Collaborative Research: Sedimentary signature of shallow and tsunamigenic megathrust ruptures: Observations and physical models from recent catastrophic events

2004年的苏门答腊和2011年的东北大地震和海啸都是灾难性的地质事件，造成了重大的社会后果。这两次地震都在俯冲构造板块边界产生了巨大的破裂，预计不会发生如此大的地震。大的断层位移到达海沟的海底，引起巨大的海啸。像这样的板块边界是海底地震最重要的来源，尤其是海啸地震，也是类似大地震的唯一来源。最近的两次地震在刺激海洋研究和进一步了解特大地震及相关海啸方面意义重大。关于早期大型事件的数据对于解决此类事件发生的地点和频率以及对它们的期望至关重要，但很少有此类事件具有仪器数据。唯一的办法是根据沉积物中的记录重建更古老的大地震。海底地震地质学的开创性深水研究现在正在使用东北冲地震的沉积特征来识别以前大地震的特征。这项研究促进了我们对与特大地震相关的沉积事件的独特特征的理解，并有助于在沉积记录中区分它们。典型的事件沉积包括层状、混沌和均质沉积物（“均质岩”）。这项研究将结合实地观察，包括沉积物和广泛的地表沉积物动员的文件，以及一种新的实验室“沉积物震动器”，以更好地了解这些罕见但具有破坏性的事件的独特属性。该项目支持学生和一名早期职业科学家的培训。在俯冲边缘最大的逆冲地震使100公里宽的裂缝上的沉积物重新活动起来，在地层学中留下了典型的事件沉积物。它们通常包括岩性、物理和地球化学均质的声学透明层（“均质岩”），以及具有多种沉积结构和组成的声学叠层（“浊积岩”）。2011年日本地震中的泥质均质岩最近被发现来自于广泛的地表（几厘米）沉积物的再活化，而富含沙子的浊积岩被认为来自于斜坡破坏和不同的来源。这种差异适用于其他地震和环境，表明不同的重建过程。这一假设将通过实验室实验来验证，这些实验将探索这些过程是否对地震谱的不同部分有反应。另一个需要解决的问题是，在M≥9.0的巨型逆冲断裂之上，海底异常大且持续的长周期振荡是否可以夹带表层沉积物，从而解释均质岩的形成。2011年日本东北地震的特殊数据集将用于进行实验。通过合作，将增加来自卡斯卡迪亚边缘、小安的列斯群岛和巴利阿里深渊平原的样本。将全球地球物理观测和岩心观测数据与物理实验数据进行比较，将为理解地震调动沉积物的方式提供重要的一步，并推动海底古地震学领域对特大地震事件沉积物的识别。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The 2004 Sumatra and the 2011 Tohoku-Oki mega-earthquakes and tsunamis were catastrophic geologic events with major societal consequences. Both produced huge ruptures in areas of subducting tectonic plate boundaries that were not expected to have such large earthquakes. The large fault displacement reached the sea-floor of oceanic trenches, causing the huge tsunamis. Plate boundaries like these are the most consequential source of submarine earthquakes, especially tsunamigenic ones, and the only source of similarly large earthquakes. These two most recent ones were particularly significant in stimulating marine research and in providing further understanding of mega-earthquakes and associated tsunamis. Data on earlier large events are critical to addressing where and how often such events occur and what to expect from them, but few such events are available with instrumental data. The only recourse is reconstruction of older mega-earthquakes from their record in sediment deposits. Pioneering deep-water research in submarine earthquake geology is now using the sedimentary signature of the Tohoku-Oki earthquake to recognize signatures of previous large earthquakes. This research is advancing our understanding about the unique characteristics of sedimentation events associated with exceptionally large earthquakes and helping to distinguish them in the sedimentary record. Typical event deposits include laminated, chaotic, and homogeneous sediments (“homogenites”). This study will combine field observations, including deposits and documentation of widespread surface sediment mobilization, with a novel laboratory “sediment shaker”, to better understand the unique attributes of these rare but devastating events. The project supports the training of students and an early career scientist.The largest megathrust earthquakes at subduction margins remobilize sediment over ruptures 100s of km wide, leaving characteristic event-deposits in the stratigraphy. They generally include an acoustically transparent layer that is lithlogically, physically and geochemically homogeneous (“homogenite”), plus acoustically laminated layers with a wide range of sedimentary structures and composition (“turbidites”). The muddy homogenites from the 2011 Japan earthquake have been recently discovered to derive from widespread remobilization of surficial (few cm) sediment, while sand-rich turbidites are thought to derive from slope failures and diverse sources. This difference holds for other earthquakes and settings, suggesting distinct remobilization processes. This hypothesis will be tested by laboratory experiments that will explore whether these processes respond to different parts of the earthquake spectrum. Another question to be addressed is whether the exceptionally large and sustained long-period oscillations of the sea floor above M≥9.0 megathrust ruptures can entrain surficial sediment and thus account for the homogenites. Exceptional data sets from the 2011 Tohoku earthquake will be available to conduct the experiments. Through collaborations, samples from the Cascadia Margin, Lesser Antilles and Balearic Abyssal plain will be added. The comparisons of global observations from geophysical and core data with the physical experiments, will provide a major step forward in understanding the ways in which earthquakes remobilize sediments and advances the field of submarine paleoseismology to recognize megaquake event deposits.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.