| 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 |
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| 引用本文： | 李志伟,黄志斌,王晓欣,韩立波,陈伟文,包丰.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[J].地球物理学报,2015,58(4):1236-1250. |
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| 作者姓名： | 李志伟 黄志斌 王晓欣 韩立波 陈伟文 包丰 |
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| 作者单位： | 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 |
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| 基金项目： | 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. |
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| 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.
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| 关 键 词： | 震源深度 sPL深度震相 短周期瑞利波 CAP |
| 收稿时间： | 2014-05-12 |
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 |
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| LI Zhi-Wei,HUANG Zhi-Bin,WANG Xiao-Xin,HAN Li-Bo,CHEN Wei-Wen,BAO Feng.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[J].Chinese Journal of Geophysics,2015,58(4):1236-1250. |
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| Authors: | LI Zhi-Wei HUANG Zhi-Bin WANG Xiao-Xin HAN Li-Bo CHEN Wei-Wen BAO Feng |
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| Institution: | 1. State Key Laboratory of Geodesy and Earth's Dynamics, Institute of Geodesy and Geophysics, Chinese Academy of Sciences, Wuhan 430077, China;2. China Earthquake Networks Center, Beijing 100045, China;3. Institute of Geophysics, China Earthquake Administration, Beijing 100081, China;4. Mengcheng National Geophysical Observatory, University of Science and Technology of China, Hefei 230026, China |
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| Abstract: | 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 upper or mid-lower crust. In contrast, the focal depths of all three earthquakes are greater than 30 km (31, 33 and 46 km 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 M4.6 and M4.9 earthquake in Longchang, Sichuan, the focal depth from sPL depth phase is about 1~2 km, which is shallower than the focal depth of 4 km from the CAP method. The independent result from amplitude ratio of Rg/P is ~2 km, which is close to the results from the sPL depth phase. Since there is 2~3 km 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 M4.6 and M4.9 earthquake in Longchang, Sichuan should be 1~2 km, 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 M4~5 earthquakes in the North-South Seismic Belt are accurately determined. The focal depths of three earthquakes are within 8~12 km, and the focal depth of the other two earthquakes are 1~2 km, 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 M4~5 earthquakes with deeper focal depths (e.g.,>30 km) in the USGS earthquake catalog need be further studied to identify their accuracy. |
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| Keywords: | Focal depth sPL depth phase Short-period Rg wave CAP |
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