Abstract:Seismic source is a main focus in seismologic research,and the focal mechanism solution and earthquake depth are important parameters.Researchers use the depth phase to enhance the precision of focal depth for M ≧ 5.0 earthquakes by using the arrival time difference between teleseismic depth sP （or pP） and reference phase P to determine the focal depth of teleseismic activity.For the small,regional earthquakes,local depth phases such as sPL,sPg,sPmP,sPn,and sSmS may be observed in the proper distance range.If the regional depth phases can be identified,the focal depth of moderate-small earthquakes may be estimated.To obtain accurate seismic parameters of the Diebu MS4.0 earthquake occurring on February 23,2010,this study uses local broadband waveforms and the cut-and-paste （CAP） method to obtain the best focal mechanism.The waveform fitting method for synthetic and observed seismograms is then used to further determine the focal depth of the earthquake.Because Minxian station （MXT） and the earthquake epicenter are 27 km apart,we use the data of that station to determine the depth phase sPL.Three component waveforms of MXT are first resolved to radial,tangential and vertical components,although the tangential component of the P wave energy is weakest for determining the best angle of rotation in obtaining the radial and tangential components.We next integrate the data of displacement and adopt a low-pass filter below 1 Hz.Finally,we use the waveform comparison method to accurately confirm focal depth.The results show that the Pg and sPL phases of time and amplitude in synthetic and observed seismograms agree stronger at 7 km,as do the three components of these seismograms.Therefore,the most accurate focal depth of the Diebu MS4.0 earthquake is 7 km,which implies that this event was a shallow earthquake in the upper crust.
