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北部延伸到不列颠哥伦比亚省，是过去大地震的发生地，可与2011年的东北Mw~9地震相媲美。上一次发生在卡斯卡迪亚的大地震发生在1700年，并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，对太平洋西北部沿海人口稠密的地区来说，这是一次重大的地震和海啸灾害。进一步了解这一俯冲带的特征具有重要的社会意义。目前，卡斯卡迪亚地震异常安静，在大多数俯冲带中很少检测到频繁的小地震，并且对该俯冲带的性质知之甚少。此外，超过3公里的沉积物掩埋了卡斯卡迪亚俯冲断层或巨型逆冲带，使其无法直接采样。海洋地震方法提供海底下类似x射线的图像，可以用来照亮断裂带，以及约束断层和边界沉积物的性质，这有助于地震发生。该项目将使用先前获得的海洋地震数据来调查大地震发生的浅层部分内的卡斯卡迪亚巨型逆冲带的结构。它将有助于阐明俄勒冈州中部近海地区断层带的性质，该地区目前可能正在蔓延，并可能在未来的地震中充当破裂的屏障。该地区将与华盛顿近海的另一个地区进行比较，该地区目前被认为是完全封闭的，在未来的地震中有可能在更大范围内破裂。从该项目中获得的对大型逆冲性质的了解将有利于该地区其他主要的正在进行和计划中的科学项目，包括利用美国国家科学基金会的海洋观测倡议对俄勒冈中部俯冲带的研究，以及利用国际海洋钻探计划计划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，帮助培养下一代海洋地震学家。本项目利用2012年在俄勒冈州和华盛顿州中部海上沿变形锋向海~40公里至距海岸~20公里的两条线获得的8公里长拖缆多通道地震和广角海底地震仪数据，以及一条位于变形锋向海~10-15公里平行于海岸的~400公里线，跨越2条倾角线。地震数据将用于详细研究卡斯卡迪亚变形前沿附近的基底沉积物剖面和顶部海洋地壳的性质，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地区俯冲带性质对比的弧前增生楔的成像。沉积物特性将通过叠前深度偏移（PSDM）和全波形反演得到的压缩波速（Vp）模型、剪切波速（Vs）和分裂研究以及振幅与偏移研究来评估。利用PSDM成像和板块界面波形建模对前弧增生楔构造进行研究。具体的项目目标是：1)确定俯冲前基底沉积剖面的地震特征（Vp、Vs、Vp/Vs、各向异性）和推导出的物理性质（孔隙度、有效应力、孔隙流体压力和裂缝密度），以及这些特征沿边缘的变化规律；2)研究控制原结带地层位置的条件，以及这些条件与弧前不同构造样式的关系；3)确定在强耦合的华盛顿地区和部分匍匐的俄勒冈中部剖面，卡斯卡迪亚板块界面带的物理性质在下倾方向上的变化，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.