Observation of unusually deep seismicity below an amagmatic ultraslow spreading Mid Ocean Ridge segment

florian.schmid [ at ] awi.de


A small array of Ocean Bottom Seismometers (OBS) has been installed for a one year deployment from late 2012 to late 2013 on the axis of an amagmatic segment of the ultraslow spreading Southwest Indian Ridge. Prior to the recovery, during RV Polarstern cruise ANT-29/8 (SWEAP) wide angle seismic profiles where shot across the instruments to obtain information on crustal and upper mantle P-wave velocities. A 1D velocity profile was established from the inverted wide angle seismic profiles and extrapolated into depth, which served as basis for the later localisation of local earthquake hypocenters. Here, one wide angle seismic profile and the to date achieved results of local seismicity analysis will be presented. Very unexpectedly, we find hypocenters down to 30 km depth below the sea floor and no seismicity in the crust and upper mantle. Local seismicity occurs to be confined to the axial rift valley and hypocenters appear to be uniformly distributed along a vertical band of seismicity with its lower limit undulating in depth along the ridge axis. The aseismicity of the crust and upper mantle might be attributed to extensive serpentinization at this ridge segment and the undulating base of the seismicity band might be associated with an isotherm paralell to the lithosphere-asthenosphere boundary (LAB). Although the source mechanism for the deep rooted hypocenters remains somewhat difficult to explain, our results indicate that the lithosphere at amagmatic segments of ultraslow Mid Ocean Ridges can be much thicker and cooler than previously assumed.

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Conference (Talk)
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Not peer-reviewed
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Jahrestagung der Deutschen Geophysikalischen Gesellschaft, 23 Mar 2015 - 26 Mar 2015, Hannover.
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Schmid, F. and Schlindwein, V. , Alfred-Wegener-Institute, Helmholtz-Zentrum for Polar- and Marine Research, POLMAR - Helmholtz Graduate Schoole for Polar and Marine Research (2015): Observation of unusually deep seismicity below an amagmatic ultraslow spreading Mid Ocean Ridge segment , Jahrestagung der Deutschen Geophysikalischen Gesellschaft, Hannover, 23 March 2015 - 26 March 2015 .


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