Focal Depth Determination From SPn Phase And Its Application

Focal depth is an important parameter for describing the source information. Focal depth plays an important role in the study of regional seismicity and seismic hazard evaluation. However, determining the focal depth accurately is always a problem in the earthquake location process. Usually, only in the case of dense seismic network and small epicentral distance, we can obtain focal depth with higher accuracy. But in the relatively sparse station network, nearby station data are lacking, and seismic wave travel time from the focus to the stations is not sensitive enough to the depth, making it difficult to accurately determine the focal depth.The purpose of this paper is mainly focusing on the method based on sPn to determine the focal depth. The Pn phases are picked by the waveform cross-correlation method, and the sPn phases are identified by using the sliding-window correlation method. Through modeling synthetic seismogram, we can research the characteristics of sPn phase, analyze the possible factors that influence the seismic phase and verify the reliability of the method using in this paper. Using the seismic waveforms recorded by the seismic array deployed in the southern section of the North-South seismic belt, we obtain the focal depths of28ML>4.0aftershocks of the Lushan Ms7.0earthquake from the arrival-time differences between sPn and Pn phase and analyze the location results. Finally, this paper gives focal depth from several earthquakes at domestic. The main conclusions are as follows:(1) sPn is a useful seismic phase to determine the focal depth in short distance (Δ<1000km). The arrival-time differences between sPn and Pn phase are almost not affected by the influence of the propagation path from upper mantle, mainly related to the focal depth, not the epicentral distance. For this reason, the focal depth from the arrival-time differences between sPn and Pn phase is quite accurate.(2) From a series of synthetic seismogram, we have obtained the characteristics of sPn phase. The results show that sPn can be observed not only in the vertical component, but also clearly in the radial component. However, in the tangential component, the energy is less. The dominant frequency range of sPn is0.1-1.0Hz. The amplitude and the polarity of first motion of sPn and the amplitude ratio of sPn and Pn are affected by several factors such as focal mechanism, azimuth angle and magnitude. Among that, the polarity of sPn with different azimuth angle may be different, and the amplitude is small in some azimuth. The depth location method based on the sPn phase is more suitable for moderate earthquake. The signal noise ratio of the stations in great distance from small magnitude earthquake (M<3) is low, and it is difficult to pick out the sPn seismic phase. If the source rupture process is complex and the source time function is long (M>6.5), it is not conducive to accurately pick out the sPn seismic phase.(3) We research the sPn phases by using the sliding-window correlation method, showing the arrival time difference with different azimuth is mainly affected by random noise, and the arrival time difference between sPn and Pn from different focal mechanism solutions is also different. On the premise of aligning accurately Pn phase, we use the sliding-window method to identify accurately the well developed sPn phase. Especially, when stations are in short distance and the waveform from adjacent stations are similar, it is reliable to use this method to extract the sPn phase.(4) Using the seismic waveforms recorded by the seismic array deployed in the southern section of the North-South seismic belt, the focal depths of28ML≥4.0aftershocks of the Lushan Ms7.0earthquake are determined from the arrival-time differences between sPn and Pn phase. The Pn phases are picked by the waveform cross-correlation method to improve the precision, and the sPn phases are identified by using the sliding-window correlation method. Considering that the focal depth given by the method is of high accuracy, the results can be used to test other earthquake location results.(5) The depth of the28ML≥4.0aftershocks given in this paper distributes mainly from10to20km, not finding shallow earthquakes near surface.. From the NW-SE profiles perpendicular to the seismic belt, the focal depths dip to NW with a dip about39°, agreeing with the thrust fault in the south of Longmen Mountain nappe belt. In the depth profile, the aftershocks show a linear characteristic in space, implying that they may occur near the rupture plane of the mainshock with dip angle of about39°, and outcrop of them is in the east side of the ShuangShi-Dachuan fault, therefore, we infer that the seismogenic fault of Lushan Earthquake may be the blind thrust fault to the east of the seismic zone rather than the Shuangshi-Dachuan fault.(6) In this paper, we determined the focal depth of several earthquakes at domestic using sPn phase, and these results can be used to test earthquake location results from other location methods.