Seafloor deformation and point convergence of and between two CORK monuments off Nicoya Peninsula, Costa Rica

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The overall goal of the proposed project is to monitor seafloor deformation (vertical movement and tilt) of the seafloor at the Middle American subduction zone off Nicoya Peninsula, Costa Rica. Two CORKs, installed during Leg 205 will be used as seafloor geodetic monuments and equipped with high resolution state-of-the-art point position monitoring devices. The devices are (1) a water level sensor, i.e. a pressure meter, that provides 0.7mm resolution at ambient pressure of the vertical position with respect to mean sea level and (2) self-leveling biaxial tiltmeters that measure the angle displacements in two perpendicular horizontal directions with a very high resolution of Snrad. To control environmental conditions high resolution temperature loggers will be included in the monitoring system. The measurements will run over approximately two years at sampling intervals of some minutes to provide a comprehensive multi-scale data set of CORK motion. Local influences on CORK motion from e.g. formation pressure changes and differential motion between both CORKs will be studied in detail. The determined values of CORK motion will be compared and linked to the global plate tectonic movement pattern, measured on land at the same time by other groups. One of the locations (Site 1253) is located in the trench about 200m seaward of the deformation front whereas the other location (Site 1255) is only 600m away from 1253 and located 400m arcward of the deformation front. By monitoring pressures and tilt at both sites we will be able to measure relative as well as absolute deformation. It is planned to install the system in the fall of 2007 with a ROV/submersible and recover it about two years later.