小震震源机制类型的地震预报作用及其力学意义

APPLICATION OF FOCAL MECHANISM TO EARTHQUAKE PREDICTION AND ITS SIGNIFICANCE

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.