Sustained Cabled Seafloor Observations of the Cascadia Subduction Zone off Central Oregon

Long-term submarine observations are critical for understanding subduction zones because the slip of great earthquakes occurs offshore.  Geophysical observations suggest that the Cascadia megathrust is locked from the coastline to the deformation front in many places, but off central Oregon they are consistent with a narrowly locked megathrust near the deformation front and creeping behavior beneath the shelf where there are two clusters of earthquakes near the plate boundary, including repeating and very low frequency earthquakes.  In this region, scientific objectives include understanding how megathrust locking transitions between the deformation front and the coastline, determining whether there is transient slip behavior, improving constraints on how shallow offshore earthquake clusters are linked to the megathrust, and characterizing the baseline deformation rate and fault slip behavior of the accretionary prism. This summer, the Cascadia Offshore Subduction Zone Observatory (COSZO), an infrastructure project funded by the US National Science Foundation, will add seismic and geodetic instruments to the Ocean Observatories Initiative (OOI) Regional Cabled Array (RCA) off Newport, Oregon. New seafloor science junction boxes, with updates to the RCA design, will be connected to three primary nodes on the continental slope and shelf that currently do not support seafloor geophysical observations. At each new junction box and a fourth site on the shelf where there is an existing science junction box but no geophysical instruments, COSZO will install a Nanometrics Atlantis Cabled Observatory ocean bottom seismic package comprising a buried broadband seismometer, a strong-motion accelerometer, a low-frequency hydrophone, and a differential pressure gauge. The project incorporates two types of calibrated absolute pressure gauges that utilize Paroscientific resonant quartz crystal sensors. The Geodetic and Seismic Sensor Module combines a triaxial accelerometer with two pressure gauges that are periodically calibrated against the internal pressure of the housing measured by a barometer. The Self-Calibrating Pressure Recorder also includes two pressure gauges but performs calibrations with a reference pressure close to ambient generated by a piston gauge. COSZO will also install uncalibrated absolute pressure gauges and Nortek Vector 3-component ocean current meters. Together with sensors already on the OOI RCA at the Slope Base and Hydrate Ridge sites and autonomous long-term geodetic observations, the COSZO infrastructure will form a critical mass observatory on the Cascadia Subduction Zone to support scientific studies and efforts to prototype offshore earthquake and tsunami early warning. COSZO will stream data into EarthScope Data Services and a workshop is planned for spring 2027 to engage early career scientists.  Looking forward, each science junction box includes open ports and any unspent COSZO funds and independent PI-driven proposals can add to the suite of cabled instruments. The OOI RCA has also hosted three short fiber sensing experiments, demonstrating the potential for single- and multi-span distributed acoustic sensing concurrent with observatory operations. Implementing permanent fiber sensing on the OOI RCA would complement COSZO by adding additional observations over an expanded footprint.