Collaborative Research: Illuminating the Cascadia plate boundary zone and accretionary wedge with a regional-scale ultra-long offset multi-channel seismic study

在卡斯卡迪亚俯冲区，胡安德富卡板块在北美西北海岸下方缓慢持续下降，过去曾引发大地震和相关海啸。地质记录表明，在上一次大地震(公元1700年)中，俯冲带断层或“巨型逆冲”的某些部分比其他部分滑动得少，而在以前的一些EQ中，只有部分俯冲带破裂。俯冲带断层从加利福尼亚州北部海岸向海延伸约35-90英里，一直延伸到不列颠哥伦比亚省南部。这些沿边缘的变化是否会在未来的EQ中持续下去，对于量化人口稠密的太平洋西北边缘地区的地震和海啸风险具有重要意义。下降的胡安德富卡板块的海山和其他地形特征等地质结构，俯冲带断层上方形成的厚褶皱和断裂包的结构和性质，或巨型逆冲断裂岩的性质，都可能导致这些沿边缘的变化。然而，目前的观测是有限的，并考虑到未来可能发生的各种地震情景。现代海洋地震反射成像技术为照亮俯冲带至震源区及以下深度提供了最好的工具。该项目的总体目标是获取跨越整个卡斯卡迪亚俯冲带的现代海洋地震反射数据的区域网格，以成像该俯冲带沿边缘和跨边缘的地质结构和性质如何变化。该项目将符合国家利益，因为所获得的数据将形成一个基础数据集，用于未来美国太平洋西北地区的危险性评估，包括地震预警网和深层钻探以采样断层带岩石，并将通过提供一个新的区域框架来促进科学进步，该框架与过去几十年在该地区进行的广泛的多学科陆上和海上研究有关。具体地说，这项研究将使用超长炮检距多通道地震(MCS)数据来表征俯冲板块和增生楔形结构，以及巨型断裂的性质。解决以下具体问题：1.进入的胡安德-富卡板块、巨型逆冲带和与推断的古破裂分段有关的增生楔体的结构和性质是否有任何系统性？2.巨型逆冲的几何形状和反射率是否存在指示断层性质转变的下倾变化，以及巨型逆冲的潜在海啸浅部的性质是什么？将沿着20条2-D剖面采集15公里长的MCS数据，间距50-100公里，方向垂直于边缘，并在推断为古破裂斑块及其边界带的地区提供覆盖。调查还将包括一条沿大陆架的连续走向线，其中心大致在重力推断的弧前盆地上方，以调查孕震带下倾界限附近可能出现的分段。边缘法线将向变形前沿向海延伸约50公里，以成像来袭来的大洋板块中的俯冲弯曲断层区域，并向陆地方向延伸至尽可能安全操纵的海岸线附近。所获得的数据将用于描述1)进入板块的变形和地形，2)巨型逆冲的深度、地形和反射率，3)沉积物性质和沉积物俯冲的数量，4)增长楔形的结构和演化，包括断层网络的几何形状和反射率，以及这些性质如何随着走向而变化，这些走向跨越整个边缘和可能是卡斯卡迪亚地震孕育区的整个宽度的下倾。这些数据将使用最先进的地震处理技术进行叠前深度偏移，并将公开提供给社区，提供一个高质量的数据集，照亮整个卡斯卡迪亚俯冲区的区域地下建筑。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

At the Cascadia Subduction Zone, the slow ongoing descent of the Juan de Fuca plate beneath the northwestern coast of North America has generated large earthquakes and associated tsunami in the past. Geologic records suggest that some sections of the subduction zone fault or "megathrust", which extends ~35-90 miles seaward from the coasts of northern California all the way to southern British Columbia, slipped less than other sections during the last large earthquake (1700 AD), and that in some prior EQ only parts of the subduction zone ruptured. Whether these along-margin variations may persist in future EQ has important implications for quantifying earthquake and tsunami hazards within the heavily populated Pacific Northwest margin. Geologic structure such as seamounts and other topographic features in the descending Juan de Fuca plate, the structure and properties of the thick folded and faulted package of sediments that forms above the subduction zone fault, or the properties of megathrust fault rocks, could contribute to these along-margin variations. However the current observations are limited and allow for a wide range of possible future earthquake scenarios. Modern marine seismic reflection imaging techniques provide the best tools available for illuminating a subduction zone to the depths of the earthquake source region and below. The overall goal of this project is to acquire a regional grid of modern marine seismic reflection data spanning the entire Cascadia Subduction zone to image how the geologic structure and properties of this subduction zone vary both along and across the margin. This project will serve the national interest in that the acquired data will form a foundational dataset for future initiatives focused on hazard assessment in the U.S. Pacific Northwest, including for an earthquake early warning network and deep drilling to sample the fault zone rocks, and it will promote the advancement of science by providing a new regional framework relevant for the wide range of multi-disciplinary onshore and offshore studies conducted in this region over the past several decades.Specifically, this study will use ultra-long-offset multi-channel seismic (MCS) data to characterize subducting plate and accretionary wedge structure, and properties of the megathrust, to address the following specific questions: 1. Are there any systematics in the structure and properties of the incoming Juan de Fuca plate, the megathrust zone, and accretionary wedge associated with inferred paleo-rupture segmentation? 2. Are there down-dip variations in megathrust geometry and reflectivity indicative of transitions in fault properties, and what are the properties of the potentially tsunamigenic shallow portion of the megathrust? Long 15-km-offset MCS data will be acquired along twenty 2-D profiles at 50-100 km spacing oriented perpendicular to the margin and located to provide coverage in areas inferred to be paleo-rupture patches and their boundary zones. The survey will also include one continuous strike line along the continental shelf centered roughly over gravity-inferred fore-arc basins to investigate possible segmentation near the down-dip limit of the seismogenic zone. The margin normal lines will extend approximately 50 km seaward of the deformation front to image the region of subduction bend faulting in the incoming oceanic plate, and landward of the deformation front to as close to the shoreline as can be safely maneuvered. The acquired data will be designed to characterize 1) the deformation and topography of the incoming plate, 2) the depth, topography and reflectivity of the megathrust, 3) sediment properties and amount of sediment subduction, 4) the structure and evolution of the accretionary wedge, including geometry and reflectivity of fault networks, and how these properties vary along strike, spanning the full length of the margin and down dip across what may be the full width of the seismogenic zone at Cascadia. The data will be processed to pre-stack depth migration using state-of-the art seismic processing techniques and will be made openly available to the community, providing a high-quality data set illuminating the regional subsurface architecture all along the Cascadia Subduction Zone.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.