伽师序列中小地震震源机制分析
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摘要
采用点源位错模型,层状介质速度结构,通过计算理论地震图,与观测的地震垂直向记录的直达P、S波最大振幅拟合,反演震源机制的方法。系统查阅了万余张地震记录图件,得到可信的小地震震源机制共计554个。有震源机制的地震完全覆盖了序列的震区范围,时间上也分布于序列的各个阶段,因此认为这些小地震对于地震序列具有一定的代表性。计算2个震源机制的2个P轴在三维空间中的夹角和2个T轴之间的夹角,再以2个夹角之和作为距离,采用系统聚类的方法对大量地震震源机制解进行分类。取和主体破裂方向(NE)一致的两类正断层地震,求其占全部类型的地震数目的比例,用来刻画应力场的变动情况。对照伽师序列的6级强震,两类正断层地震出现3次显著的增加。3组强震都在其后发生,这就给出了确切的前兆信息。小地震震源机制显示出的变化,表明它们对于强震孕育发生非常灵敏。文章最后提出伽师强震序列的块体旋转成因模型。
Using the dislocation model of the point source and layered velocity structure in crustal medium,through computing the theoretical seismic seismograms,we synthesized the maximum amplitude of P and S wave in vertical components with the observation earthquake in order to inverse the focal mechanism.About ten thousands of earthquake record have been studied,and 554 focal mechanism solutions of credible small earthquakes are obtained.The earthquakes with the focal mechanism solution cover completely the seismic region of the sequence.It has also distributed in all time duration in the sequences.So we think that these small earthquakes can be represented for the whole sequence.We calculate the angles between the P axis and the T axis of two focal mechanism solutions in 3 dimensions,and take the sum of two angles as the distance.By using the system cluster method,the classification of focal mechanism solutions has been carried out.We chose two types of normal fault which is consistent with the main fracture direction(NE).And compute the proportion of earthquake numbers in all types.Then,using it to reflect the change of the stress field.Comparing the type of M6 earthquakes in Jiashi sequence,two types of the normal fault earthquakes occur remarkablely before M6 earthquakes.It seems the change of the focal mechanism of the small earthquake may indicate some precursory meanings.Finally,the model of the block body revolving for the Jiashi strong earthquake sequence has been suggested.
Using the dislocation model of the point source and layered velocity structure in crustal medium,through computing the theoretical seismic seismograms,we synthesized the maximum amplitude of P and S wave in vertical components with the observation earthquake in order to inverse the focal mechanism.About ten thousands of earthquake record have been studied,and 554 focal mechanism solutions of credible small earthquakes are obtained.The earthquakes with the focal mechanism solution cover completely the seismic region of the sequence.It has also distributed in all time duration in the sequences.So we think that these small earthquakes can be represented for the whole sequence.We calculate the angles between the P axis and the T axis of two focal mechanism solutions in 3 dimensions,and take the sum of two angles as the distance.By using the system cluster method,the classification of focal mechanism solutions has been carried out.We chose two types of normal fault which is consistent with the main fracture direction(NE).And compute the proportion of earthquake numbers in all types.Then,using it to reflect the change of the stress field.Comparing the type of M6 earthquakes in Jiashi sequence,two types of the normal fault earthquakes occur remarkablely before M6 earthquakes.It seems the change of the focal mechanism of the small earthquake may indicate some precursory meanings.Finally,the model of the block body revolving for the Jiashi strong earthquake sequence has been suggested.
引文
[1]梁尚鸿,李幼铭,束沛镒,等.利用区域地震台网P、S振幅比资料测定小震震源参数.地球物理学报,1984,27(3):247-257
[2]刁桂苓,王海涛,高国英,等.伽师强震系列应力场的转向过程.地球物理学报,2005,48(5):1062-1068
[3]刁桂苓,于利民,李钦祖.震源机制解的系统聚类分析.中国地震,1992,8(3):86-92
[4]王琪,丁国瑜,乔学军,等.用GPS研究南天山(伽师)地区现今地壳变动.地震学报,2000,22(2):263-270
[5]乔学军,王琪,王晓强,等.新疆伽师地区现今构造运动和地形变GPS监测的初步研究.地壳形变与地震,2000,20(3):49-57
[6]刘志,张先康,周雪松,等.帕米尔东北侧地壳物性结构及其发震环境探讨.地震学报,2003,25(3):242-249
[7]苏乃秦.新疆伽师强震群强震主破裂面初探.西北地震学报,2003,25(3):269-274
[8]苏乃秦.从1997～1998年伽师强震群到2003年巴楚—伽师岳普湖震群震群演化特征及趋势分析.内陆地震,2004,18(1):29-38
[2]刁桂苓,王海涛,高国英,等.伽师强震系列应力场的转向过程.地球物理学报,2005,48(5):1062-1068
[3]刁桂苓,于利民,李钦祖.震源机制解的系统聚类分析.中国地震,1992,8(3):86-92
[4]王琪,丁国瑜,乔学军,等.用GPS研究南天山(伽师)地区现今地壳变动.地震学报,2000,22(2):263-270
[5]乔学军,王琪,王晓强,等.新疆伽师地区现今构造运动和地形变GPS监测的初步研究.地壳形变与地震,2000,20(3):49-57
[6]刘志,张先康,周雪松,等.帕米尔东北侧地壳物性结构及其发震环境探讨.地震学报,2003,25(3):242-249
[7]苏乃秦.新疆伽师强震群强震主破裂面初探.西北地震学报,2003,25(3):269-274
[8]苏乃秦.从1997～1998年伽师强震群到2003年巴楚—伽师岳普湖震群震群演化特征及趋势分析.内陆地震,2004,18(1):29-38
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