2013 MW7.8 South Scotia Ridge Earthquake: Focal mechanism inversion of the multi-point sources
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摘要:
2013年11月17日，在南极南奥克尼群岛北、南极板块与斯科舍板块之间发生了一次MW7.8级地震(2013年南斯科舍海岭MW7.8地震)，我们利用全球分布的长周期和宽频带地震记录反演确定了这次地震随时间和空间变化的震源机制，验证了提出的一种多点震源机制反演的新方法.首先利用长周期记录的W震相反演了这次地震的矩心矩张量解并利用体波提取了视震源时间函数，同时利用台阵反投影技术从宽频带记录中获得了这次地震的高频源的时空分布，然后基于矩心矩张量解、视震源时间函数以及高频源的时空分布，实现了采用新方法对2013年南斯科舍海岭MW7.8地震的多点震源机制反演.矩心矩张量解表明，地震矩心在44.50°W/60.18°S，矩心深度19 km，半持续时间49 s，释放标量地震矩4.71×1020 N·m，发震断层走向104°，倾角54°，滑动角8°.视震源时间函数清楚地揭示了地震矩随时间变化的方位依赖性，总体上可以将时间过程分为前60 s和后50 s两个阶段，但前60 s可细分为两次子事件.根据台阵反投影结果，这次地震为沿海沟从西到东的单侧破裂，破裂长度达311 km，可以分为5次子事件，能量释放的峰值点依次为13 s、30 s、51 s、64 s和84 s，平均破裂速度分别为0.6 km·s-1、2.6 km·s-1、2.3 km·s-1、2.8 km·s-1和3 km·s-1.多点震源机制反演显示，5次子事件的矩震级分别为MW7.57，MW7.48，MW6.80，MW7.53和MW7.08，半持续时间依次为21 s，17 s，6 s，16 s和8 s，走向分别为95°，105°，81°，98°和98°，倾角依次为57°，49°，86°，46°和64°，滑动角-9°，1°，-17°，13°和-4°.这些在震源机制、能量释放以及持续时间方面的变化都是当地构造和应力环境复杂性的反映.
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关键词:
- 南斯科舍海岭MW7.8地震 /
- 矩心矩张量解 /
- 高频源时空分布 /
- 视震源时间函数 /
- 多点震源机制反演
Abstract:On 17 November, 2013, an MW7.8 earthquake occurred between the Scotia and Antarctic plates (2013 MW7.8 South Scotia Ridge Earthquake) in north of the South Orkney islands. We determined the spatiotemporal variation of focal mechanism solutions of the event based on long-period and broadband teleseismic waveform data worldwide,and validated a new approach to focal mechanism inversion of multi-point sources. Firstly,the centroid moment tensor solution (CMT) was determined by inverting the W-phase data of long-period seismic recordings,the apparent source time functions (ASTFs) were retrieved from the P-waveform data of long-period seismic recordings,and the spatiotemporal distribution of high-frequency sources (SDHS) was imaged by back-projecting the array-recorded broadband P-waveform recordings. Then,taking into account the CMT solution,ASTFs and the SDHS,we obtained the focal mechanism solutions of multi-point sources (FMSMS) for this event by adopting the new approach. The CMT solution shows a centroid location of 44.50°W/60.18°S,a depth of 19 km,a half duration of 49 s,a scalar seismic moment release of 4.71×1020 N·m,and fault parameters of strike 104°,dip 54° and rake 8°. The ASTFs reveals an azimuth-dependant variation of the moment-rate time functions,by which the source process was isolated into two stages,the first 60s and the rest 50s. Furthermore,the first stage includes two subevents. In contrast,the back-projection identifies a unilaterally eastward rupture along the trench north of the South Orkney microcontinent,with a length of ~311 km. The rupture process consisted of 5 subevents,with peaks of the energy release at 13 s、30 s、51 s、64 s and 84 s,which had rupture velocities of 0.6 km·s-1、2.6 km·s-1、2.3 km·s-1、2.8 km·s-1 and 3 km·s-1,respectively,on average. According to the FMSMS,the five subevents had moment magnitudes of MW7.57,MW7.48,MW6.80,MW7.53 and MW7.08,half-durations of 21 s,17 s,6 s,16 s and 8 s,strikes of 95°,105°,81°,98ånd 98°,dips of 57°,49°,86°,46ånd 64°,and rakes of -9°,1°,-17°,13ånd -4°in order. These spatiotemporal variations in focal mechanism,moment release and duration times typically indicate the complexity of local tectonic and stress situation.
