Estimating stress drop of the 2013 Lushan earthquake sequence based on the empirical Green’s function spectral ratio method
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摘要: 本文以芦山地震余震为例，分析了基于强震动观测记录的经验格林函数（EGF）谱比法估计地震的拐角频率和应力降的可行性。首先给出能够可靠估计地震拐角频率和应力降的经验格林函数谱比曲线的质量标准；然后利用其估算出17次ML3.8—5.4芦山强余震的拐角频率；最后，参考其它研究给出的地震矩震级，计算出地震应力降。结果显示，芦山强余震的拐角频率主要介于1.0—2.0 Hz之间，应力降平均值为9.98 MPa，且地震应力降表现出明显的震级相关性。Abstract: We adopted the 2013 Lushan aftershocks as a typical example, to investigate the feasibility of estimating corner frequency and stress drop by the empirical Green’s function （EGF） spectral ratio method based on the strong motion observation data. Firstly, we suggested the quality standard on the EGF spectral ratio curve to guarantee the reliable estimation of seismic corner frequency and stress drop. The corner frequencies for the 17 Lushan aftershocks with magnitude in the range of 3.8−5.4 were then estimated. Referring to the seismic moment magnitude given by other studies, we further computed the seismic stress drops.The results show that the corner frequency of Lushan strong aftershocks is mainly in the range of 1.0−2.0 Hz, the average stress drop is 9.98 MPa, and seismic stress drop presents obvious dependency on seismic magnitude.
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Keywords:
- Empirical Green’s function /
- spectral ratio /
- Lushan earthquake /
- stress drop
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图 1 目标地震及其经验格林函数地震分布
图中数字编号为筛选后的目标地震编号（按发震时间排序）
Figure 1. Distribution of target earthquakes and its EGF earthquakes
The numbers in the figure mean the selected target earthquakes’ number （sorted by occuring time）
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图 2 EGF谱比法计算目标地震拐角频率算例
（a） 目标地震及其EGF地震和触发台站位置分布；（b） 水平向傅氏谱；（c） EGF谱比曲线拟合；（d） M01/M0j，ƒcj随ƒc1可能值的变化；（e） Var随ƒc1可能值（颜色与图（c）对应）的变化；（f） 基于多个EGF谱比曲线的目标地震拐角频率估计值
Figure 2. An example of estimating target earthquake 's corner frequency by spectral ratio method
（a） Target earthquakes ，EGF earthquakes and trigger stations location distribution ； （b） Horizontal fourier spectrum ；（c） EGF spectral ratio curve fitting；（d） M01/M0j，fcj vary with the possible value of fc1；（e） Var varies with the possible value of fc1 ，color corresponding to figure （c）；（f） Estimated value of target earthquake’s corner frequency based on multiple EGF spectral ratio curves
表 1 目标地震的震源参数
Table 1 Source parameters of target earthquakes
地震序号 发震时间 ML 震中位置 EGF地震个数 记录数 ƒc1/Hz MW Δσ/MPa 年-月-日 时:分:秒 北纬/° 东经/° 1 2013-04-20 08:06:38 4.8 30.18 102.88 16 22 1.56 2 2013-04-20 08:31:34 4.1 30.39 103.01 2 3 1.91 5 2013-04-20 09:02:57 4.6 30.28 102.93 9 9 1.28 4.67* 4.41 6 2013-04-20 09:11:51 4.3 30.25 102.83 9 14 1.67 4.76* 13.35 7 2013-04-20 09:20:09 4.6 30.17 102.99 3 3 2.48 4.59* 24.31 8 2013-04-20 09:26:00 4.4 30.19 102.95 3 4 1.66 4.21† 1.96 9 2013-04-20 09:37:28 4.9 30.28 102.99 18 20 1.87 4.78* 20.09 10 2013-04-20 09:39:34 4.5 30.13 102.84 5 5 1.44 4.54* 4.00 14 2013-04-20 11:34:17 5.3 30.24 102.94 23 114 1.16 5.15* 17.21 16 2013-04-21 11:59:37 4.9 30.26 103.00 7 8 1.34 4.85* 9.41 17 2013-04-21 12:39:18 3.8 30.22 102.89 2 2 1.64 3.95* 0.77 18 2013-04-21 17:05:22 5.4 30.34 103.00 15 91 1.14 5.09* 13.28 19 2013-04-21 17:30:24 4.2 30.28 102.98 14 15 2.21 4.27* 5.70 21 2013-04-21 22:16:54 4.3 30.33 102.93 20 28 1.91 4.32* 4.37 22 2013-04-23 05:54:49 4.5 30.35 103.00 6 6 1.67 4.32* 2.92 23 2013-04-23 22:07:15 3.8 30.29 102.96 4 4 5.14 3.76* 12.31 24 2013-05-01 02:14:13 4.1 30.25 102.89 9 9 3.2 4.24* 15.59 注：标*数据来自易桂喜等（2016），标†数据来自温瑞智等（2015）。 
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表 2 部分EGF地震的拐角频率
Table 2 Corner frequency of partial EGF earthquakes
发震时间 ML 震中位置 记录数 ƒcj/Hz 年-月-日 时:分:秒 北纬/° 东经/° 2013-04-20 22:10:32 3.1 30.28 102.39 4 6.78 2013-04-29 13:45:40 3.0 30.26 102.88 3 2.79 2013-04-25 01:20:15 3.2 30.29 102.88 5 3.58 2013-04-25 14:11:22 3.0 30.28 102.92 12 6.51 2013-04-22 19:04:52 3.1 30.21 102.96 3 2.42 2013-04-24 21:36:35 3.1 30.26 102.90 12 3.91
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