We report on a feasibility study for an offshore instrument network in the Cascadia subduction zone to improve earthquake and tsunami early warning. The global DART buoy network provides effective warning for far-field tsunamis but near-field tsunami warning is challenging because the lead time is short and near-source observations are rarely available to directly measure the sea surface disturbance and evolution. Near-field tsunami warnings presently rely on rapid point source seismic inversions that do not estimate tsunami wave height. Efforts are underway to incorporate GNSS data into rapid source inversions that would support an initial near-field tsunami prediction. Offshore observations would contribute further to near-field tsunami warnings by providing: first, direct observations of seafloor and sea surface displacements during earthquake rupture and second, ongoing measurements for continued forecast refinement. Offshore instruments could also detect tsunamis triggered by submarine landslides and by so-called “tsunami” or “slow” or “silent” earthquakes that can generate unexpectedly large tsunamis but are characterized by shaking intensity so low as to be undetected or ignored. Pressure observations in the source zone will be challenging to interpret because they are dominated by seafloor accelerations and hydroacoustic waves rather than changes in hydrostatic pressure. In an effective system, pressure observations may need to be complemented by other observations such as inertial measurements of seafloor displacement, GNSS buoys and high-frequency coastal radar. It may also be important to place pressure sensors just seaward of the source zone to measure the developing tsunami in a region with an undisturbed seafloor. We will discuss alternative design options for an offshore instrument network in Cascadia, the research and development that must to be completed to determine the best approach, and the role of offshore observations in a holistic plan for tsunami mitigation.

Measurements of ground tilt are a critical geodetic tool for monitoring active volcanoes because they provide multidimensional data that can resolve complex deformation signals. We are developing a Self-Calibrating Tilt Accelerometer (SCTA) for use in the marine environment and present results from two deployments: on land at the Scripps Institution of Oceanography Cecil and Ida Green Piñon Flat Observatory and on the seafloor at Axial Seamount on the Juan de Fuca Ridge. The SCTA utilizes a Quartz Sensor Solutions triaxial accelerometer on a gimbal system to periodically rotate the horizontal channels into the vertical to calibrate against the local g vector, achieving high precision and stability within 1 microradian. The SCTA tiltmeter has the added benefit of simultaneously measuring ground accelerations and recording seismic signals. We compare the SCTA performance at the center of the summit caldera at Axial Seamount against a co-located Jewell Instruments LILY tiltmeter on the OOI Cabled Array. The tilt measurements in one direction are consistent, but the data suggest that the deployment platform for the SCTA may be settling in the other direction. We are using data from the ensemble of 4 cabled pressure sensors and 5 tilt sensors at Axial, including the SCTA, to study its inflation behavior since its eruption in 2015. We have identified several significant, cm-scale deflation events of durations of tens of days. The tilt and relative elevations of instrument sites are asymmetric about their turning points, suggesting a more complex mechanism than a simple inflation reversal. We are conducting forward modeling of the deformation signals to determine if the geodetic signals are consistent with differential slip rates, normalized to the rate of inflation/deflation, on the caldera’s outwardly dipping ring faults between these periods. Another plausible mechanism that we plan to investigate is the lateral transport of magma from beneath the southern caldera to either the northern caldera or to a secondary reservoir, located 5 km to the east. These deflation events are potentially important for understanding the mechanisms of magma supply, storage, and transport at Axial Seamount, as well as for accurately forecasting future eruptions, which have been shown to be inflation-predictable.