A Rotating Tiltmeter for Marine Geodesy: Development and Testing at Axial Seamount on the Ocean Observatories Initiative Cabled Array

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Measurements of ground deformation due to the motion of the Earth's tectonic plates are important for scientific research aimed at understanding volcanoes and earthquakes. Tiltmeters are instruments that measure changes in the tilt or slope of the ground that can occur for example, when a volcano inflates prior to an eruption or when the stresses that cause earthquakes lift up one part of the earth relative to another. A challenge with designing tiltmeters is that the measurements tend to drift with time due to instability in the sensors ¡V this limits their ability to measure tilt signals that are changing slowly. This project will develop a new type of tiltmeter that will correct for instrument drift by calibrating the sensor against the Earth's gravitational force, which can be considered constant at any point on the Earth. The tiltmeter will be deployed on a real-time cabled seafloor observatory at Axial Seamount, an active volcano in the Northeast Pacific Ocean. This is a site of significant scientific interest; the volcano has erupted three times in 20 years. It is also a site where tilt is being measured by other methods which allows validation of the new instrument. The data collected by this instrument will be made available to the public and other scientists. In the future, the tiltmeter may contribute to studies that seek to understand the geological processes that lead to great subduction zone earthquakes or that cause hazardous volcanic eruptions. The project will train a graduate student and provide field opportunities for undergraduates to help deploy the instrument at sea.Geodetic measurements of seafloor deformation are essential to understand geodynamic processes at oceanic plate boundaries but are quite challenging. The project will employ a novel approach to creating a short-baseline tiltmeter. The tiltmeter is based on a high-resolution three-component quartz crystal accelerometer. If the accelerometer is deployed on a stable platform that is coupled to the seafloor, changes in tilt of the horizontal channels of the accelerometer will lead to changes in the measured accelerations with time but so will the drift of the sensors. The approach to correcting the horizontal channels for sensor drift is to conduct a periodic calibration by rotating (or "flipping") each horizontal channel into the vertical for a short interval to measure the acceleration of gravity, g. Since g is to a high degree of accuracy invariant at any location, changes in the measurement of g between successive rotations can be attributed to sensor drift. This measurement of drift can then be used to correct each horizontal accelerometer channel to obtain a time series of true tilt changes between calibrations. The resulting tiltmeter will have unique characteristics ¡V a high-dynamic range (it cannot go off scale), high precision (~ a few nrads) and good long-term stability after calibration (10 micro rads). At least a year of data will be collected at the Ocean Observatories Initiative cabled observatory on Axial Seamount.