This project addresses techniques to measure present-day deformation of seafloor crust in the vicinity of mid-ocean ridges. By measuring the acoustic range directly between seafloor mounted transducers on the Juan de Fuca Ridge, the rate at which crustal plates move over long time periods can be precisely measured. To continue an ongoing plate monitoring effort, the existing installation will be replaced prior to 2002 at which time the batteries will likely be depleted to a level that will not permit intra-transponder ranging. This project allows construction of a second generation instrument that will address shortcomings in the existing installation while maintaining an unbroken time series of crustal motion measurements from 1994 through 2001 to 2006. The proposed design will be a new type of seafloor instrument that can be configured to extend the temporal and spatial sampling to hourly/daily measurements over 10-km scale distances. The new design will incorporate both horizontal and vertical crustal deformation measurements in an intelligent controller that will respond to changes in background measurements of temperature, pressure, tilt and travel times as sustained deviations from background measurements may indicate the onset of tectonic activity. The multiple sensors and extended spatial coverage of the new system will be applicable to other ridges or across other seafloor faults e.g., on continental shelves at subduction zones. In year one, the instruments will be constructed and tested during a month-long deployment in waters off-shore San Diego. In year two, the instruments will be deployed across axial Cleft of the South Cleft Segment on the Juan de Fuca Ridge.