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

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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.