宁夏十五数字地震观测台网自投入使用以来,积累了大量的地震数字波形.利用宁夏台网中震中距在 1 000 km范围内不同台站记录的 9次M≥3.0 地震的 sPn 与 Pn 震相进行分析、计算这些地震的震源深度.研究表明,利用 sPn与Pn震相的到时差计算地壳内近震深度的方法简捷、准确.%Focal depth,the vertical distance from an earthquake source to the ground (the epicen-ter),is one of the basic earthquake parameters and is one of the most difficult of the seismic pa-rameters to accurately determine.The accurate determination of the focal depth involves a series of important problems including a correct understanding of the earthquake preparation process, the earthquake mechanism,its geological structure,and the stress field.Since the fifteen stations of the Ningxia digital seismic observation network were officially established in 2009,a large vol-ume of digital seismic waveform data has accumulated,and much research work has been possi-ble.On the basis of previous studies,using the lag of the sPn and Pn phases,in this study,we calculated the focal depth of nine earthquakes with M≥3.0,which occurred in the area surround-ing Ningxia in recent years.The results of the sPn method are mainly dependent on the velocity model selection and precision of the seismic phase.We selected the approved and widely used Ningxia local velocity model.The Pn and sPn phases and reading accuracy are particularly impor-tant,because the initial amplitude of this type of wave phase is generally weak.As such,if there is an arrival time difference of 0.5~1 s,it may result in a calculation error of 1.5~3 km in the fo-cal depth.With the availability of multiple records of the sPn phase,using the averages of each calculation depth,it is possible reduce the error caused by the reading.Research shows that it is a relatively simple matter to accurately calculate the near-earthquake depth within the crust using the lag of the sPn and Pn phases.Determining the exact value of the focal depth can provide an important criterion in earthquake research with respect to the causal factors of the formation and structure of deep crustal activities.
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