Determining the Locking State of the Updip end of the Cascadia Megathrust using Real Time Seafloor Geodetic data

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Large earthquakes and tsunamis can occur due to the motion of Earth's tectonic plates at plate boundaries where oceanic crust is subducted under overlying crust in places such as Cascadia, which is offshore of the northwestern US near Vancouver Island, or Japan. Faults causing these earthquakes sometimes rupture from deep in Earth's crust all the way up to the surface of the seafloor. When that happens, displacement of the seafloor occurs; and, depending on the size of the up-thrust, this can generate a megatsunami. One such example would be the magnitude 9 earthquake that struck Japan in 2011 and generated a major tsunami that devastated communities along Japan's northeastern shore. Not all subduction ruptures, however, create such large tsunamis because, in some, displacements are confined along parts of faults that are well below the ocean floor. This can be seen in the 2014 magnitude 8.3 Chile Iquique earthquake which did not cause any major tsunami because most displacement in the Earth occurred at depths around 10 km. This research focuses on collecting and improving seismic data in the Cascadia region that is streaming from a unique subseafloor borehole geophysical observatory. These data are being used to study fault mechanics in the Cascadia area and provide tsunami hazard forecasts. Cascadia is an area of interest because it is close to the US and is a place where active subduction is occurring and for which there are no known large subduction-related earthquakes. It is important to understand if this "locking" of the fault is real, thus raising concern that accumulating stress is building up and result in a major earthquake or if the stress is being released through a series of slow and low, slip events. To help understand the fault dynamics in the Cascadia area, in 2016 a unique borehole geophysical installation was established in the seafloor off Vancouver Island on the Ocean Networks Canada cabled observatory. This research calibrates and validates data streaming from the borehole installation to test its reliability and provide real time-seismic and geodetic data from the shallowest part of the Cascadia subduction zone. This new system is designed to be especially sensitive to ruptures and properties in the shallow parts of the Cascadia fault zone. Broader impacts of the work include increasing infrastructure for science in terms of developing the unique capabilities of this new installation and providing important hazards-related data for earthquakes and potential mega tsunamis generated by continued subduction of the ocean plate under the northwest US and Canada. It also represents an important collaboration between the US and Canadian scientists running the Ocean Networks Canada cabled undersea observatory and the leveraging of infrastructure from the NSF International Ocean Discovery Program. In 2016, an international team of US and Canadian scientists and engineers installed a borehole geophysical observatory in the seafloor near the up-dip end of the Cascadia subduction zone offshore of Vancouver Island on the Canadian cabled observatory. Since 2017, this system has been returning high quality, real-time, seismic and borehole tilt data. Sensors are positioned about 300 m below the seafloor and about 4 km above the Cascadia plate boundary fault. Initial analysis of the data indicates the borehole system should be able to detect low magnitude, slow slip, earthquake events as small as magnitude 4. However, many small signals and excursions in the data still remain to be investigated to determine their sources and establish the stability of the instruments and the reliability of their data. This research continues the improvement of streaming data and determining its reliability. It involves optimizing the borehole instrumentation, with a focus on improving the de-tiding algorithm, cataloging potential transients at different time scales, and investigating instrument performance and stability. These activities ensure the best data quality for all users, ranging from scientists to managers of real-time warning systems who study earthquakes and tsunamis. Data from the borehole observatory can also be used to detect and track marine mammals and provide information on gas hydrate stability. The work will also ensure the dataset collected is archived if there are outages in telemetry.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.