Collaborative Research: The Aleutian megathrust from trench to base of the seismogenic zone; integration and synthesis of laboratory, geophysical and geological data

地球上最大和最具破坏性的地震和海啸是沿着俯冲巨型逆冲构造产生的。这些板块构造边界断层在地震中破裂的部分被称为造震带。最近对传播到近地表的高滑动的观察，以及对异常慢滑动事件的新发现，对广泛持有的解释孕震区行为的假说提出了根本问题。尤其是，许多俯冲断层的孕震带似乎是“斑驳的”，一些区域在大地震中突然坍塌，另一些区域则通过稳定的无地震蠕变滑动。此外，在某些深度范围内，特别是在孕震带的浅部和深部边缘，在许多边缘观察到了缓慢滑动事件和具有异常低频能量的地震。目前对导致这些不同滑动方式的断裂带条件和过程的了解有限，这在很大程度上是因为对地下原地条件的定量约束很少。因此，与之相关的地震和海啸危险也同样缺乏约束。该项目将把阿留申俯冲边缘的高质量区域地球物理研究与相关岩石和沉积物的实验室实验测量相结合，以校准地球物理数据，并量化俯冲巨型逆冲带沿线的原地孔隙流体压力和应力。最终，通过提供从海沟到孕震带沿板块边界的地下条件的定量估计，这项研究将检验大范围地震行为的假设机制。为了实现这一点，该研究将把现代海洋沉积物和出土的变质泥岩的实验室数据与现有的多分辨率地球物理数据结合起来，以提高我们对从海沟到~30-40公里深度的板块边界巨型逆冲的就地条件和过程的理解。该项目将解决以下问题：1)低Vp、高Vp/VS比和高反射率的俯冲巨型逆冲带沿线的原地条件、材料和性质是什么？在近静岩压力和低有效应力下，地震资料在多大程度上成像了弱的沉积相与高孔的通道或斑块？2)板块边界的性质如何随深度和沿走向变化，以及它们与地震活动和上板块变形的关系？这两个主要问题将通过以下方法解决：(1)从海沟到~30-40公里深度的主动和被动源数据集提取对巨型逆冲的地球物理性质(Vp、Vp/Vs、反射率)的详细约束；(2)通过对阿拉斯加近海IODP岩心和科迪亚克岛出土的断裂带样品的实验室实验，定义与阿拉斯加边缘原位俯冲界面相关的沉积物和岩石的弹性和水文性质；(3)综合地球物理和实验室成分，验证物质性质和状态变量对俯冲构造地球物理特征的作用的假设；(4)通过对微震活动的精确定位，研究巨型逆冲性质与地震位置的相关性。这项工作将利用几个现有的高质量地球物理数据集、一套已经收集的样品以及DSDP/IODP岩心和数据。总体而言，这项协调研究将极大地提高我们对巨型逆冲带沿线条件和物质及其与地震活动的关系的了解，并为其他边缘地区的类似研究提供模板。

Earth's largest and most destructive earthquakes and tsunamis are generated along subduction megathrusts. The portion of these plate tectonic boundary faults that ruptures in earthquakes is known as the siesmogenic zone. Recent observations of high slip that propagates to the near surface, and new discoveries of anomalously slow slip events, have raised fundamental questions about widely held hypotheses that explain seismogenic zone behavior. In particular, the seismogenic zone of many subduction faults appears to be 'patchy', with some regions that fail suddenly in large earthquakes and others that slide by stable, aseismic creep. Additionally, in certain depth ranges, typically at the shallow and deep fringes of the seismogenic zone, slow slip events and earthquakes with anomalous low frequency energy have been observed at many margins. Current knowledge of the fault zone conditions and processes that cause these different modes of slip is limited, largely because quantitative constraints on in situ conditions in the subsurface are scarce. As a result, the associated earthquake and tsunami hazards are similarly poorly constrained. This project will combine high quality regional geophysical studies from the Aleutian subduction margin with laboratory experimental measurements on relevant rock and sediment, to calibrate the geophysical data and quantify in situ pore fluid pressure and stress along the subduction megathrust. Ultimately by providing quantitative estimates of the subsurface conditions along the plate boundary from the trench through the seismogenic zone, this study will test hypothesized mechanisms for the wide range of earthquake behavior.To accomplish this, the study will integrate laboratory data from modern oceanic sediment and exhumed metapelites with existing, multi-resolutional geophysical data to improve our understanding of in situ conditions and processes along the plate boundary megathrust from the trench to ~30-40 km depth. The project will address the following questions: 1) What are the in situ conditions, materials, and properties along the subduction megathrust that are sensed by low Vp, high Vp/Vs ratio, and high reflectivity? To what extent do seismic data image weak metasedimentary material vs. high-porosity channels or patches at near-lithostatic pressure and low effective stress? 2) How do the properties of the plate boundary change with depth and along-strike, and how do they relate to seismicity and upper plate deformation? These two overarching questions will be addressed by: (1) extracting detailed constraints on geophysical properties of the megathrust (Vp, Vp/Vs, reflectivity) from active and passive source datasets from the trench to depths of ~30-40 km; (2) defining elastic and hydrologic properties of sediment and rock relevant to the in situ subduction interface at the Alaska margin, via lab experiments on IODP cores from offshore Alaska and samples from exhumed fault zones on Kodiak Island; (3) integrating the geophysical and laboratory components to test hypotheses about the roles of material properties and state variables on geophysical signatures along the subduction thrust; and (4) investigating the correlation of megathrust properties with earthquake locations via precise relocation of microseismicity. The work will leverage several existing high-quality geophysical datasets, a suite of already collected samples, and DSDP/IODP cores and data. As a whole, this coordinated study will vastly improve our understanding of the conditions and materials along the megathrust, their relationship to seismicity, and serve as a template for similar studies at other margins.