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，胡安·德富卡板块在北美西北海岸之下缓慢下降，过去曾产生过大地震和相关的海啸。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（从北方加州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）的滑动量小于其他部分，并且在之前的一些EQ中，只有部分俯冲带破裂。无论这些沿利润率的变化可能会持续在未来的EQ量化地震和海啸的危险在人口稠密的太平洋西北边缘有重要的意义。下降的胡安·德富卡板块中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层褶皱和断层沉积物的结构和性质，或巨型逆冲断层岩的性质，都可能促成这些沿边缘的变化。然而，目前的观测是有限的，并允许范围广泛的可能的未来地震的情况。现代海洋地震反射成像技术提供了最好的工具，可用于照亮震源区深处及以下的俯冲带。该项目的总体目标是获得一个跨越整个卡斯卡迪亚俯冲带的现代海洋地震反射数据区域网格，以描绘该俯冲带的地质结构和性质如何沿沿着和沿边缘变化。该项目将服务于国家利益，因为所获得的数据将形成未来计划的基础数据集，重点是美国太平洋西北部的灾害评估，包括地震预警网络和深层钻探，以采样断层带岩石，它将通过提供一个新的区域框架来促进科学的进步，本项研究将利用超长偏移距多道地震（MCS）资料研究俯冲板块和增生楔构造特征，以及巨型逆冲断层的性质，解决以下具体问题：1.有没有任何系统学的结构和性质的传入胡安德富卡板块，巨型逆冲带，加积楔与推断的古破裂分割？2.是否有向下倾斜的变化，在巨型逆冲断层的几何形状和反射率的过渡断层性质的指示，以及什么是潜在的海啸浅部的巨型逆冲断层的性质？将沿沿着20个二维剖面以50-100公里的间距采集15公里偏移距的长MCS数据，这些剖面垂直于边缘，并定位在推断为古破裂斑及其边界带的区域。勘测工作还将包括沿大陆架沿着一条连续走向线，该走向线大致以重力推断的弧前盆地为中心，以调查孕震带下倾极限附近可能存在的分段。边缘法线将从变形前缘向海方向延伸约50 km，以反映传入海洋板块中的俯冲弯曲断层区域，并从变形前缘向陆方向延伸至尽可能靠近海岸线的安全位置。 所获得的数据将被设计用于表征：1）传入板块的变形和地形，2）巨型逆冲断层的深度、地形和反射率，3）沉积物特性和沉积物俯冲量，4）增生楔的结构和演变，包括断层网络的几何形状和反射率，以及这些特性如何沿沿着变化，横跨整个边缘，向下倾斜，可能是卡斯卡迪亚地震带的全部宽度。这些数据将被处理为使用最先进的地震处理技术进行叠前深度偏移，并将向社会公开，提供一个高质量的数据集，阐明整个沿着卡斯卡迪亚俯冲带的区域地下结构。该奖项反映了NSF的法定使命，并被认为值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估来支持。

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.