Relocations and focal mechanism solutions characteristics of 2012 Yiliang MS5.7 and MS5.6 earthquake sequence
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摘要: 利用地震科学探测台阵在云南、 贵州地区的17个流动台站的地震记录， 采用双差定位法对2012年9月7日云南彝良MS5.7和MS5.6地震及其余震序列(ML≥1.0)进行重定位． 在获得精确的震源位置后， 采用CAP法反演了MS≥4.0地震的震源机制解． 结果显示, 彝良MS5.7主震位于(27.509°N， 103.971°E)， 震源深度为9.7 km, 震源机制解节面Ⅰ走向251°、 倾角66°、 滑动角150°， 节面Ⅱ走向354°、 倾角63°、 滑动角27°; 彝良MS5.6主震位于(27.563°N， 104.034°E)， 震源深度为10.0 km, 震源机制解节面Ⅰ走向235°、 倾角39°、 滑动角147°， 节面Ⅱ走向352°、 倾角70°、 滑动角56°． 反演结果显示断层的几何形态、 余震分布特征、 震源机制解特征及构造应力场等均有很好的一致性． 综合断层的运动学特征、 地震活动规律和地质构造背景， 推测彝良地震的发震断裂为昭通断裂带的前缘断裂， 即NE走向的石门断裂． 导致震区受灾严重的主要原因是由于彝良地震震源深度较浅， 能量释放多发生在地壳浅部所致．
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关键词:
- 彝良MS5.7和MS5.6地震 /
- 双差定位法 /
- CAP法 /
- 震源机制解
Abstract: Based on the recordings of 17 portable seismographs of ChinArray deployed in Yunnan-Guizhou region, the Yiliang MS5.7 and MS5.6 earthquakes happened on September 7, 2012 and the aftershocks with ML≥1.0 were relocated by double-difference hypocenter location algorithm (hypoDD), and then the focal mechanism solutions of earthquakes (MS≥4.0) were determined by CAP (cut and paste) method. Our results show that the Yiliang MS5.7 mainshock is located at (27.509°N, 103.971°E) with initial rupture depth of 9.7 km. The best double-couple solutions for the Yiliang MS5.7 earthquake have one nodal plane with strike 251°, dip 66°, rake 150°, and another plane with strike 354°, dip 63° and rake 27°. The Yiliang MS5.6 mainshock is relocated at (27.563°N, 104.034°E) with initial rupture depth of 10.0 km, and the mechanism solutions of two nodal planes are 235°, 39°, 147° and 352°, 70°, 56°, respectively. The fault geometry, aftershock distribution characteristics, focal mechanism solutions and the tectonic stress field are in good consistency. Combining with the characteristic of fault kinematic, seismicity and the geological structure, we infer that Yiliang MS5.7 and MS5.6 earthquakes occurred on the NE-striking Shimen fault which is the leading edge of Zhaotong fracture. The shallow focal depth of Yiliang earthquake and the energy release occurred in shallow crust are the major reasons for serious damage in the hypocentral region. -
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图 1 彝良MS5.7和MS5.6地震震中及其邻区构造图(闻学泽等，2013)黑色框所示范围为昭通断裂带. F1： 昭通—鲁甸断裂； F2： 石门断裂； F3： 龙树断裂； F4： 会泽—彝良断裂
Figure 1. Epicenters of Yiliang MS5.7 and MS5.6 earthquakes and the surrounding regional tectonic settings(after Wen et al，2013)The black rectangle delineates the Zhaotong fault zone. F1 ： Zhaotong-Ludian fault； F2 ： Shimen fault； F3： Longshu fault； F4： Huize-Yiliang fault
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图 2 彝良MS5.7和MS5.6地震震中及台站分布
Figure 2. Epicenters of Yiliang MS5.7 and MS5.6 earthquakes and stations distribution
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图 3 彝良MS5.7和MS5.6地震序列重定位前(a)、 后(b)的震中分布
Figure 3. Epicentral distribution of Yiliang MS5.7 and MS5.6 earthquake sequence before(a) and after(b)relocation
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图 4 彝良MS5.7和MS5.6地震序列沿AA′(a)和BB′(b)剖面的震源深度分布
Figure 4. Focal depth distribution of Yiliang MS5.7 and MS5.6 earthquake sequence along the profiles of AA′(a) and BB′(b)
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图 5 彝良MS5.7地震的拟合差和震源机制解(下半球投影)随矩心深度的变化
Figure 5. Misfit plots as a function of centroid depth for Yiliang MS5.7 earthquake(lower-hemisphere projection)
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图 6 彝良MS5.6地震的拟合差和震源机制解(下半球投影)随矩心深度的变化
Figure 6. Misfit plots as a function of centroid depth for Yiliang MS5.6 earthquake(lower-hemisphere projection)
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图 7 彝良MS5.7地震的理论地震波形(红色)与观测波形(黑色)对比 波形下方第一行数字为理论波形相对于观测波形的时移(单位： s)，正值为理论波超前； 第二行数字为两波形的相关系数； 波形左侧数字为台站名及其震中距(单位： km)
Figure 7. Comparison between the theoretical(red lines) and observed(black lines)waveforms of the Yiliang MS5.7 earthquake The numbers of the first line below the waveforms are the time shift(in s)of theoretical waveforms relative to observed ones，and the positive values indicate the theoretical waveform being ahead of the observed one. The numbers of the second line indicate the correlative coefficients between them. The name of stations and corresponding epicentral distances(in km)are given at the left side of waveforms
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表 1 地壳速度和密度参考模型
Table 1 Crustal velocity and density reference model
地壳厚度/km vP/(km·s-1) vS/(km·s-1) ρ/(g·cm-3) 0—5 5.680 3.114 2.645 5—10 5.889 3.387 2.829 10—20 6.008 3.395 2.832 20—32 6.344 3.590 2.906 32—36 6.408 3.625 2.920 36—40 6.345 3.590 2.906 40—45 6.800 3.863 3.000 ＞45 7.900 4.350 3.310 
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表 2 彝良MS5.7和MS5.6地震序列中4次MS≥4.0地震的震源机制解
Table 2 Focal mechanism solutions of four earthquakes with magnitude MS≥4.0 of Yiliang MS5.7 and MS5.6 earthquake sequence
地震编号 发震时刻 东经/° 北纬/° 矩心深度/km MS MW 走向/° 倾角/° 滑动角/° 年-月-日 时:分:秒 1 2012-09-07 11:19:41.27 103.971 27.509 7 5.7 5.36 251 66 150 2 2012-09-07 12:16:29.71 104.034 27.563 9 5.6 5.25 235 39 147 3 2012-09-07 11:58:00.41 104.032 27.553 7 4.4 4.36 216 25 105 4 2012-09-07 13:12:46.19 104.041 27.561 6 4.4 4.38 190 74 22
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表 3 本文结果与不同文献给出的震源机制解结果对比
Table 3 Comparison of focal mechanism solutions by this paper with those from other studies
文献 走向/° 倾角/° 滑动角/° 矩心深度/km 韩立波和蒋长胜(2012) 248/233 59/39 149/151 10/10 吕坚等(2013a) 243/241 62/37 149/162 6/6 Global CMT(2012) 236/235 52/45 139/158 12/12 本文 251/235 66/39 150/147 7/9 注： 数据为同一节面的走向、 倾角和滑动角. “/”前、 后分别为彝良MS5.7和MS5.6地震的震源机制解结果.
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