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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS016 Oral Presentation 2350
Long-term Seismic Monitoring of the East Pacific Rise at 950 N ---
Scientific Findings and Implications for Future Permanent Ocean
Observatories
Dr. Felix Waldhauser
Lamont-Doherty Earth Observatory Columbia University IASPEI
Maya Tolstoy
An array of up to twelve short period Ocean Bottom Seismometers (OBSs) (Scripps L-CHEAPO) have
been deployed at the Ridge 2000 East Pacific Rise Integrated Studies Site Bulls Eye at 950 N since
October 2003. The array, occupying an area of approximately 4 by 4 km, has been turned-around on an
approximately annual basis. It has been pulled out in January 2007, ending the longest continuous
recording of local microearthquake activity at a Mid Ocean Ridge (MOR). The array captured the onset
of a ramp up in the seismicity rate from ~36 events per day in 2003 to ~600 events per day in 2005
leading up to January 2006, when an eruption occurred within and near the array. While eight OBSs
were either lost or buried in fresh lava, four were recovered, and at least two and possibly all four OBSs
recorded the eruption and its seismic aftermath. Data from the first deployment (Oct 2003 - April 2004)
has been analyzed and a total of 16,079 events were detected and located, of which approximately
7,300 events locate within the array (azimuthal station gap less than 180). High-resolution event
locations based on hand-picked P- and S-wave arrival times image, for the first time, the basic structure
of a hydrothermal circulation cell. In particular, we interpret a near vertical pipe of seismicity as an onaxis
down-flow zone. The lateral positions of the shallow earthquakes in the pipe correlate with a kink in
the axial summit trough, and an absence of nearby active hot vents. The deeper events are consistent
with measured axial magma chamber depths (~1.4 km) and widths in the 950 N area. Data from later
deployments are currently being analyzed and we expected that they provide insight into the temporal
and spatial evolution of the eruption that occurred in January 2006. We use this experiment and
associated data and results to highlight the scientific value of long-term local-scale seismic monitoring of
MOR systems to further our understanding of the strongly time-dependent processes that drive
divergent plate boundaries. We report on the characteristics and quality of the seismic data collected
and our experience with handling and processing continuous OBS recordings with peak rates of over
2000 earthquakes per day.
Keywords: mid ocean ridge, seismicity, eruption