Replacement of temperature loggers in CORKed ODP boreholes off Costa Rica and analysis of long-term temperature records
Zusammenfassung der Projektergebnisse
Our primary objective for the ReCORKed project was to continue the long-term monitoring of borehole temperatures and pressures in ODP Holes 1253A and 1255A off Nicoya Peninsula, Costa Rica, for the next decade. We planed to reach this goal by recovering the existing miniaturized temperature loggers (MTLs) from these holes and by installing new loggers with a expanded temperature resolution. We hoped to prevent a gap in the more than six years long record of continuous temperature measurements since the memory capacity of the deployed MTLs would have been exceeded in August 2009. The new MTLs were specifically customized for the temperature conditions in the two boreholes. Therefore, they provide an enhanced temperature resolution which allows the detection of very small transient signals which are for instance generated by slow slip events. Using the recovered temperature data, we planed to test tentative conclusions drawn by Davis and Villinger (2006) regarding the hydrological state of the of the subduction zone. Moreover, we hope to gain additional insight in the correlation between transient temperature and pressure events in the boreholes and recent slow slip events (e.g. May 2007) that have been observed using onshore GPS and seismic networks. The recovery and re-deployment of the loggers was done with the manned submersible Alvin, operated from R/V Atlantis. The cruise took place from February 7, 2009 until February 14, 2009. Two data loggers (MTLs) were installed at Site 1253 and two recovered from the sea floor at the same site. Unfortunately, we were not able to replace the MTLs at Site 1255 since the original OsmoSampler package could not be removed from the hole. Seafloor temperature records from Site 1253 reveal periodic events during which bottom water temperatures rise slowly before they rapidly drop back to background values. We were able to correlate these events with close-by bottom current observations; the event timing generally tracked the envelope of the tidal current modulation and had a mean duration of 30 to 40 days. Findings indicate that the episodic flows were likely rotationally modified, autosuspending turbidity currents. Tidal currents that resuspend sediments above the shoaling trench floor to the southeast of the mooring site probably initiate these turbidity currents, periodically. Pressure data from ODP hole 1253A and 1255A show several formation pressure transients which follow ’episodic tremor and slip events’ (ETS) observed on shore. The signatures of the transients are consistent with local slip on the shallow part of the thrust interface, and the temporal relationship between them and the deep ETS provides strong evidence that slow slip can propagate with a rate of about 10 km/day across the full width of the seismogenic zone. Contrasting polarity of the pressure transients at the two boreholes suggests volumetric expansion of the subducting plate and contraction of the overlying prism when slip reaches the prism toe. Such slip may serve to relieve stress at subduction zones, but could also increase stress in parts of the plate boundary where interseismic slip does not occur.
Projektbezogene Publikationen (Auswahl)
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(2010), Observations of long-duration episodic bottom currents in the Middle America Trench: Evidence for tidally initiated turbidity flows, J. Geophys. Res.
Thomson, R. E., E. E. Davis, M. Heesemann, and H. Villinger
