Lowermost Mantle Velocity Estimations Beneath the Central North Atlantic Area from Pdif Observed at Balkan, East Mediterranean, and American Stations
- 164 Downloads
Lowermost mantle velocity in the area 15°S–70°N latitude/60°W–5° W longitude is estimated using two groups of observations, complementary to each other. There are 894 Pdif observations at stations in the Balkan and Eastern Mediterranean areas from 15 major earthquakes in Central and South America. Another 218 Pdif observations are associated with four earthquakes in Greece/Turkey and one event in Africa, recorded by American stations. A Pdif slowness tomographic approach of the structures immediately above the core-to-mantle boundary (CMB) is used, incorporating corrections for ellipticity, station elevation and velocity perturbations along the ray path. A low-velocity zone above CMB with a large geographical extent, approximately in the area (35–65°N) × (40–20°W), appears to have the velocity perturbations exceeding the value actually assumed by some global models. Most likely, it is extended beneath western Africa. A high-velocity area is observed west of the low-velocity zone. The results suggest that both Cape Verde and Azorean islands are located near transition areas from low-to-high velocity values in the lowermost mantle.
KeywordsPdif slowness tomography Cape Verde Azores Super-plume root D″ layer Central North Atlantic area
- Alexander S.S. & Phinney R.A., 1966. A study of the core-mantle boundary using P waves diffracted by the earth’s core, J. Geophys. Res., 71, 5943–5958.Google Scholar
- Brown, R.J., 1984. On the determination of source-receiver distances using a new equidistant latitude, Geophys. J. R. astr. Soc., 76, 445–459.Google Scholar
- Garnero, E.J., Revenaugh, J.S., Williams, Q., Lay, T. & Kellogg, L.H., 1998. Ultralow velocity zone at the core-mantle boundary. In: The Core-Mantle Boundary, pp. 319–334, eds Gurnis, M., Wysession, M.E., Knittle, E. and Buffet, B.A., American Geophysical Union.Google Scholar
- Gilbert, F. & Backus, G.E., 1966. Propagator matrices in elastic and vibration problems, Geophys., 31, 326–332.Google Scholar
- Karason, H. & van der Hilst, R.D., 2001. Tomographic imaging of the lowermost mantle with differential times of refracted and diffracted core phases (PKP, Pdiff), J. Geophys. Res., 106, 6569–6587.Google Scholar
- Kennett, B.L.N. & Engdahl, E.R., 1991. Travel times for global earthquake location and Phase identification, Geophys. J. Int., 106, 429–465.Google Scholar
- Kennett, B. L. N., Engdahl, E. R. & Buland, R., 1995. Constraints on seismic velocities in the Earth from traveltimes, Geophys. J. Int., 122, 108-124.Google Scholar
- Knopoff L. & Gilbert F., 1961. Diffraction of elastic waves by the core of the earth, Bull. Seis. Soc Am., 51, 35-49.Google Scholar
- Li, C., van der Hilst, R.D., Engdahl, E.R. & Burdick, S., 2008. A new global model for P wave speed variations in Earth’s mantle, Geochem. Geophys. Geosyst., 9, Q05018, doi:10.1029/2007GC001806.
- Loper, D.E. & Lay, T., 1995. The core-mantle boundary region, J. Geophys. Res., 100, 6397–6420.Google Scholar
- Masters, G., Laske, G., Bolton, H. & Dziewonski, A., 2000. The Relative Behavior of Shear Velocity, Bulk Sound Speed, and Compressional Velocity in the Mantle: Implications for Chemical and Thermal Structure in: Earth’s Deep Interior Mineral Physics and Tomography From the Atomic to the Global Scale,Vol. 117: Geophysical Monograph Series, pp. 63–87, eds Karato, S., Forte, A., Liebermann, R., Masters, G. & Stixrude, L., American Geophysical Union.Google Scholar
- McNamara, A. K., Garnero, E.J. & Rost, S., 2010. Tracking deep mantle reservoirs with ultra-low velocity zones, Earth Planet. Sci. Lett., 299, 1–9, doi:10.1016/j.epsl.2010.07.042.
