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

合作研究：北加州海岸地震拜尔利伪 S 相模型

• 批准号: 0745902
• 负责人: Douglas Dreger
• 金额: --
• 依托单位: University of California-Berkeley
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2008
• 资助国家: 美国
• 起止时间: 2008-05-01 至 2010-04-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0745902&HistoricalAwards=false
• 关键词: Collaborative; Research; Modeling; Byerly; False

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.

1937年，Bylerly在北方加州观测到了在S波之前到达的早期能量，他称之为假S波。70年后，仍然没有足够的解释这个不寻常的观察。原因之一是该地区的地震台站覆盖面相当稀疏。最近安装的USAray台站补充了现有的北方加州地震台网（NCSN）和伯克利数字地震台网（BDSN），大大改善了这种情况，并在过去几年中提供了许多新的假S相位观测结果。我们使用双差算法重新定位近海事件，然后反演地震矩张量。在位置和源参数的约束下，我们使用二维和三维速度结构来模拟宽带波形，以找到假S波的起源。我们的二维模拟显示，俯冲戈尔达板块的几何形状和速度结构在产生旅行时间在直接P和S之间的强相位中起着重要作用。在这项工作中，假S观测进一步建模，通过开发一个网络的2D结构，以解释假S的所有站导致3D结构建模。还研究了假S可能的源机制依赖性。模拟结果将提供重要的约束的几何形状和速度结构的俯冲戈尔达板块的门多西诺三重结（MTJ）的北部，以及上覆方济各地壳的结构。在MTJ以南，结果可以提供对戈尔达板块残留物的深度、形状和速度结构的限制。更详细地了解戈达板块的几何形状和大MTJ区域地壳的速度结构，将有助于了解北美西缘的构造演化。研究结果还将为未来的三维区域速度模型提供约束，该模型将为北方加州和南俄勒冈州可能发生的海上和陆上事件提供更可靠的地面运动估计。