Research Infrastructure: Mid-scale RI-1 (M1:IP): Creating an Offshore Subduction Zone Observatory in Cascadia with the Ocean Observatories Initiative Regional Cabled Array

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Earth’s greatest geological hazards are concentrated in its subduction zones, the places where two tectonic plates converge and collide. All known earthquakes with magnitudes ≥8.8 have taken place at subduction zones, as have the most devastating tsunamis. The Cascadia subduction zone, spanning the offshore coasts from northern California to British Columbia, hosts earthquakes up to magnitude 9 every few hundred years, the last of which was in 1700. While there are dense seismic and geodetic sensor networks today on land in the Pacific Northwest, the offshore region, where almost all the locked plate boundary and expected earthquake slip will occur, is largely devoid of such instruments in US waters. This project will take advantage of the existing NSF-funded Regional Cabled Observatory that brings power and the internet into the oceans on the margin offshore Newport, Oregon. Instruments, including seismic sensors and seafloor pressure gauges, will be added to the cabled infrastructure. Real-time data streaming from the resulting subduction zone observatory will help answer fundamental questions about how subduction zone faults work and can be used to enhance existing systems for earthquake and tsunami warning. A fundamental “Grand Challenge” in the geosciences today is to improve understanding of how subduction zone plate interface faults work. This requires sustained offshore observations in multiple subduction zones with contrasting properties to characterize how deformation is partitioned among large earthquakes, slow slip, and aseismic creep. The Cascadia subduction zone is a global endmember, owing to the young age of the subducting plate and thick sediment cover. The Ocean Observatories Initiative Regional Cabled Array includes a southern cable located on the slope and shelf of the Cascadia subduction zone off Newport, Oregon. This array has great potential for subduction zone studies because it is in an ideal central location in Cascadia where the gradient in megathrust coupling extends well offshore and there are ongoing clusters of seismicity and low frequency earthquakes beneath the shelf. This project will add cabled seafloor science junction boxes to each of three Primary Nodes on the continental slope and shelf, none of which are presently instrumented for geophysics. Each junction box will host a suite of geophysical sensors, comprising a buried broadband seismometer, a low-frequency hydrophone, strong motion accelerometers, calibrated pressure gauges and a current meter, with room for future expansion. In conjunction with nodes that already have geophysical sensors, this will create a world class offshore geophysical observatory to study fault coupling and deformation of the Cascadia megathrust and the overlying accretionary prism across a ~100 km section offshore. The observatory can address the following questions: (1) How does the locking of the Cascadia megathrust transition between the deformation front and the coastline off central Oregon?(2) Is there transient slip behavior – slow slip, tremor, and/or very low frequency earthquakes – offshore spanning the locked zone and its downdip transition? (3) How are the clusters of shallow earthquakes offshore linked to the megathrust?(4) What is the baseline deformation rate and fault slip behavior of the accretionary prism? Because the submarine cable streams data to shore at the speed of light, this observatory can provide the nation’s first offshore real-time earthquake and tsunami early warning detector, adding to and complementing the USGS’s ShakeAlert earthquake warning system and NOAA’s tsunami buoys. It can also support research into new warning technologies. Both undergraduate summer interns and graduate students will be trained on cabled geophysical observatories, including at-sea experiential learning programs prior to and during installation of the observatory. A workshop will introduce early career researchers to the scientific goals and infrastructure, provide hands-on experience with the tools for accessing and visualizing the data streams, and explore ideas for future analysis.Because the submarine cable streams data to shore at the speed of light, this observatory can provide the nation’s first offshore real-time earthquake and tsunami early warning detector, adding to and complementing the USGS’s ShakeAlert earthquake warning system and NOAA’s tsunami buoys. It can also support research into new warning technologies. Both undergraduate summer interns and graduate students will be trained on cabled geophysical observatories, including at-sea experiential learning programs prior to and during installation of the observatory. A workshop will introduce early career researchers to the scientific goals and infrastructure, provide hands-on experience with the tools for accessing and visualizing the data streams, and explore ideas for future analysis.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.