A seafloor long-baseline tiltmeter

Long-term monitoring of seismicity and deformation has provided constraints on the eruptive behavior and internal structure and dynamics of subaerial volcanoes, but until recently, such monitoring of submarine volcanoes has not been feasible. Little is known about the formation of oceanic crust or seamounts, and we have therefore developed a stand-alone long-baseline tiltmeter to record deformation on active seafloor volcanoes. The instrument is a differential pressure, two-fluid sensor adapted for use on the seafloor, combined with an autonomous data logger and acoustic navigation/release system. The tiltmeter can be Installed without use of remotely operated vehicles or manned submersibles and, to first order, is insensitive to noise driven by temperature or pressure gradients. We recorded 65 days of continuous data from one of these tiltmeters on Axial Seamount on the Juan de Fuca Ridge during a multidisciplinary experiment that included ocean bottom seismographs, magnetotelluric instruments, and short-baseline tiltmeters. After instrument equilibration the 100-m-long tiltmeter provided a record with long-term drift rates of 0.5-5 mu rad day(-1) and higher frequency variations of the order of 5-10 mu rad. Comparison with records of subaerial volcanic tilt shows that this instrument can discriminate volcanic deflation events, though none occurred during our deployment, a conclusion supported by nearby short-baseline tilt and bottom pressure recordings. The short-and long-baseline data constrain volcanic inflation of Axial Seamount to be below 0.5-1 mu rad day(-1) during mid-1994. Analysis of the long-baseline tilt data in conjunction with electric field, temperature, and short-baseline tiltmeter data shows that high-frequency signals are largely driven by ocean currents. Improved coupling between the tiltmeter and seafloor should reduce this noise, improve stability and drift, and further enhance our ability to record tilt related to active submarine volcanism.