素对SmS震相的影响.结果表明,倾角和滑动角对其振幅比影响呈现比较复杂的关系,而SmS与S振幅比随地壳浅层衰减的增大而减小,说明浅层衰减对SmS震相影响较大.综合分析认为,除当地地壳速度结构的影响外,SmS震相的发育受到震源机制解的倾角、滑动角,震源深度以及地壳浅层衰减等多种因素的影响.

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 compa- ring the amplitude ratio between SmS and S. The amplitude ratio between SInS and S becomes smaller with larger attenuation in the shallow crust, indicating SmS is sensitive to that. Overall, the SInS 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.