Maya Tolstoy & Felix Waldhauser
Lamont-Doherty Earth Observatory of Columbia University

From October 2003 through January 2007 an array of up to 9 OBSs continuously recorded earthquake activity near 9°50’N at the East Pacific Rise ISS.   With hundreds of thousands of individual earthquakes recorded, the data processing task has been substantial.  We anticipate that picking of events will be completed prior to the meeting with preliminary event locations being calculated in the months immediately following.

These data can provide information on the depth, width and temporal variability of the hydrothermal cracking front as well as possible zones of hydrothermal recharge and discharge.   Work already published suggests that hydrothermal flow is dominantly along-axis with hydrothermal cells on the scale of about 2 km, and recharge occurring on-axis at locations of increased tectonic stress (and hence increase permeability) (Tolstoy et al., 2008).   Additionally, work to date has shown that the earthquakes are occurring preferentially during times of increased tidal stress (Stroup et al., 2007) and that systematic spatial patterns in this triggering can be used to estimate permeability (Stroup et al., 2009; Crone et al., submitted).   Stress and permeability within the hydrothermal cracking front are seen to be quite spatially heterogeneous (Bohnenstiehl et al, 2008; Crone et al., submitted).   As processing of the entire data set is completed we expect to resolve temporal variability in properties of the cracking front as seismicity rates increased building toward an eruption (Tolstoy et al., 2006).

We hope to be able to integrate these interpreted properties and changes in the hydrothermal and magmatic system with geochemical, biological, geological and modeling work in the same area to build an integrated picture of a fast spreading ridge system.  We also hope that detailed seismicity patterns from the fully processed data set will illuminate magmatic processes leading up to the eruption and help resolve differences between the geophysical and geochemical indicators of eruption timing.

An overarching goal of the R2K program included an integrated understanding of the mid-ocean ridge system from mantle to microbe.  While much research is on-going at this site, and many collected data sets have yet to be fully analyzed, the East Pacific Rise ISS with it’s bull’s-eye at 9°50’N arguably represents the best studied and best understood section of mid-ocean ridge on the planet.  As such it is an ideal site to define our current state of knowledge from mantle to microbe of an archetypal section of fast-spreading mid-ocean ridge.