HR: 1340h
AN: T33B-1378    [Abstracts]
TI: Investigation of Deep Microearthquakes at 9 deg 50' N on the East Pacific Rise
AU: * Doermann, L
EM: doermann@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964, United States
AU: Tolstoy, M
EM: tolstoy@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964, United States
AU: Waldhauser, F
EM: felixw@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964, United States
AB: An array of nine closely-spaced ocean bottom seismometers (OBSs) recorded microseismicity at 9 deg 50' N on the East Pacific Rise (EPR) between October 2003 and April 2004. Precise double-difference earthquake locations for more than 7,200 events were calculated from analyst-reviewed arrivals. Important insights have been gained about the anatomy of this fast spreading ridge and the implications for hydrothermal circulation above the axial magma chamber (AMC) located at approximately 1.5 km depth. Nearly 10% of the events, however, were located below the AMC and require further investigation, as they are counter to models suggesting that brittle fracture below the melt zone is not possible. Still, other data point to a solid floor to the AMC with high P-wave velocities (Singh et al., 1999). We evaluate the robustness of deep-located events on the 9 deg 50' N segment by reviewing their waveforms for excessive noise and poor phase picks, spatial distribution, time difference in P and S arrivals, event magnitude, and spectral content. Although S arrivals can be difficult to discern and easily mispicked, many events have clear S arrivals with consistent picks. Deep-located events show a similar along-axis distribution to shallow events, predominantly located near the center of the OBS array. The difference between S- and P-wave arrival times increases steadily with earthquake depth, supporting accurate locations even at greater depth. Further, the magnitude of events beneath the melt lens decreases with depth, consistent with a warming trend. Our preliminary analysis does not reveal drastic differences in spectral slope, although we might expect less high frequency content for deep events due to waves passing through partial melt or because the earthquakes originate in semi-brittle material. However, since the AMC at this location is only 500 m wide and most of the OBSs are off-axis, the seismic waves of deep events may not travel through melt at all.
DE: 3025 Marine seismics (0935, 7294)
DE: 3035 Midocean ridge processes
DE: 7245 Mid-ocean ridges
SC: Tectonophysics [T]
MN: 2007 Fall Meeting