Collaborative Research: Inferences on Cascadia Deformation Front and Plate Interface Properties from Advanced Studies of Active Source Seismic Data

卡斯卡迪亚俯冲带从加利福尼亚州北部延伸到不列颠哥伦比亚省，从北美西部近海延伸到不列颠哥伦比亚省，过去曾发生过可与2011年东北9级地震相媲美的大地震。卡斯卡迪亚上一次大地震发生在1700年，引发了日本有记录的跨洋海啸。据估计，它的复发间隔为300-500年，对太平洋西北部人口稠密的沿海地区来说，是一种重大的地震和海啸危险。进一步了解这一俯冲带的特征具有重大的社会意义。目前，卡斯卡迪亚的地震异常平静，几乎没有在大多数俯冲带检测到的频繁发生的小地震，人们对这一俯冲带的性质知之甚少。此外，超过3公里的沉积物埋藏着卡斯卡迪亚俯冲断层或巨型逆冲带，使其无法直接采样。海洋地震方法在海底下提供类似X射线的图像，可以用来照亮断裂带，并限制断层和边界沉积物的性质，这有助于地震成因。该项目将使用以前获得的海洋地震数据来调查大地震发源地浅部卡斯卡迪亚巨型逆冲带的结构。这将有助于阐明俄勒冈州中部近海地区断裂带的性质，该地区目前可能正在爬行，并可能在未来的地震中充当破裂的屏障。这一地区将与华盛顿近海的另一个地区进行比较，据信该地区目前已完全封锁，未来地震可能会在更大范围内破裂。从该项目中获得的对巨型逆冲性质的了解将有助于该地区正在进行和计划中的其他重大科学计划，包括利用美国国家科学基金会的海洋天文台倡议对俄勒冈州中部俯冲带的研究，以及利用国际大洋钻探计划计划进行的未来俯冲带监测研究。这项研究将支持两名研究生和两名早期职业科学家，帮助建设下一代海洋地震学家。该项目利用了2012年获得的8公里长的流光多道地震和广角海底地震仪数据，这两条线横跨俄勒冈州和华盛顿州中部近海边缘，从变形锋向海延伸约40公里，一直延伸到距离海岸约20公里，以及与海岸平行的一条线，位于变形前缘向海约10-15公里，跨越两条倾斜线。地震数据将用于详细研究卡斯卡迪亚变形前锋附近的基底沉积物剖面和顶部洋壳的性质，包括速度衍生的有效应力和孔隙流体压力，并用于改进华盛顿州和俄勒冈州俯冲带性质截然不同的弧前吸积楔的成像。通过叠前深度偏移(PSDM)和全波形反演得到的纵波速度(Vp)模型、横波速度(Vs)和分裂研究以及振幅与偏移距研究，将评估沉积物的性质。将使用板块界面的PSDM成像和波形模拟来检查弧前吸积楔形结构。具体的项目目标是：1)确定俯冲前基底沉积物剖面的地震特征(波速、波速、波速/波速、各向异性)和导出的物理特性(孔隙度、有效应力、孔隙流体压力和裂缝密度)，以及这些特性如何沿边缘变化；2)研究什么条件控制着原生消融的地层位置，这些条件如何与弧前的不同构造样式有关；3确定卡斯卡迪亚板块界面带的物理特性如何在强烈耦合的华盛顿地区以及部分爬行的俄勒冈中部剖面中向下倾斜。以及它们与上板块弧前变形的关系。

The Cascadia subduction zone extends offshore western North America from northern California to British Columbia and has been the site of past great earthquakes comparable to the Tohoku Mw~9 earthquake in 2011. The last great earthquake at Cascadia occurred in 1700 and produced a trans-oceanic tsunami recorded in Japan. With estimated recurrence intervals of 300-500 yrs it represents a significant seismic and tsunami hazard for the dense coastal population of the Pacific Northwest. Furthering our understanding of the characteristics of this subduction zone is of significant societal interest. At present, Cascadia is unusually quiet seismically, with few of the frequent small earthquakes detected at most subduction zones, and the properties of this subduction zone are poorly understood. Furthermore, over 3 kilometers of sediment buries the Cascadia subduction fault or megathrust zone making it inaccessible to direct sampling. Marine seismic methods provide x-ray like images beneath the seafloor that can be used to illuminate the fault zone as well as constrain the properties of the fault and bounding sediments, which contribute to seismogenesis. This project will use previously acquired marine seismic data to investigate the structure of the Cascadia megathrust zone within the shallow portion where great earthquakes initiate. It will help elucidate properties of the fault zone in a region offshore central Oregon that may currently be creeping and which may act as a barrier to rupture in future earthquakes. This area will be compared with another region offshore Washington that is believed to be fully locked at present with potential for rupture over a larger area in future earthquakes. The understanding of megathrust properties obtained from the project will benefit other major ongoing and planned scientific programs in the region including studies of the subduction zone at central Oregon using NSF's Ocean Observatory Initiative and planned future subduction zone monitoring studies making use of the International Ocean Drilling Program. This study will support two graduate students and two early career scientists, helping to build the next generation of marine seismologists.The project makes use of 8 km long-streamer multi-channel seismic and wide-angle ocean bottom seismometer data acquired in 2012 along two lines that cross the margin offshore central Oregon and Washington extending from ~40 km seaward of the deformation front to ~20 km from the coast, and one ~400 km line parallel to the coast located ~10-15 km seaward of the deformation front and spanning the 2 dip lines. The seismic data will be used for detailed studies of the properties of the basal sediment section and top oceanic crust near the Cascadia deformation front, including velocity-derived effective stress and pore fluid pressures, and for improved imaging of the forearc accretionary wedge within the Washington and Oregon regions of contrasting subduction zone properties. Sediment properties will be evaluated from compressional-wave velocity (Vp) models derived from Pre-Stack Depth Migration (PSDM) and Full Waveform Inversion, shear wave velocity (Vs) and splitting studies, and Amplitude vs Offset studies. Forearc accretionary wedge structure will be examined using PSDM imaging and waveform modeling of the plate interface. Specific project goals are: 1) to determine the seismic characteristics (Vp, Vs, Vp/Vs, anisotropy) and derived physical properties (porosity, effective stress, pore fluid pressure, and crack density) of the basal sediment section prior to subduction, and how these vary along the margin, and 2) to examine what conditions control the stratigraphic position of the proto-décollement and how these conditions are related to the different structural styles of the forearc and 3 to determine how the physical properties of plate interface zone at Cascadia vary in the down-dip direction both in the strongly-coupled Washington region as well as in the partially creeping central Oregon section, and how they relate to upper plate forearc deformation.