小震震源机制类型的地震预报作用及其力学意义
|
摘要
人们在分析应力场类型时往往忽略σ_2的作用。只从σ_1与σ_3的分布情况出发讨论可能出现的震源机制类型。 实际上,地壳中存在的是三维应力状态,而不是二维应力状态。在走滑应力状态下,不仅可以在包含σ_2轴的断层面上出现走滑型地震,也可以在包含σ_3和σ_1轴的面上出现正断型和逆断型地震。而能否出现走滑型以外的地震类型取决于应力比φ值及断层面的摩擦系数。其中φ=(σ_2-σ_3)/(σ_1-θ_3)。在滑动判据保持某一定值的情况下,当应力比φ较小时,预存断层中包含σ_1轴、σ_3轴和σ_2轴的断层分别可发生正断型、逆断型和走滑型错动,当φ值较大时,则只可能发生走滑型和正断型,以至只有走滑型地震。 作者结合华北地区、北京地区及其它地区的震源机制结果,进行分析发现逆断型地震数较高的年份,均为区域地震活动增强的年份。代表区域应力场中σ_1增大(或者是σ_2减小)造成φ值变小。由此可见,区域小震震源机制解的分析有利于对区域应力状态的判断和地震活动状态预测。
The function of σ2 is usually neglected in considering local stress field. The possible types of focal mechanism are usually prediced only based on the orientations of σ1 and σ3. For example, the sign of first arrival not in accordance with strike sliping is regarded as contradictions sign in a strike slip regime.
In fact, it is a three dimensional stress field rather than a two dimensional one in the earth crust. It could be occurred not only the earthquakes of strike sliping containing σ2 direction, but also zones of normal sliping containing σ3 and reverse sliping containing σ1 in a strike sliping regime. Occurrence of nonstrike sliping earthquake depends on the stress ratio Φ and the frictional coefficient, where Φ = (σ2-σ3) / (σ1-σ3).
Along preexisting faults containing σ1,σ3 and σ2 may occur earthquakes of normal, reverse and strike sliping, respectively, when stress ratio Φis lower than a critical value Φ0in a strike sliping regime. When Φ is larger than Φ0, may occur only the earthquake of strike sliping and perhaps that of normal sliping.
Analyzed the focal mechanisms in North China, Beijing and the other regions, we have found that the period of frequent occurrence of reverse sliping earthquake is in accordance with the period of active seismicity. It may be related with the decrease of Φ by the σ1, growth (or drop of σ2). obviously, analysis of focal mechanism of microseismicity is meaningful to assess the stress state and the situation of seismicity.
The function of σ2 is usually neglected in considering local stress field. The possible types of focal mechanism are usually prediced only based on the orientations of σ1 and σ3. For example, the sign of first arrival not in accordance with strike sliping is regarded as contradictions sign in a strike slip regime.
In fact, it is a three dimensional stress field rather than a two dimensional one in the earth crust. It could be occurred not only the earthquakes of strike sliping containing σ2 direction, but also zones of normal sliping containing σ3 and reverse sliping containing σ1 in a strike sliping regime. Occurrence of nonstrike sliping earthquake depends on the stress ratio Φ and the frictional coefficient, where Φ = (σ2-σ3) / (σ1-σ3).
Along preexisting faults containing σ1,σ3 and σ2 may occur earthquakes of normal, reverse and strike sliping, respectively, when stress ratio Φis lower than a critical value Φ0in a strike sliping regime. When Φ is larger than Φ0, may occur only the earthquake of strike sliping and perhaps that of normal sliping.
Analyzed the focal mechanisms in North China, Beijing and the other regions, we have found that the period of frequent occurrence of reverse sliping earthquake is in accordance with the period of active seismicity. It may be related with the decrease of Φ by the σ1, growth (or drop of σ2). obviously, analysis of focal mechanism of microseismicity is meaningful to assess the stress state and the situation of seismicity.
