Collaborative Research: 3D onshore-offshore seismic investigation of Japan's megathrusts

俯冲带，一个构造板块沿着断裂带滑动到另一个板块之下，产生地球上最大和最具破坏性的地震和海啸。目前，人们还不清楚是什么使一些俯冲断层能够稳定地蠕动，从而降低发生大地震的风险，而另一些断层则保持了数百年的锁定状态，积累了大量能量，将在未来的大地震中释放出来。该项目将分析来自日本两个俯冲带的现有地球物理数据，其规模和分辨率在地球上尚属首次。这一20年的数据集既包括海底地震仪和多道地震调查收集的近海数据，也包括世界上密度最大的永久陆上地震仪网络被动记录的数据。地震波通过地球传播到地震仪阵列，可以以类似于计算机辅助层析成像(CAT)扫描的方式使用，以形成地壳深部和上地幔的三维图像。这些断层图像将与地震行为的详细知识相结合，以研究1)断层两边的岩石类型；2)断层表面的粗糙度；以及3)沿断层表面的沉积物和流体的分布如何影响地震行为。在日本获得的结果将广泛适用于其他俯冲带，并将用于改善日本、太平洋西北部和新西兰的地震危险性评估。该项目将支持两名早期职业科学家。这项研究的结果将在美国、英国、日本和新西兰的研讨会和会议上公布，并将为斯克里普斯白桦水族馆的高中课程开发计划做出贡献。该项目将分析有史以来最大的陆上-近海地震数据集，以产生地球上任何地方俯冲带的两张最高分辨率的3D图像。20年的数据集横跨南开和日本东北部俯冲带，包括由最密集的陆上地震仪永久网络被动记录的200条近海地震剖面(50万炮)的地震能量。将在两个~38万平方公里的区域内进行多相层析成像反演，将这一无与伦比的陆上-近海数据集与日本海洋地球科学技术厅提供的密集主动和被动(地震)震源数据整合在一起。由此产生的3-D图像将推进俯冲带研究的三个领域的知识。首先，在逆冲板块中，前弧内的岩性变化将被视为南开和日本东北部9.0级东北地震现场地震破裂带大小的可能关键控制因素。其次，将对俯冲太平洋和菲律宾海板块的结构和粗糙度进行成像，以约束水化状态的差异，并分析俯冲海山/海脊对弧前结构和巨型逆冲滑移行为的影响。第三，对孕震带物理性质的原位约束将被用来更好地了解南开和日本东北部在慢震现象上的显著差异。这是一个独特的机会，因为地球上没有其他板块边界拥有以这种分辨率进行区域成像所需的数据。

Subduction zones where one tectonic plate slides beneath another along a fault zone produce the largest and most destructive earthquakes and tsunamis on Earth. At present, it is poorly known what enables some subduction faults to creep steadily, thereby reducing risk of major earthquakes, while others remain locked for centuries accumulating large amounts of energy to be released in giant future earthquakes. This project will analyse existing geophysical data from two subduction zones in Japan on a scale and resolution that is the first of its kind anywhere on Earth. This 20-year dataset includes both offshore data collected by ocean bottom seismometers and multichannel seismic surveys as well as data passively recorded by the densest permanent network of onshore seismographs in the world. Seismic waves, travelling through the Earth to the array of seismometers, can be used in a fashion similar to CAT (computed-assisted-tomography) scanning to form 3-D images of the deep crust and upper mantle. These tomographic images will be integrated with detailed knowledge on earthquake behavior to study how this is influenced by 1) the type of rocks on each side of the fault; 2) the roughness of the fault surface; and 3) the distribution of sediments and fluids along the fault surface. The results obtained in Japan will be widely applicable to other subduction zones, and will be used to improve assessments of seismic hazard in Japan, the Pacific Northwest and New Zealand. This project will support two early career scientists. The results of the study will be presented at workshops and conferences in the US, UK, Japan and New Zealand, and will contribute to the high school curriculum development program at the Birch Aquarium at Scripps.This project will analyze the largest dataset of onshore-offshore seismic ever recorded to produce the two highest resolution 3-D images of subduction zones anywhere on Earth. The 20-year dataset spans the Nankai and NE Japan subduction zones, and includes seismic energy from 200 offshore seismic profiles (0.5 million shots) was passively recorded by the densest permanent network of onshore seismographs. A multi-phase tomographic inversion will be conducted within two ~380,000 km2 regions, integrating this unparalleled onshore-offshore dataset with dense active and passive (earthquake) source data provided by the Japan Agency for Marine-Earth Science and Technology. The resultant 3-D images will advance knowledge in three areas of subduction zone research. First, in the overthrusting plate, lithological variations within the forearc will be examined as possible key controls on the size of earthquake rupture zones in Nankai and in NE Japan, site of the MW 9.0 Tohoku-oki earthquake. Second, the structure and roughness of the subducting Pacific and Philippine Sea plates will be imaged to constrain differences in hydration state, and analyze the influence of subducting seamounts/ridges on the structure of the forearc and slip-behavior of the megathrust. Third, in situ constraints on physical properties both up-dip, and most importantly, down-dip of the seismogenic zone will be used to better understand the marked differences in slow-earthquake phenomena between Nankai and NE Japan. This is a unique opportunity as no other plate boundary on Earth has the requisite data to permit regional imaging at this resolution.