山东长岛震群震源机制解一致性参数时空演化特征
Spatial-temporal Characteristics of the Focal Mechanism SolutionsConsistency Parameter of Changdao Earthquake Swarm

借助山东数字地震台网记录到的长岛海域的中小地震波形资料，采用P波初动求取小震震源机制解的方法对研究区内293个ML ≥ 2.0地震进行震源机制反演，并根据震源机制解的资料对长岛地区进行应力张量反演，计算各节点震源机制解一致性参数的时空演化，进而分析长岛地区的应力状态以及震源机制一致性参数的时空演化特征。结果表明：（1）研究区地震震源以走滑型为主，正断型次之，逆断型最少，初步判断发震断层为SE走向的高角度的走滑断层，主要表现为水平运动。（2）通过反演得到研究区区域平均应力场主压应力方向为101.4°，倾角29.1°；中等应力方向为280.8°，倾角60.8°；主张应力方向为11.3°，倾角0.3°，应力张量方差为0.085。（3）震源机制一致性参数的时空分布与中等地震活动有一定的关系，具体表现为：在中强地震发生前，震源机制和应力场一致，震源区附近的构造应力场的作用增强，此时震源机制一致性参数处于较低水平。空间上，研究区震源机制一致性参数整体上呈现出中间低两边高的特点，所有ML ≥ 4.0地震都发生在极低值区或者高值区向低值区变化的过渡带上。
This paper applies the P-wave initial motion to invert the focal mechanisms of 293 earthquakes with ML ≥ 2.0 in the Changdao area, Shandong Province. According to focal mechanism solutions, the stress tensor of Changdao was retrieved to calculate the spatial-temporal evolution of the focal mechanism consistency parameter of each node. We then analyzed the stress condition in the study area and the spatial-temporal characteristics of the consistency parameters. Our results showed that (1) The type of earthquake source was strike-slip type > normal faulting type > thrust faulting type, and we judged that the seismogenic fault stretches along the SE and exhibits nearly vertical strike-slip movement, which mainly showed horizontal motion. (2) The inversion showed that the azimuth and plunge of the principal compressive stress axis were 101.4° and 29.1°, respectively; 280.8° and 60.8°, respectively, for the middle compressive stress axis; and 11.3° and 0.3°, respectively, for the tensile stress axis. The stress tensor variance was 0.085. (3) There was a certain correlation between the time-space distribution of focal mechanism consistency parameters and the seismicity of moderate-strong earthquakes. Before the occurrence of a moderate-strong earthquake, the focal mechanism consistency parameters were relatively low, and the tectonic stress field near the earthquake source zones increased. In general, the focal mechanism consistency parameters of the study area exhibited low values in central part and high values on both edges, and all ML ≥ 4.0 earthquakes occurred in areas with extremely low values and in the transitional areas from high values to low values.