Long-term observations of seismic activity and ground deformation at mid-ocean ridges and submarine volcanoes are required for an understanding of the spatial and temporal characteristics of magma transport and intrusion. To make precise records of tilt on the seafloor we have installed short baseline tiltmeters in six ocean bottom seismometers (TILT-OBS) and developed a long baseline (100-500 m) two-fluid tiltmeter (LBT). In the TILT-OBS, the seismometer platform is levelled to better than 1 degrees after deployment. The tiltmeter consists of a pair of electrolytic bubble sensors mounted on a secondary levelling stage on the seismometer platform. The levelling stage uses two motor-driven micrometers on a triangular mounting plate to bring the sensors to null. The sensitivity of these tiltmeters is 0.05 mu rad, at a dynamic range of 0.2 mrad. A long baseline instrument was developed to achieve a better spatial average of deformation. Most approaches used on land to measure stable long baseline tilt cannot be applied to a submarine instrument, but tiltmeters in which the pressure of a fluid in tubes is measured are amenable to installation on the seafloor. The development resulted in a device that is essentially a center-pressure instrument folded back on itself, with fluids of different densities in the two tubes. During July to September 1994, these instruments were deployed on Axial Seamount, on the Juan de Fuca Ridge off Washington state, for a test of their relative performance on volcanic terrain, yielding 9 weeks of continuous data (seismic, tilt, and temperature) from five TILT-OBS and one long baseline instrument. Drift on all instruments was of the order of 1 mu rad/day, with higher frequency variations of order 5-10 mu rad. Initial drift on the TILT-OBS is shown to be associated with platform settling rather than with the sensor or its mounting. High frequency noise is coherent across instruments and tidal in character, and we conclude that tidal currents moving the sensors are responsible. (C) 1998 Elsevier Science B.V. All rights reserved.
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