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图 1
2013年南斯科舍海岭MW7.8地震的构造与地震活动背景
Figure 1.
Tectonic and seismicity settings of the 2013 MW7.8 South Scotia Ridge earthquake
图 2
本研究中使用的地震台站分布及其长周期波形记录
Figure 2.
Distribution of the seismic stations with long-period recordings used in this study
图 3
矩心矩张量反演过程与结果
Figure 3.
The inversion of centroid moment tensor and the inverted results
图 5
依赖于台站方位的视震源时间函数分析
Figure 5.
Analysis of the azimuth-dependant apparent source time functions (ASTFs)
图 7
不同台站的视震源时间函数(填充部分)以及用视震源时间函数合成的理论地震图与观测地震图的比较
Figure 7.
The apparent source time functions (ASTFs) retrieved from various stations (color-filled) and the comparison of the observed data (black) with the synthesized data (red) using the ASTFs
图 11
不同时窗内聚束能量的空间分布.五角星表示破裂起始点，红色圆圈表示瞬时能量最大点
Figure 11.
Spatial distribution of the beam energy within various time windows, where red stars refer to the initial points and the red dots denote the points of the maximal energy within the windows
图 13
基于高频源的时空特征辨识的5次子事件
Figure 13.
Identification of 5 subevents based on the spatiotemporal distribution of the high-frequency sources
图 14
子事件A、B的半持续时间与矩率时间函数的确定
Figure 14.
Determination of the half-duration times and the moment rate time functions of subevents A and B
图 16
五次子事件的震源机制与震源时间函数
Figure 16.
Focal mechanism solutions and source time functions of the five subevents
图 18
频率为2 Hz时台阵响应的比较
Figure 18.
Comparison of the array response functions at frequency of 2 Hz
表 1
2013年斯科舍海MW7.8地震震源机制解
Table 1.
Focal mechanism solutions of the 2013 MW7.8 South Scotia Ridge earthquake
Sources Scalar Moment(N·m) τc(s) Centroid information Plane Latitude/(°) Longitude/(°) Depth(km) Strike/(°) Dip/(°) Rake/(°) GCMT 5.820e+20 45.5 -60.49 -45.32 23.8 103 44 -3 USGS 5.151e+20 - -60.29 -45.55 11 99 50 -1 Ye 6.45e+20 44.4 - - 11.5 97 46 -3 GFZ 2.229e+20 - - - 16 90 66 -14 
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表 2
与GCMT矩心矩张量解对比
Table 2.
Comparison of our result with the global centroid moment tensor solution
DC Centroid Location Moment Tensor (1020N·m) Lat/(°) Lon/(°) depth/(km) Mrr Mtt Mpp Mrt Mrp Mtp WCMT (This study) 88% -60.18 -44.50 19 0.015 1.229 -1.244 1.752 2.562 3.484 GCMT 93% -60.49 -45.32 23.8 0.059 1.210 -1.270 1.560 3.950 3.860
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表 3
双力偶解
Table 3.
Double-couple solutions
Time/s Location/(°) Focal mechanism (DC) τc τh Lat Lon Scalar moment (N·m) strike/(°) dip/(°) rake/(°) WCMT(This study) 49 49 -60.18 -44.50 4.708×1020 104 54 8 Event A 34 28 -60.40 -45.93 4.218×1020 99 55 -3 Event B 81 20 -60.46 -43.26 2.519×1020 88 53 -3 Event Ⅰ 23 21 -60.39 -46.73 2.585×1020 95 57 -9 Event Ⅱ 44 17 -60.41 -45.13 1.890×1020 105 49 1 Event Ⅲ 65 6 -60.38 -44.41 1.791×1019 81 86 -17 Event Ⅳ 81 16 -60.53 -43.36 2.239×1020 98 46 13 Event Ⅴ 102 8 -60.48 -42.01 4.701×1019 98 64 -4 Ⅰ—Ⅴ total - - - - 7.363×1020 98 52 0
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