地震研究

2018年云南通海2次M_S5.0地震震源机制解及深度测定

李姣,姜金钟,王光明,付虹

(云南省地震局,云南昆明 650224)

关键词：通海地震; CAP方法; sPn深度震相; 震源机制解; 震源深度

Focal Mechanism and Focal Depth Determination of the Two Tonghai,Yunnan,M_S5.0 Earthquakes in 2018

LI Jiao,JIANG Jinzhong,WANG Guangming,FU Hong

(Yunnan Earthquake Agency,Kunming 650224,Yunnan,China)

Keywords：Tonghai earthquake; CAP method; sPn depth-phase; focal mechanism; focal depth

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准确的震源机制解和震源深度是深入理解地震发震机理的基础。针对2018年8月13、14日在云南通海发生的2次MS5.0地震,利用云南地震台网30多个宽频带地震仪的波形记录,采用3个不同的区域速度模型和CAP方法获得了2次地震的震源机制解及矩心深度,并选取方位角约为300°的不同震中距的多个台站记录到的sPn深度震相准确测定了2次地震的初始破裂深度。结果表明:①基于数量更多、方位角覆盖较好的近台宽频带地震波形,在不同频率域内利用3个速度模型反演得到了2次地震的矩心深度分别为8 km和7 km; ②利用震中距大于260 km、方位角约300°的多个地震台记录波形的sPn和Pn震相的到时差,测定2次地震的初始破裂深度约为10 km,2种深度的差异可能是反映了主震沿断层面自下向上扩展的破裂过程; ③结合双差重定位及CAP反演结果,初步认为2次MS5.0地震的震源机制解应该是一致的,2018年2次通海MS5.0地震的发震断层应该为同一条左旋走滑型断层,其断层面走向/倾角/滑动角约为20°/75°/-10°。

: Accurate earthquake parameters(e.g.focal mechanism and depth)are crucial for understanding the seismogenic mechanism.Based on waveforms of the 2018 Tonghai,Yunnan,MS5.0 earthquakes recorded by more than 30 broadband seismic stations of the Yunnan Earthquake Networks,we invert their focal mechanisms and depths by using Cut-and-Paste(CAP)method and three different regional velocity models.We also employ sPn depth-phases recorded by different stations situated at the azimuth of ～300° to estimate the focal depth of the Tonghai MS5.0 double earthquakes.The results are as follows:① Based on more broadband waveforms of near-earthquakes with better azimuth coverage,the obtained centroid depths(8 km and 7 km)of the two earthquakes inverted from different velocity models and frequency bands are more reliable.② By using sPn depth-phases at different stations with similar azimuth(～300°),we determine that the initial rupture depth of the two earthquakes are both about 10 km,which may indicate the rupture directivity(from bottom to top)of the earthquakes.③ According to the Double-difference relocation and our CAP inversions,we preliminarily infer that the Tonghai double earthquakes should occur on the same fault plane,whose parameters,strike/dip/rake,could be 20°/75°/-10°.

引言
1 数据选取和研究方法
2 利用CAP方法反演震源机制解及矩心深度
3 利用sPn深度震相测定初始破裂深度
4 讨论
5 结论

图1 2018年2次通海地震及周边断层、]固定地震台站分布<br/>Fig.1 Distribution of the 2018 two Tonghai earthquakes,and faults and stationary seismic stations in the vicinity of Tonghai area

图1 2018年2次通海地震及周边断层、]固定地震台站分布
Fig.1 Distribution of the 2018 two Tonghai earthquakes,and faults and stationary seismic stations in the vicinity of Tonghai area

表1 图1中的台站代码及其对应的台站名称<br/>Tab.1 Station code and corresponding station name marked in Fig.1

表1 图1中的台站代码及其对应的台站名称
Tab.1 Station code and corresponding station name marked in Fig.1

图2 通海地震序列重定位后的震中(a)及震源深度分布(b)(重定位结果来自王光明等,2018; 区域断层信息来自安晓文等,2018)<br/>Fig.2 Distribution of relocated epicenters(a)and focal depths(b)of the 2018 Tonghai MS5.0 earthquake sequence by Double-difference method(the relocation results are from Wang et al,2018,and the regional faults are from An et al,2018)

图2 通海地震序列重定位后的震中(a)及震源深度分布(b)(重定位结果来自王光明等,2018; 区域断层信息来自安晓文等,2018)
Fig.2 Distribution of relocated epicenters(a)and focal depths(b)of the 2018 Tonghai MS5.0 earthquake sequence by Double-difference method(the relocation results are from Wang et al,2018,and the regional faults are from An et al,2018)

图3 利用3个区域速度模型反演通海第一(a)、二(b)次MS5.0地震的震源深度拟合误差对比<br/>Fig.3 Focal depths fitting errors of the first(a)and the second(b)Tonghai MS5.0 earthquake inverted through three regional velocity models

