The ELSC Seismic Experiment

 

This large collaborative project involves researchers from Wash U., LDEO, U. Hawaii, SIO  and LDEO. The Eastern Lau Spreading center (ELSC) is a back arc spreading center lying close by the Tonga trench. Fluids expelled from the subducting slab cause melting of the upper mantle above the slab, producing the volcanos of the Tonga Ridge, and strongly influencing the structure of the southern ELSC. The ELSC seismic experiment used two large deployments of ocean bottom seismometers to image the oceanic crust near the spreading center and the structures connecting the ELSC to the down going slab.


The “passive” component of the experiment detected abundant earthquakes in the down going slab that were recorded by 52 ocean bottom seismometers (OBS).  These OBS were deployed from the R/V Revelle in Nov. 2009 and were recovered with the R/V Kilo Moana in Nov. 2010.

The data have been combined with ambient noise tomography to image the structure in the upper mantle. The “active source” component of the experiment was conducted during early 2009  by Columbia’s ship the R/V Langseth.  Seismic arrivals from pneumatic sound sources were recorded by a large number of OBSs to image in glorious detail the structure of the crust and uppermost mantle beneath the spreading center.


Zha, Yang, Spahr C. Webb, S. Shawn Wei, Douglas A. Wiens, Donna K. Blackman, William Menke, Robert A. Dunn, and James A. Conder, Seismological imaging of ridge–arc interaction beneath the Eastern Lau Spreading Center from OBS ambient noise tomography, Earth and Planet. Science Letters 408 : 194-206, (2014).


Menke, W., Zha, Y., Webb, S. C., & Blackman, D. K. . Seismic Anisotropy Indicates Ridge‐parallel Asthenospheric Flow Beneath the Eastern Lau Spreading Center. J. .Geophys .R, (2015).


Wei, S. Shawn, D.A. Wiens, Y. Zha, T. Plank, S.C. Webb, D.K. Blackman, R.A. Dunn. and J.A. Conder, Seismic evidence of effects of water on melt transport in the Lau back-arc mantle, Nature, doi:10.1038/nature14113, (2015).


Figure from Zha et al., 2014 showing the shear velocity at 20km depth. The low velocity (magma rich) region beneath the arc is connected with the ELSC in the southern part of the region, but disconnected to the north, consistent with a more robust magma supply to the southern segment of the ELSC.