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

一个构造板块沿着断裂带滑到另一个板块下面的俯冲带会产生地球上最大、最具破坏性的地震和海啸。目前，人们还不太清楚是什么使一些俯冲断层稳定地蠕动，从而降低了发生大地震的风险，而另一些则保持了几个世纪，积累了大量的能量，准备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任何地方都是首次。这个20年的数据集包括海底地震仪和多通道地震调查收集的海上数据，以及世界上最密集的陆上地震仪永久网络被动记录的数据。地震波穿过地球到达地震仪阵列，可以用一种类似于CAT（计算机辅助断层扫描）扫描的方式来形成地壳深处和上地幔的三维图像。这些层析成像图像将与地震行为的详细知识相结合，以研究地震行为如何受到以下因素的影响：1)断层两侧的岩石类型；2)断层表面的粗糙度；3)断层表面沉积物和流体的分布。在日本获得的结果将广泛适用于其他俯冲带，并将用于改进对日本、太平洋西北地区和新西兰地震危险性的评估。这个项目将支持两位早期职业科学家。研究结果将在美国、英国、日本和新西兰的研讨会和会议上发表，并将为斯克里普斯桦树水族馆的高中课程发展计划做出贡献。该项目将分析有史以来最大的陆上-海上地震数据集，生成两张地球上任何地方俯冲带的最高分辨率3d图像。20年的数据集跨越南开和日本东北部俯冲带，包括200个海上地震剖面（50万次）的地震能量，由最密集的陆上地震仪永久网络被动记录。将在两个~38万平方公里的区域内进行多阶段层析反演，将这一无与伦比的陆上-海上数据集与日本海洋-地球科学技术厅提供的密集主动和被动（地震）源数据相结合。由此产生的三维图像将促进三个领域的俯冲带研究。首先，在逆冲板块中，弧前的岩性变化将被视为南开和日本东北部地震断裂带大小的可能关键控制因素，日本东北部发生了MW 9.0级东北大地震。其次，对太平洋和菲律宾海俯冲板块的构造和粗糙度进行成像，以约束水化状态的差异，并分析海山/海脊俯冲对前弧构造和大逆冲构造滑动行为的影响。第三，震源带上倾和下倾物理性质的原位约束将有助于更好地理解南开和日本东北部慢震现象的显著差异。这是一个独特的机会，因为地球上没有其他板块边界有必要的数据，以这种分辨率进行区域成像。

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.