引文
[1] Sadovsky M. A. et. al., The Processes proceding strong earthquakes insome region of Middle Asia, Tectonophysics, 14, 295-307, 1972．
[2] 华祥文,唐山强震前后北京、天津周围地区应力的变化过程,地震学报,2,2,1980年。
[3] 孙加林,p轴转向是前兆地震的一个可能判据,地震科学研究,1,1983年。
[4] 孙加林,中强地震前兆的综合判定及预报,地震,5,1985年。
[5] 成尔林,1976年四川省松潘-平武7． 2级地震前后主压应力轴的方向特征,地震学报,4,2,1983年。
[6] 成尔林、庞明虎,1976年盐源-宁蒗6． 7级地震前后平均应力轴取向的时间特征,地震,6,1985年。
[7] Gupta I. N., Precursory reorientation of stress axes due to vertical migration of seismic activity, JGR, 80, 2, 272-273, 1975．
[8] Brady B. T., Theory of earthquake Ⅱ, inclusion theory of crustal earthquake, Pure & Appl. Geophy. 113, 149-168, 1975．
[9] 刘力强等,雁列式断层变形与失稳过程的实验研究,地震学报,8,4,1986年。
[10] 魏柏林等,新丰江水库地震震源机制解和构造应力场,地震学报,13,4,1991年。
[11] 李钦祖,华北地壳应力场的基本特征,地球物理学报,23,4,1980年。
[12] 华祥文,大震前地震活动的异常变化,地震,3,1985年。
[13] 阚荣举等,龙陵地区大震前后平均应力轴取向的时间变化,地球物理学报,26,6,1983年。
[14] 宋惠珍等,地应力场综合研究,石油工业出版社,1990年。
[15] Ma Jin et al, Deformation of fault-block system and seismicity in Beijing area, 423-430, in ″Continental Earthquake″, Seismological Press, 1993．
[16] 中国科学院地质研究所破裂与震源力学组,新丰江水库区微震震源力学的初步研究,地质科学,3,1974年。
[17] Scotti, O., Nur, A. and Estevez, R., Distributed deformation and block rotation in three dimensions, JGR, 96, No. B7, 1991．
[2] 华祥文,唐山强震前后北京、天津周围地区应力的变化过程,地震学报,2,2,1980年。
[3] 孙加林,p轴转向是前兆地震的一个可能判据,地震科学研究,1,1983年。
[4] 孙加林,中强地震前兆的综合判定及预报,地震,5,1985年。
[5] 成尔林,1976年四川省松潘-平武7． 2级地震前后主压应力轴的方向特征,地震学报,4,2,1983年。
[6] 成尔林、庞明虎,1976年盐源-宁蒗6． 7级地震前后平均应力轴取向的时间特征,地震,6,1985年。
[7] Gupta I. N., Precursory reorientation of stress axes due to vertical migration of seismic activity, JGR, 80, 2, 272-273, 1975．
[8] Brady B. T., Theory of earthquake Ⅱ, inclusion theory of crustal earthquake, Pure & Appl. Geophy. 113, 149-168, 1975．
[9] 刘力强等,雁列式断层变形与失稳过程的实验研究,地震学报,8,4,1986年。
[10] 魏柏林等,新丰江水库地震震源机制解和构造应力场,地震学报,13,4,1991年。
[11] 李钦祖,华北地壳应力场的基本特征,地球物理学报,23,4,1980年。
[12] 华祥文,大震前地震活动的异常变化,地震,3,1985年。
[13] 阚荣举等,龙陵地区大震前后平均应力轴取向的时间变化,地球物理学报,26,6,1983年。
[14] 宋惠珍等,地应力场综合研究,石油工业出版社,1990年。
[15] Ma Jin et al, Deformation of fault-block system and seismicity in Beijing area, 423-430, in ″Continental Earthquake″, Seismological Press, 1993．
[16] 中国科学院地质研究所破裂与震源力学组,新丰江水库区微震震源力学的初步研究,地质科学,3,1974年。
[17] Scotti, O., Nur, A. and Estevez, R., Distributed deformation and block rotation in three dimensions, JGR, 96, No. B7, 1991．
版权所有：© 2023 中国地质图书馆 中国地质调查局地学文献中心