【摘要】 在南北地震带地区,USGS全球地震目录中存在一些震源深度大于30km的地震.这些地震的震源深度是否可靠,对于研究这一地区的孕震机制、岩石圈强度和构造演化等科学问题具有重要意义.本文以南北地震带2012年发生的5个4~5级地震为例,利用区域地震台网的波形数据,基于sPL深度震相、短周期瑞利面波以及CAP等独立方法测定了其震源深度.结果表明:sPL深度震相和CAP方法给出的震源深度比较一致,差别小于2~3km,能够得到比较可靠的震源深度;短周期瑞利面波及其与P波振幅比也确定了地震震源深度较浅的特征.本文研究结果显示:宁夏会宁4.7级、云南富民4.8级和四川会东4.7级地震的震源深度约为8~12km左右,仍为发生于上地壳的地震,USGS地震目录给出的30km甚至更深的震源深度存在明显偏差;对于四川隆昌4.6和4.9级地震,本文给出的震源深度为1~2km,属于极浅源地震,USGS地震目录给出的10km和35km的震源深度结果尚需进一步改进.更多还原

【Abstract】 The USGS earthquake catalog indicates that a few earthquakes with >30km focal depths occurred in the North-South Seismic Belt of China.Whether these focal depths are reliable or not is an important issue for the studies on the seismogenic mechanism,lithosphere strengthand tectonic implications.The focal depths of five M4~5earthquakes in the North-South Seismic Belt are carefully studied with broadband three-component waveforms from local and regional stations.With the traveltime difference between first P and sPL depth phase,the qualitative relations between the focal depth and the short-period Rayleigh wave(Rg wave),the amplitude ratio of Rg/P,and the focal mechanism inversion by Cut and Paste(CAP)methods,the focal depths of five M4~5earthquakes in the North-South Seismic Belt are accurately determined.Synthetic seismic waveforms are calculated by frequency-wavenumber(FK)method.In the CAP method,three-component waveforms from local and regional stations with good azimuthal distributions are used to invert for the focal mechanisms and focal depth.The Pnl and surface wave are separately fitted to avoid the effects of the lateral variations of the crustal structure.The grid search scheme is used in the CAP method to find the best-fit focal depth and focal mechanisms.For the Rg wave,when the earthquakes have shallow focal depth(e.g.,<10km)and the stations is beyond~5times epicentral distances,the Rg wave becomes strong in the radial or vertical components of the seismic waveforms.Together with the amplitude ratio of Rg/P,the Rg wave can be used to quantitatively determine the focal depth.The focal depths of the M4.7earthquake in Huining,Ningxia(10~11km),the M4.8earthquake in Fumin,Yunnan(11~12km)and the M4.7earthquake in Huidong,Sichuan(8~9km),are all within 8~12km.The differences in focal depth from the sPL depth phase and the CAP method are about 2~3km,which should be accurate enough to confirm that the earthquakes occurred in the upper or mid-lower crust.In contrast,the focal depths of all three earthquakes are greater than 30km(31,33 and 46km depth)in the USGS earthquake catalog,which mistakenly means the three earthquakes all occurred in the mid-lower crust rather than upper crust.For the M4.6and M4.9earthquake in Longchang,Sichuan,the focal depth from sPL depth phase is about 1~2km,which is shallower than the focal depth of 4km from the CAP method.The independent result from amplitude ratio of Rg/P is~2km,which is close to the results from the sPL depth phase.Since there is 2~3km difference in the focal depth between results from the sPL depth phase,Rg/P and the CAP method,we inferred that the regional 1-D velocity model used in the synthetic waveform calculation in the CAP method could induce the bias on the focal depth due to the thick low-velocity sediments and/or strong lateral variations in the upper crustal structure.In contrast,the sPL depth phase method is not strongly affected by the bias of the velocity model.Therefore,for the extremely shallow earthquakes,the sPL depth phase and the amplitude ratio of Rg/P methods should be used to identify the focal depth rather than the CAP method only.In final,the focal depths of the M4.6and M4.9earthquake in Longchang,Sichuan should be 1~2km,which belong to the extremely shallow earthquakes.There is still large bias in the USGS earthquake catalog compared to the focal depth of 10 km for the two earthquakes.It′s meaningful to determine the focal depth with seismic waveforms at only one or two local stations for the regions with sparse seismic stations.With the sPL depth phase,the Rg wave,and the CAP methods,the focal depths of 5 M4~5earthquakes in the North-South Seismic Belt are accurately determined.The focal depths of three earthquakes are within 8~12km,and the focal depth of the other two earthquakes are 1~2km,which indicates quite large discrepancies with the focal depths of the USGS earthquake catalog.Therefore,at least in the North-South Seismic Belt of China,the M4~5earthquakes with deeper focal depths(e.g.,>30km)in the USGS earthquake catalog need be further studied to identify their accuracy.更多还原