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加利福尼亚州北部延伸到北美西部近海，一直延伸到不列颠哥伦比亚省，是过去发生过可与 2011 年东北 Mw~9 地震相媲美的大地震的地方。卡斯卡迪亚最近一次大地震发生在 1700 年，并产生了跨洋地震日本记录到的海啸预计复发间隔为 300-500 年，对太平洋西北部人口稠密的沿海地区来说，这是一次重大的地震和海啸灾害。进一步了解该俯冲带的特征具有重要的社会意义。目前，卡斯卡迪亚的地震异常安静，大多数俯冲带很少发生频繁的小地震，而且对该俯冲带的性质知之甚少。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断层，使其无法直接采样，海洋地震方法提供了海底下方的类似 X 射线的图像，可用于照亮断层带。限制断层和边界沉积物的特性，这有助于地震发生。该项目将利用先前获得的海洋地震数据来研究发生大地震的浅部卡斯卡迪亚逆冲断层带的结构，这将有助于阐明断层的特性。俄勒冈州中部近海地区目前可能正在蠕动，可能会成为未来地震破裂的屏障，该地区将与华盛顿近海的另一个地区进行比较，该地区目前被认为已完全锁定，有可能发生破裂。从该项目中获得的对更大区域的逆冲断层特性的了解将有利于该地区其他正在进行和计划中的重大科学项目，包括利用国家科学基金会海洋观测计划对俄勒冈州中部俯冲带进行的研究以及计划中的未来俯冲带监测研究。利用国际海洋钻探计划，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 8 公里长的拖缆多道地震和广角。 2012 年沿穿过俄勒冈州和华盛顿州中部近海边缘的两条线采集的海底地震仪数据，从变形前缘向海约 40 公里延伸至距海岸约 20 公里，以及一条与海岸平行的约 400 公里线，位于约 10变形前缘向海-15公里并跨越2条倾角线的地震数据将用于详细研究卡斯卡迪亚变形附近的基底沉积物部分和顶部洋壳的特性。前沿，包括速度导出的有效应力和孔隙流体压力，以及改进华盛顿和俄勒冈地区对比俯冲带特性的弧前增生楔的成像，将根据源自压缩波速（Vp）模型来评估沉积物特性。将使用叠前深度偏移（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量研究来检查。板块界面的 PSDM 成像和波形建模具体项目目标是： 1) 确定地震特征（Vp、Vs、Vp/Vs、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）。 ) 俯冲前的基底沉积物部分，以及它们如何沿边缘变化，以及 2) 检查哪些条件控制着原始滑脱的地层位置以及这些条件如何与不同的构造样式相关前弧和 3 以确定卡斯卡迪亚板块界面区的物理性质在强耦合华盛顿地区以及部分蠕动的俄勒冈州中部部分在下倾方向上的变化，以及它们与上部板块的关系前弧变形。