Collaborative Research: Modeling of the Byerly's False S Phase for the Earthquakes off the Coast of Northern California

1937年，拜尔利对北加州近海地震的S波之前到达的早期或前兆能量进行了观测，他称之为假S。70年后，仍然没有对这种不寻常的观测做出充分的解释。其中一个原因是该地区的地震台覆盖范围相当稀疏。最近安装的美国国家地震台网和伯克利数字地震台网对现有的北加州地震台网和伯克利数字地震台网进行了补充，极大地改善了这种状况，并在过去几年里提供了许多关于假S震相的新观测结果。我们使用双差算法重新定位近海事件，然后对地震矩张量进行反演。在位置和震源参数约束的情况下，利用二维和三维速度结构对宽带波形进行模拟，以找出假S波的来源。我们的二维模拟表明，俯冲的哥达板块的几何和速度结构在产生走时在直接P和S之间的强相中起着重要作用。在这项工作中，通过发展一个二维结构网络来解释所有台站上的假S，进一步模拟了假S观测，从而实现了三维结构模拟。还研究了假S可能的震源机制依赖性。模拟结果将对Mendocino Triple Junction(MTJ)以北俯冲Gorda板块的几何和速度结构以及上覆Franciscan地壳的结构提供重要约束。在MTJ以南，结果可以提供对戈尔达板块残留物的深度、形状和速度结构的约束。更详细地了解戈尔达板块的几何形状和大MTJ地区地壳的速度结构，将有助于理解北美西部边缘的构造演化。这一结果还将为未来的3D区域速度模型提供约束，这些模型将为加利福尼亚州北部和俄勒冈州南部可能发生的离岸和陆上事件提供更可靠的地面运动估计。

Bylerly in 1937 made the observation of early or precursory energy arriving before the S-wave for offshore earthquakes in Northern California, which he referred to as False S. 70 years later there is still no adequate explanation of this unusual observation. One of the reasons is that the seismic station coverage in the region was rather sparse. The recently installed USArray stations that complemented the existing Northern California Seismic Network (NCSN) and Berkeley Digital Seismic Network (BDSN) have greatly improved the situation and have provided numerous new observations of the False S phase in just the past few years. We are relocating the offshore events using a double-difference algorithm, and then inverting for the seismic moment tensor. With the location and source parameters constrained we then model the broadband waveforms using 2D and 3D velocity structures in order to find the origin of the False S arrivals. Our 2D modeling is showing that the geometry and velocity structure of the subducting Gorda plate plays an important role in the generation of strong phases with travel times between direct P and S. In this work the False S observations are further modeled by developing a network of 2D structures to explain False S on all the stations leading to 3D structure modeling. Possible source mechanism dependence of False S is also studied. The modeling results will provide important constraints on the geometry and velocity structure of the subducting Gorda plate to the north of the Mendocino Triple Junction (MTJ) as well as on the structure of the overlying Franciscan crust. To the south of the MTJ, the results could provide constraints on the depth, shape, and velocity structure of Gorda plate remnants. A more detailed knowledge of the geometry of the Gorda plate and velocity structure of the crust in the greater MTJ region will contribute to the understanding of the tectonic evolution of the western edge of the North America. The results will also provide constraints for future 3D regional velocity models that will provide more reliable ground motion estimates for possible offshore and onshore events in northern California and southern Oregon.