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由地震纵波初动符号的资料,求得了海城地震系列中Ms≥4.0的24个地震的断层面解。主震发生于1975年2月4日,它的一个节面走向N70°W,倾向NE,倾角81°;另一个节面走向N23°E,倾向SE,倾角75°。根据余震的空间分布以及地面形变资料选取N70°W的节面为断层面,主震是发生在这个近乎直立的断层面上的左旋走向滑动,略具正的倾向滑动分量。前震及大多数余震的震源机制和主震的相似,有四个Ms≥4.0的余震的震源机制和主震的迥然不同,表现出滑动向量和主震的滑动向量相反的断层错动方式。这种情况的一种可能的解释是主震时在断层的一些地段发生错动过头。 由野外资料及余震的空间分布资料计算了主震的震源参数。主震断层长70公里,宽20公里,平均错距45厘米,地震矩2.1×10~(26)达因·厘米,应力降4.8巴,应变降7.3×10~(-6)。它是发生在不能积累起较高应力的薄弱地带的一次低应力降的地震。 由地震纵波初动的半周期和振幅的资料计算了81个前震和余震的震源尺度、地震矩、应力降和平均错距。结果表明前震和余震的应力降都比较低,一般在0.1—1巴之间。余震区中有两个应力降相对说来比较高(高于0.8巴)的地区,它们恰好对应于主破裂错动过头的部位。这些结果意味着震前高应力、错动过头、相对高应力降和震源机制反向四者之间?
From the data of the initial sign of longitudinal compressional wave, the fault surface solutions of 24 earthquakes with Ms≥4.0 in Haicheng earthquake series have been obtained. The main shock occurred on February 4, 1975 with one nodal plane heading for N70 ° W with a dip in NE of 81 ° and the other in N23 ° E with a dip in SE of 75 °. According to the spatial distribution of aftershocks and the ground deformation data, the nodal plane of N70 ° W is selected as the fault plane. The main shock is the leftward strike-slip and slightly positive dip-slip component occurring on this nearly upright fault plane. The focal mechanism of the front earthquakes and most of the aftershocks is similar to that of the main shocks. The focal mechanism of the aftershocks with Ms ≥ 4.0 is quite different from that of the main shocks, showing the fault-slip mode opposite to the slip vector of the main shock. A possible explanation for this is that the mainshock misdirection over some sections of the fault. The source parameters of the main shock are calculated from the field data and the spatial distribution data of aftershocks. The main fault has a length of 70 km and a width of 20 km. The average fault distance is 45 cm and the seismic moment is 2.1 × 10 ~ (26) dyne cm. The stress drop is 4.8 bar and the strain is decreased by 7.3 × 10 -6. It is a low stress drop earthquake that occurs in weak areas where it can not accumulate higher stresses. The source scale, seismic moment, stress drop and average offset of 81 foreshocks and aftershocks are calculated from the data of half-period and amplitude of the initial seismic wave. The results show that the stress drop of foreshocks and aftershocks are relatively low, generally between 0.1 and 1 bar. In the aftershock zone, there are two areas where the stress drop is relatively high (above 0.8 bar), which corresponds exactly to the area where the main rupture moves too far. These results mean that before the earthquake high stress, over-staggering, relatively high stress drop and focal mechanism inversion between the four?