sPL,一个近距离确定震源深度的震相

实际地震波形观测表明,对于大陆结构相对简单的地壳中的地震而言,有一震相出现在P 波和S波之间.一般在30~50 km附近发育得较好,其能量主要集中在径向分量,而垂向分量的振幅相对径向要小,切向分量上的振幅很弱,且波形以低频为主,通常没有P波尖锐.在利用FK方法计算合成地震图的基础上,发现该震相是由S波入射到自由地表形成水平传播的P波(文献称为surface P wave,自由地表P波)或者包括S波入射到地表后形成的多次P波或其散射震相.由于该震相是由S波和P波之间耦合而形成,本文将其定义为sPL(s coupled into P) 震相.理论波形研究表明,sPL相对直达P波的到时差对震中距离不敏感,而随着震源深度的增加几乎呈线性增加,因此可以很好的约束震源深度.本文以2005年江西九江地震为例,证实了sPL确定震源深度的可行性和可靠性.在观测到sPL震相的情况下,离震源50 km以内的一个三分量地震台站的波形就可以帮助获得可靠的震源深度,而不需要精确的震中距离.由于sPL震相出现距离较近,对于较小(三级以上)的地震也可以应用,因此在稀疏台网布局情形下sPL对于确定中小地震深度应该具有很好的应用意义.

sPL, an effective seismic phase for determining focal depth at near distance

A seimic phase is often observed between P and S of continental earthquakes for epicentral distances less than 50km in regions with simple crustal structure. It is stronger on radial component than on vertical component, and its tangential amplitude is weak. The seismic phase is also less impulsive than P waves. From synthetic waveforms, we find that it is generated when S wave goes upward and is post-critically converted to P wave traveling along the free surface. It also may contain multiple conversion or reverberations of P wave near free surface. Because it results from coupling between S and P waves, we define this phase as sPL. For a half-space velocity model, sPL degenerates to surface P wave as defined by Aki and Richards, and it contains less high frequency energy due to its similar nature as head waves. But it may contain multiple reverberations of P wave near critical distance when there is substantial sub-surface velocity gradient. Waveform study shows that differential time between sPL and direct P is insensitive to epicentral distance, but increases almost linearly with the focal depth. It is used to determine the focal depths for a 2005 earthquake sequence in Jiujiang, China. Error analysis with synthetic waveforms indicates that 10% uncertainty in velocity structure will lead to depth error of about 2km or less. Focal depth from sPL has the advantage that it can be obtained from even only one seismic record in distance less than 50km and accurate epicentral distance is not required. It may help a lot to determine focal depth of small earthquakes with magnitude less than M3, thus will be helpful when only sparse seismic stations are available.