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

在卡斯卡迪亚俯冲带，北美西北海岸下方的胡安·德·富卡板块缓慢下降，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）期间，俯冲带断层或“巨型逆冲断层”的某些部分（从加利福尼亚北部海岸一直延伸到不列颠哥伦比亚省南部，向海延伸约35-90英里）的滑动幅度小于其他部分，并且在一些先前的EQ中，只有部分俯冲带破裂。这些沿边缘的变化是否会在未来的EQ中持续存在，对于量化人口稠密的太平洋西北边缘的地震和海啸灾害具有重要意义。地质构造，如海底山和下行胡安·德·富卡板块的其他地形特征，俯冲带断层上方形成的厚褶皱和断裂沉积层的结构和性质，或大型逆冲断层岩石的性质，都可能导致这些沿边缘变化。然而，目前的观测是有限的，并且考虑到未来可能的地震情景的范围很广。现代海洋地震反射成像技术提供了最好的工具，可以照亮震源区及以下深处的俯冲带。该项目的总体目标是获取横跨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。该项目将为国家利益服务，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灾害评估，包括地震预警网络和深部钻探，以取样断层带岩石。通过为过去几十年来在该地区进行的广泛的多学科陆上和海上研究提供一个新的区域框架，它将促进科学的进步。具体而言，本研究将利用超长距多道地震（MCS）数据来表征俯冲板块和增生楔构造，以及大型逆冲构造的性质，以解决以下具体问题：进入的胡安·德·富卡板块、巨型逆冲带和与推断的古破裂分割相关的增生楔的结构和性质是否有任何系统性？2. 巨型逆冲构造的几何形状和反射率是否存在下倾变化，表明断层性质的转变？巨型逆冲构造的潜在海啸成因浅部的性质是什么？长15公里偏移的MCS数据将沿着50-100公里垂直于边缘的20个二维剖面获得，并定位在推断为古破裂斑块及其边界带的区域提供覆盖。该调查还将包括沿大陆架的一条连续走向线，该走向线大致位于重力推断的弧前盆地之上，以研究在孕震带下倾极限附近可能的分段。变形锋的边缘法线将向海方向延伸约50公里，以成像进入海洋板块的俯冲弯曲断层区域，并向陆地方向延伸，以尽可能接近海岸线。获得的数据将用于描述1)进入板块的变形和地形，2)大逆冲断层的深度、地形和反射率，3)沉积物性质和沉积物俯冲量，4)增生楔的结构和演化，包括断层网络的几何形状和反射率，以及这些性质如何沿走向变化。横跨整个边缘，向下倾斜，可能是卡斯卡迪亚发震带的整个宽度。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处理，并将公开提供给社区，提供高质量的数据集，照亮整个Cascadia俯冲带的区域地下结构。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值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.