Replacement of temperature loggers in CORKed ODP boreholes off Costa Rica and analysis of long-term temperature records

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Since 2002, long-term temperature and pressure monitoring in marine boreholes have been part of a larger coordinated effort to investigate subduction fluxes and fluid flow through the toe of the subduction-zone prism off Nicoya Peninsula, Costa Rica. Our immediate goals are to recover miniaturized temperature loggers (MTLs) before their memory capacity is exceeded (August 2009), to replace them with new MTLs that will ensure continuous temperature data for the next decade, and to download pressure data recorded by Circulation Obviation Retrofit Kit (CORK)-IIs that seal the boreholes. Subsequently, tentative conclusions regarding the hydrology across the margin—based on the first two years of the time series—will be verified using the new temperature and pressure data of the following four years. Specifically, we anticipate new observations of transient temperature and pressure events correlated with slow slip events that have been detected by onshore Global Positioning System (GPS) and seismic networks. In temperature and pressure data analysis, tidally induced variations—separated by frequency analysis—provide information about hydrological formation parameters and the remaining signal represents the hydrological state of the frontal subduction zone. In addition to slow slip events, future MTL and CORK data may also document the response of a large earthquake (Mw >7:5) that is anticipated as one of a sequence of historical events (1853, 1900, and 1950).