转换断层地震触发机制对比研究:以东太平洋脊和圣安德列斯断层为例

The lithologic components and geological structure diversity between oceanic and continental transform fault lead to differences factors in earthquake triggering, such as friction properties, faults and earthquakes distributions, wave propagation characteristics. Comparative study of their seismicity, static and dynamic stress changes help to analysis and understand the basic mechanism of earthquake stress triggering. Since the fault distribution is concise, the geological structure is clear and earthquakes are plenty, oceanic transform fault region is an ideal area for the study of earthquake triggering. Currently, there are still many difficulties need to be resolved in the mechanism of earthquake triggering, particularly dynamic stress triggering. Using Coulomb stress changes and rate and state friction law, this project will contrast the transform faults in East Pacific Ridges and the San Andreas Fault in: 1. Temporal and spatial variations in seismicity; 2. The differences in static Coulomb stress changes and earthquakes triggering; 3. The difference in distribution and propagation of dynamic Coulomb stress changes, and earthquake triggering mechanism due to dynamic stress changes. We have already found some difference in aftershock and static stress distributions between oceanic and continental transform fault, and the tectonic significance. Further studies will focus on: The quantitative analysis of observations and static/dynamic Coulomb stress changes modeling; The comparison between oceanic/continental transform fault on the spreading and propagation of Coulomb stress changes; And difference in aftershocks triggering and fault interaction. This study will help us to understand the stress triggering mechanism of earthquakes, explore the geologic structures and geodynamic mechanisms of the plate boundary around transform fault, provide research basis for earthquake prediction and disaster reduction.

洋-陆转换断层在岩性组分与地质构造上的差异，导致其断层形态、摩擦性质与地震特征的不同。对比研究洋-陆地震及应力变化分布，对揭示地震触发的基本机制具有重要意义。海洋转换断层分支少、地质构造特征鲜明、地震频发，是地震触发研究的理想区域，而目前国际上对海洋地震尤其动态应力触发机理的研究刚起步，为前沿课题。本项目将对比研究东太平洋脊的海洋转换断层和圣安德列斯断层，聚焦如下关键研究任务：1）定量分析洋-陆转换断层高精度定位地震的时空分布特征；2）建模分析洋-陆静态应力变化及地震触发的异同；3）开拓模拟海洋地震动态应力变化的创新研究，对比洋-陆动态应力变化传播的异同及对地震触发的影响。我们的前期研究已发现洋-陆转换断层在余震与应力变化特征上的明显差异，本研究将系统进行定量分析与建模计算，研究结果将加深我们对地震触发基本机制的认识，揭示洋-陆转换断层板块边界的地球动力学特征，为地震减灾提供科学依据。

海洋与大陆转换断层在岩性组分与地质构造上有着重要差异，因此导致其断层形态、摩擦特征与地震时空分布特征有所不同。对比研究其地震时空分布特性以及静态、动态应力变化特征，对揭示地震应力触发的基本机制以及其如何受控制于各种构造因素有着重要意义。本研究分析了海洋和大陆转换断层的地震时空分布规律，研究结果显示东太平洋脊（EPR）海洋转换断层的b值（0.66–0.89）总体上小于SAF走滑断层（0.81–1.11）。同时应用时-空"传染型余震序列"(ETAS)模型，我们发现EPR转换断层上的余震占比（22–52%）除了Siqueiros和Wilkes分别为60%和68%以外，总体上低于SAF大陆走滑断层余震占比（56–75%）。通过拟合余震持续时间，我们发现除了Wilkes转换断层上的序列3其持续时间为201天以外，EPR的余震持续时间范围为2-55天。而SAF上余震持续时间分布范围较广为20天至14年。本研究计算了静态及动态应力变化，除Wilkes和Siqueiros上的三个主余震序列以外，EPR上65%的余震分布在库伦应力增加大于0.1bar的范围内。而三个特殊的主余震序列发生在Wilkes的超微板块区域和Siqueiros的转换断层内扩张中心，且其余震个数远远大于走滑型余震序列。通过分析转换断层地震的库伦应力变化所反应的断层地震触发机制差异，发现海洋转换断层除走滑断层触发的走滑型余震以外，由于存在转换断层内扩张中心，余震触发类型同时存在走滑断层触发正断层型地震，该类型以Siqueiros和Wilkes两个转换断层为典型，且余震数量比走滑型主余震序列的余震个数多。同时通过b值的比较和地质条件分析，我们认为，海洋转换断层地震b值较低，余震持续时间较短，地震活动背景场丰富而触发的余震较少的原因为断层面上蛇纹石化的结果。蛇纹石的存在改变了断层面的摩擦性质，阻碍了部分余震的孕育和触发，研究结果可以为地震灾害的触发和预测问题提供重要的依据。