USGS地震目录中4～5级震源深度异常地震可靠性初步研究:以南北地震带若干地震为例  

A study on the reliability of M4～5 earthquakes with anomalous focal depth in the USGS earthquake catalog:several earthquakes in the North-South Seismic Belt

作　　者：李志伟[1] 黄志斌[2] 王晓欣[2] 韩立波[3] 陈伟文[4] 包丰[1] 

机构地区：[1]中国科学院测量与地球物理研究所大地测量与地球动力学国家重点实验室,武汉430077 [2]中国地震台网中心,北京100045 [3]中国地震局地球物理研究所,北京100081 [4]中国科学技术大学蒙城地球物理国家野外科学观测研究站,合肥230026

出　　处：《地球物理学报》2015年第4期1236-1250,共15页Chinese Journal of Geophysics

基　　金：国家重点基础研究发展计划项目(2014CB845903);中国科学院先导专项(XDB06030203);地震行业科研专项(201308013);国家自然科学基金(41304045;41210005)资助

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.The USGS earthquake catalog indicates that a few earthquakes with 〉30 km 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 strength and tectonic implications. The focal depths of five M4~5 earthquakes 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 M4~5 earthquakes 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., 〈10 km） and the stations is beyond ~5 times 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 M4.7 earthquake in Huining, Ningxia （10~11 km）, the M4.8 earthquake in Fumin, Yunnan （11~12 km） and the M4.7 earthquake in Huidong, Sichuan （8~9 km）, are all within 8~12 km. The differences in focal depth from the sPL depth phase and the CAP method are about 2~3 km, which should be accurate enough to confirm that the earthquakes occurred in the

关 键 词：震源深度 sPL深度震相 短周期瑞利波 CAP 

分 类 号：P315[天文地球—地震学]