GEOCE

We are just beginning to construct an instrument capable of making geodetic seafloor motion measurements from oceanographic moorings and observatories, as they are expected to be installed as part of the future ORION program in both coastal and open-ocean sites. The GEOCE system would integrate several components, most of which have been proven separately in different applications already. It consists of continuous raw-waveform differential GPS measurements on the surface buoy of the mooring, to determine the horizontal and vertical motion of the buoy with subcentimer accuracy. Horizontal seafloor motion is measured via acoustic transponder interrogations relative to the surface buoy, a variant of an existing ship-based approach. Vertical seafloor motion is determined via high-precision bottom pressure measurements, after removing sea surface height motion (using the GPS) and internal ocean density fluctuations (using water column CTD sensors in the mooring). The residual bottom pressure signal is seafloor motion or sensor drift. The accuracy of the sensors except for a long-term drift has been demonstrated in prior applications. Here an in-situ calibration system will be added to determine the drift. Irregular vertical motion or sudden events would be detectable even without this calibration system. Some of the data would be telemetered in real-time. The subsurface part of the system relies on existing and proven robust technologies, such as acoustic time-delay recovery of bottom pressure data, inductive communication with the CTD sensors in the mooring wire, while the transponders are interrogated with buoy-mounted transducers.

The ability to measure seafloor motion with subcentimeter accuracy on long timescales (months to years) in a continuous mode using moored installations would represent a significant advance over our current technological and scientific capabilities. It would significantly enhance our ability to track where long term stress/strains are accumulating in subduction zone environments and thus coupling patterns on seismogenic faults (i.e. the location of seismic asperities and the up-dip limit of locking). Even though based on existing elements, the integration of various geophysical and oceanographic methods into a stand-alone system is pushing all the elements to their limits and together results in novel tools for geophysics and physical oceanography. The oceanographic benefit of the system (thus the acronym “geophysical and oceanographic” observatory system) comes from the ability to perform bottom pressure observations without long-term drift, which until now has hampered the detection of long-period signals in bottom pressure which is important for determining interannual, interdecadal and climatic changes in ocean circulation.

The proposed system is a step towards merging expertises and technologies and implementing multi-use ocean moorings in sites of common interest between geophysics and oceanography (such as those identified in ORION planning letters and conceptual proposals). It demonstrates how existing or planned water column observatories can be upgraded to support additional scientific objectives. In addition, the real-time aspect of the system allows for monitoring of potential events, such as seafloor motions which can proceed major quake slip events and landslides. Thus, the system could contribute to early warning strategies for undersea earthquakes and tsunamis. When connected to a data network, multiple cross-disciplinary benefits should also result from the system for studies ranging from coastal process, to broader physical oceanographic, hazard, and climatic studies. In the future, particularly if incorporated into ORION, arrays of GEOCE instruments could potentially track the geodetic response and propagation rate of major rupture events by incorporating an event-driven high data rate response, similar to the present NOAA tsunami warning system.