李春燕, 徐 辉
(甘肃省地震局,甘肃 兰州 730000)
地震监测
2014年阿拉善块体西缘两次中强地震震源机制解①
李春燕, 徐辉
(甘肃省地震局,甘肃 兰州 730000)
摘要:2014年2月12日新疆于田发生了MS7.3地震,之后在阿拉善块体西缘相继发生了蒙古4.9级、新疆哈密5.1级地震。利用甘肃“十五”数字地震台网的波形资料,采用CAP方法反演这两次地震的震源机制解。结果显示蒙古4.9级地震为走滑型,而新疆哈密5.1级属于逆冲型,表明青藏块体与阿拉善块体之间的作用增强引发了这两次地震活动。
关键词:阿拉善块体; CAP方法; 震源机制解
0引言
阿尔金断裂带是印度板块向欧亚板块俯冲造成青藏高原隆起并向东滑移而形成的,是一条大型的板内走滑活动断裂带,具有挤压、大幅度水平位移的左旋走滑特征,是青藏高原岩石圈物质向东蠕散的一条重要边界,丁国瑜[1]提出,阿尔金断裂带的左旋走滑运动是自西向东阶段式发展的,阿尔金断裂带东段的走滑时代要晚于西段。王萍等[2]的研究表明,在印度板块与欧亚板块的碰撞挤压作用下,青藏块体不断隆升并向四周扩展,青藏高原东北缘浅层物质向阿拉善块体挤入并形成传递应力的通道,青藏块体成为阿拉善块体活动的主要施力源。2014年新疆于田MS7.3地震震中位于巴颜喀拉块体与塔里木块体之间的阿尔金断裂带西南段上,Global CMT给出的震源机制解参数显示该地震为左旋走滑型地震。受于田地震的影响,阿拉善块体西缘的地震活动增强,2月28日在阿拉善块体西缘发生了蒙古4.9级地震(43.0° N,96.6° E,CEIC)和4月30日新疆哈密5.1级地震(43.1° N,94.3° E, CEIC)。
利用“十五”数字地震台网的资料,采用CAP(Cut and Paste)方法[3-7]反演了这两次中强地震的震源机制解。CAP方法将近震宽频带波形数据分割为Pnl和面波部分进行反演,在给定参数空间中进行网格搜索,得出二者全局差异最小的震源机制解和震源矩心深度。其优势是反演结果对速度模型和地壳横向变化的依赖性相对较小,可以得到可靠的震源机制解与准确的震源矩心深度[5-9]。
反演中使用CRUST2.0模型确定每个地震事件震源周围的地壳速度模型,结果如表1所列。蒙古4.9级地震[图1(a)、2(a)]最佳双力偶解节面Ⅰ走向273°,倾角61°,滑动角8°;节面Ⅱ走向179°,倾角83°,滑动角151°;震源矩心深度为11.5 km,地震类型属于走滑兼逆冲型。新疆哈密5.1级地震[图1(b)、2(b)]最佳双力偶解节面Ⅰ走向304°,倾角60°,滑动角77°;节面Ⅱ走向148°,倾角32°,滑动角111°;震源矩心深度为8 km,是以逆冲为主兼少量走滑分量的地震破裂事件。
表 1 两次地震震源机制解
图1 反演方差和震源机制解随不同深度的变化图Fig.1 Variation of the error plots and focal mechanism solutions with the focal depth
图2 理论地震图与观测地震图(黑线为观测地震图,灰线为理论地震图)Fig.2 Comparison between the synthetic and the observed seismograms (Black lines indicate the observed seismograms,and ash lines indicate thetheoretical ones).
致谢:本文使用了中国地震局甘肃区域地震台网提供的波形资料,在此表示感谢!
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Focal Mechanism Solutions of Two Moderate-strong
Earthquakes of 2014 in the West Alax Block
LI Chun-yan, XU Hui
(EarthquakeAdministrationofGansuProvince,Lanzhou730000,Gansu,China)
Abstract:After the Yutian, Xinjiang earthquake ofMS7.3 that occurred on February 12, 2014, the Mongolia earthquake ofMS4.9 and Hami, Xinjiang earthquake ofMS5.1 occurred on the western margin of the Alax block. This shows that seismic activity is strengthening on the western margin of the Alax block. The cut-and-paste (CAP) method performs inversion of the entire profile of an earthquake, the result of which has advantages of relatively small merit that rely on the modeling of speed and lateral movement of the earth’s crust. Its basic principle is splitting seismic broadband waveform data into Pnl wave and surface-wave parts, calculating the objective error function of the observed and theoretical seismograms, performing a grid search in the assigned parameter space, and searching for the focal mechanism and focal (centroid) depth using the two global minimum variances. In this study, using wave data from the Digital Seismological Network of Gansu Province and adopting the CAP method, we determined the focal mechanisms of the two earthquakes. The result shows that the focal mechanisms of the MongoliaMS4.9 and Hami, XinjiangMS5.1 earthquakes were strike-slip and thrust, respectively.Our analysis concludes that the occurrence of the MongoliaMS4.9 and Hami, XinjiangMS5.1 earthquakes exhibits a correlation with the interaction of the Qinghai-Tibet block and the Alax block.
Key words:Alxa block; CAP method; focal mechanism solution
DOI:10.3969/j.issn.1000-0844.2015.04.1129
中图分类号:P315.332
文献标志码:A
文章编号:1000-0844(2015)04-1129-03
通信作者:徐辉,女,工程师,主要从事地震活动性分析。
作者简介:李春燕,女,工程师,主要从事地震分析预测工作。
基金项目:国家自然科学基金项目(41304048);甘肃省科技计划(145RJZA186)
收稿日期:①2014-08-01