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

A seimic phase is often observed between P and S of continental earthquake 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 a along the free surface It also may contain multiple conversion or reverberations of P wave bear 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 is 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 I 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