We present observations from two field deployments of a calibrated tiltmeter that we name the Self-Calibrating Tilt Accelerometer (SCTA). The tiltmeter is based upon a triaxial quartz crystal accelerometer; the horizontal channels measure tilt and are periodically rotated into the vertical to obtain a measurement of the acceleration of gravity. Changes in the measured total acceleration are ascribed to drift in the vertical channel and used as calibrations for removing that same drift from the tilt time series observed between calibrations. Changes in the span (sensitivity) of the accelerometer channels can also be measured by calibrating them pointing up and down. A 3-year test on the seafloor at Axial Seamount show that the calibrations are consistent with a linear-exponential model of drift to ~0.5 μg (μrad). The calibrated tilt time series was impacted by platform settling for the first 2 years, but after repositioning the tiltmeter, the calibrated observations were consistent for the final year with the tilt observed on a nearby LILY tiltmeter. A separate 15-month test in a stable vault at Piñon Flat Observatory was complicated by seasonal temperature variations of >5°C; the calibrations are consistent with a linear-exponential model of drift to ~2 μg when temperature and temperature time-derivative dependence is included. Similarly, the calibrated tilt time series was impacted by thermal deformation of the SCTA assembly. A future test in a thermally and tectonically stable borehole will be required to assess the accuracy of the SCTA.

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.