图3 利用3个区域速度模型反演通海第一(a)、二(b)次MS5.0地震的震源深度拟合误差对比
Fig.3 Focal depths fitting errors of the first(a)and the second(b)Tonghai MS5.0 earthquake inverted through three regional velocity models

图4 采用2号速度模型在较高滤波频段内反演得到第一次MS5.0地震、震源深度8 km(a)和第二次MS5.0地震、震源深度7 km(b)的部分理论(红线)及观测波形(黑线)的对比<br/>Fig.4 Comparison between observed(black line)and synthetic(red line)seismograms of the first Tonghai MS5.0 event at 8 km depth(a),and the second Tonghai MS5.0 event at 7 km depth(b)by using No.2 velocity model and higher frequency band filter

图4 采用2号速度模型在较高滤波频段内反演得到第一次MS5.0地震、震源深度8 km(a)和第二次MS5.0地震、震源深度7 km(b)的部分理论(红线)及观测波形(黑线)的对比
Fig.4 Comparison between observed(black line)and synthetic(red line)seismograms of the first Tonghai MS5.0 event at 8 km depth(a),and the second Tonghai MS5.0 event at 7 km depth(b)by using No.2 velocity model and higher frequency band filter

图5 利用较高(a)和较低(b)滤波频段波形记录和2号速度模型反演得到震源深度为8 km时第一次通海MS5.0地震的部分理论(红线)及观测波形(黑线)对比<br/>Fig.5 Comparison between observed(black line)and synthetic(red line)seismograms of the first Tonghai MS5.0 event at 8 km depth by using No.2 velocity model,higher(a)and lower(b)frequency band filters

图5 利用较高(a)和较低(b)滤波频段波形记录和2号速度模型反演得到震源深度为8 km时第一次通海MS5.0地震的部分理论(红线)及观测波形(黑线)对比
Fig.5 Comparison between observed(black line)and synthetic(red line)seismograms of the first Tonghai MS5.0 event at 8 km depth by using No.2 velocity model,higher(a)and lower(b)frequency band filters

表2 利用两种滤波频段反演2次通海地震的震源机制解<br/>Tab.2 Focal mechanisms of the two Tonghai MS5.0 earthquakes inverted by using two different frequency band filters

表2 利用两种滤波频段反演2次通海地震的震源机制解
Tab.2 Focal mechanisms of the two Tonghai MS5.0 earthquakes inverted by using two different frequency band filters

图6 利用2号速度模型计算得到的不同震源深度和震中距的sPn及Pn震相理论走时(a)及理论走时差(tsPn-Pn)变化(b)<br/>Fig.6 Theoretical travel times of sPn and Pn phases(a)and differential travel times of sPn and Pn phases tsPn-Pn(b)calculated by using No.2 velocity model

图6 利用2号速度模型计算得到的不同震源深度和震中距的sPn及Pn震相理论走时(a)及理论走时差(tsPn-Pn)变化(b)
Fig.6 Theoretical travel times of sPn and Pn phases(a)and differential travel times of sPn and Pn phases tsPn-Pn(b)calculated by using No.2 velocity model

表3 利用sPn深度震相测定震源深度的台站震中距和方位角<br/>Tab.3 Epicentral distance and azimuth of seismic stations used in focal depth determination

表3 利用sPn深度震相测定震源深度的台站震中距和方位角
Tab.3 Epicentral distance and azimuth of seismic stations used in focal depth determination

图7 不同震中距、方位角约为300°的地震台记录到通海第一(a)、二(b)次MS5.0地震的Pn及sPn震相<br/>Fig.7 Pn and sPn phases recorded by different stations situated at azimuth of ～300° of the first(a)and the second(b)Tonghai MS5.0 earthquake

图7 不同震中距、方位角约为300°的地震台记录到通海第一(a)、二(b)次MS5.0地震的Pn及sPn震相
Fig.7 Pn and sPn phases recorded by different stations situated at azimuth of ～300° of the first(a)and the second(b)Tonghai MS5.0 earthquake

表4 不同机构(研究)给出的2018年2次通海MS5.0地震的震源初始破裂深度<br/>Tab.4 Focal depths of the 2018 two Tonghai MS5.0 earthquakes provided by different institutes/studies

表4 不同机构(研究)给出的2018年2次通海MS5.0地震的震源初始破裂深度
Tab.4 Focal depths of the 2018 two Tonghai MS5.0 earthquakes provided by different institutes/studies

图8 不同方位角的台站记录到2次通海MS5.0地震在不同频率范围内的波形对比图及其互相关系数<br/>Fig.8 Comparison between waveforms of two Tonghai MS5.0 earthquakes recorded by different stations and their cross-correlate coefficient(CC)

图8 不同方位角的台站记录到2次通海MS5.0地震在不同频率范围内的波形对比图及其互相关系数
Fig.8 Comparison between waveforms of two Tonghai MS5.0 earthquakes recorded by different stations and their cross-correlate coefficient(CC)