Absence of Ultra-Low Velocity Zones at the CMB

Absence of Ultra-Low Velocity Zones at the CMB A) Seismic section (ground velocity) of the recorded aftershock recorded at the Southern California seismic network. Data have been bandpass filtered with corner frequencies of 0.7Hz and 2 Hz. Data have been aligned on the theoretical PcP arrival for IASP91. Theoretical arrival times for several body waves are marked. B) Map of source (star) and receiver (inverted triangle) combination. PcP CMB reflection points are marked by crosses. Background shows S-wave seismic velocities from a tomographic model (Ritsema and van Heijst, 2002). Great circle paths are marked as thin grey lines and the double couple solution for the earthquake is also given.
Seismological studies of Earth’s core mantle boundary (CMB) show evidence for heterogeneities and structures on many scales. Among the most enigmatic structures detected at the CMB are ultra-low velocity zones (ULVZs). ULVZs are thin layers of strongly reduced seismic velocities that have been detected at the CMB in several locations. Typical ULVZ thicknesses are on the order of 5 to 40 km with velocity reductions of up to 10 and 30% relative to 1D velocity models for P- and S-waves, respectively. Some studies also indicate strongly increased ULVZ density relative to the surrounding mantle. Several models for ULVZ have been proposed including partial melting of mantle material and iron enrichment of perovskite and/or post-perovskite. The knowledge of the global distribution of ULVZs on Earth is essential for distinguishing between different ULVZ hypotheses. Unfortunately, only a limited number of regions covering roughly half of the CMB area have been probed to date.
</p><p>Here we use an aftershock of the relatively large Hawaiian earthquake on October 15, 2006 (moment magnitude, Mw ~ 6.7) that was recorded by roughly 1100 stations from several networks in the western United States and Canada. Using core reflected P-waves (PcP) allows the study of a previously unprobed region just north of the large low shear velocity province (LLSVP) beneath the Pacific Ocean. The large amount of high quality stations sampling this patch of CMB allows unprecedented waveform quality to </p><p>sample the fine scale structure of the CMB.
Stacked seismograms show PcP amplitudes clearly out of the background noise level, but no evidence for a PcP precursor that would indicate ULVZ existence. Synthetic modelling of a large parameter space of ULVZ properties indicates that this region of the CMB is likely devoid of ULVZ, although ULVZs with thicknesses below the vertical resolution level (~5 km) of PcP might exist.
</p><p>Rost, S., Garnero, E., Thorne, M., Hutko, A., (2010). On the absence of an ultralow-velocity zone in the North Pacific. J. Geophys. Res., 115, doi:10.1029/2009JB006420.
Acknowledgements: This work was supported by CSEDI grant EAR-0456356 (SR) and NERC New Investigator grant NE/F000898/1 (SR), and grant NSF EAR-0453944 (EJG).</p>


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