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

合作研究：从主动源地震数据的高级研究中推断卡斯卡迪亚变形前缘和板块界面特性

• 批准号: 1657839
• 负责人: Adrien Arnulf
• 金额: $ 16.35万
• 依托单位: University of Texas at Austin
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2017
• 资助国家: 美国
• 起止时间: 2017-07-01 至 2020-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1657839&HistoricalAwards=false
• 关键词: Collaborative; Research; Inferences; Cascadia; Deformation

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.

卡斯卡迪亚俯冲带从北方加州到不列颠哥伦比亚省延伸到北美西部近海，过去曾发生过与2011年东北部Mw~9地震相当的大地震。卡斯卡迪亚上一次大地震发生在1700年，并引发了日本有记录的跨洋海啸。据估计，它的重现间隔为300-500年，对太平洋西北部密集的沿海人口来说，这是一个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，在大多数俯冲带很少检测到频繁的小地震，而且人们对这个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大型逆冲带，因此无法直接采样。海洋地震方法在海底下提供类似X射线的图像，可用于照亮断层带，并限制断层和边界沉积物的性质，这有助于地震成因。该项目将利用以前获得的海洋地震数据，调查卡斯卡迪亚巨型逆冲带浅层的结构，大地震就是在该区域发生的。它将有助于阐明俄勒冈州中部近海地区断层带的性质，该断层带目前可能正在蠕动，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锁定，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巨型逆冲断层特性的了解将有益于该区域正在进行和计划进行的其他重大科学方案，包括利用国家科学基金会的海洋观测倡议对俄勒冈州中部的俯冲带进行研究，以及利用国际大洋钻探方案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，帮助培养下一代海洋地震学家。该项目利用2012年沿着两条线采集的8公里长拖缆多道地震和广角海底地震仪数据，这两条线穿过俄勒冈州中部和华盛顿州近海边缘，从变形锋向海方向约40公里延伸到海岸约20公里，一条长约400 km的平行于海岸线，位于变形锋向海方向约10-15 km处，横跨两条倾向线。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洋壳的特性，包括速度导出的有效应力和孔隙流体压力，并用于改进俯冲带特性截然不同的华盛顿和俄勒冈州地区内弧前增生楔的成像。沉积物特性将根据压缩波速度（Vp）模型进行评估，该模型来自叠前深度偏移（PSDM）和全波形反演、剪切波速度（Vs）和分裂研究以及振幅与偏移距研究。弧前增生楔结构将使用PSDM成像和板块界面的波形模拟进行检查。具体项目目标是：1）确定地震特性（Vp、Vs、Vp/Vs、各向异性）和导出的物理性质（孔隙度、有效应力、孔隙流体压力和裂缝密度），以及这些参数沿着边缘如何变化，（2）探讨控制原岩地层位置的条件。滑脱和这些条件是如何与不同的构造风格的弧前和3，以确定如何在卡斯卡迪亚板块界面区的物理性质变化，在下倾方向，无论是在强烈的，耦合华盛顿地区以及部分蠕动的中部俄勒冈州部分，以及它们如何与上板块弧前变形。