- Montagner, J.-P. & Kennett, B. L. N., 1996. How to reconcile body-wave and normal-mode reference Earth models? Geophys. J. Int., 125, 229–248.Google Scholar
- Montelli, R., Nolet, G., Dahlen, F.A. & G. Masters, G., 2006. A catalogue of deep mantle plumes: New results from finite frequency tomography, Geochem. Geophys. Geosyst., 7, Q11007, doi:10.1029/2006GC001248.
- Mooney, W., Laske, G. & Master, G., 1998. CRUST 5.1: a global crustal model at 5° x 5°, J. Geophys. Res. 103, 727–747.Google Scholar
- Mula A.H. & Muller G., 1980. Ray parameters of diffracted long period P and S waves and the velocities at the base of the mantle, Pageoph, 118, 1272–1292.Google Scholar
- Ni, S. & Helmberger, D.V., 2001. Horizontal transition from fast to slow structures at the core-mantle boundary; South Atlantic, Earth Planet. Sci. Lett., 187, 301–310.Google Scholar
- Pearce, J. & Mittleman, D., 2002. Defining the Fresnel zone for broadband radiation, Phys. Rev., E 66, 1–4.Google Scholar
- Ritsema, J., Ni, S., Helmberger, D.V. & Crotwell, H.P., 1998. Evidence for strong shear velocity reductions and velocity gradients in the lower mantle beneath Africa, Geophys. Res. Lett., 25, 4245–4248.Google Scholar
- Rost, S. & Garnero, E.J., 2006. Detection of an ultralow velocity zone at the core-mantle boundary using diffracted PKKPab waves, J. Geophys. Res., 111, B07309, doi:10.1029/2005JB003850.
- Sacks, I.S., 1967. Diffracted P-wave studies of the earth’s core, 2. Lower mantle velocity, core size, lower mantle structure, J. Geophys. Res., 72, 2589–2594.Google Scholar
- Sidorin, I., Gurnis, M. & Helmberger, D.V, 1999. Dynamics of a phase change at the base of the mantle consistent with seismological observations, J. Geophys. Res., 104, 15005–15023.Google Scholar
- Souriau, A. & Poupinet G. (1994). Lateral variations in P velocity and attenuation in the D” layer from diffracted P waves, Phys. Earth. Planet. Int., 84, 227–234.Google Scholar
- Sylvander, M., Ponce, B. & Souriau, A., 1997. Seismiv velocities at the core-mantle boundary inferred from P waves diffracted around the core, Phys. Earth Planet. Inter., 101, 189-202.Google Scholar
- Takeuchi, H. & Saito, M., 1972. Seismic surface waves. In: Bolt, B.A. (Ed.) Seismology: Surface Waves and Earth Oscillations, Methods in Computational Physics, v.11, 217-295. New York, Academic Press.Google Scholar
- Taylor, J.R., 1982. An Introduction Error Analysis, University Science Books.Google Scholar
- Tkalčić, H., Romanowicz, B. & Houy, N., 2002. Constraints on D″ structure using PKP(AB-DF), PKP(BC-DF) and PcP-P travel time data from broadband records, Geophys. J. Int., 149, 599–616.Google Scholar
- Thorne, M.S. & Garnero, E.J., 2004. Inferences on ultralowvelocity zone structure from a global analysis of SPdKS waves, J. Geophys. Res., 109, B08301, doi:10.1029/2004JB003010.
- Valenzuela, R.W. & Wysession, M.E., 1998. Illuminating the base of the mantle with diffracted waves, in The Core-Mantle Boundary Region, Geodyn Ser, Vol. 28, edited by M. Gurnis, M.E. Wysession, E. Knittle, & B.A. Buffett, pp. 273–297, AGU, Washington, D.C.Google Scholar
- Wessel, P. & Smith, W.H.F., 1996. A global, self-consistent, hierarchical, high-resolution shoreline database, J.Geophys.Res., 104, 4795–4809.Google Scholar
- Wang, R., 1999. A simple orthonormalization method for the stable and efficient computation of Green’s functions, Bull. Seismol. Soc. Amer., 89, 733–741.Google Scholar
- Williams, Q., Revenaugh, J. & Garnero, E., 1998. A correlation between ultra-low basal velocities in the mantle and hot spots, Science, 281, 546–549.Google Scholar
- Wysession, M.E. & Okal, E.A., 1988. Evidence for lateral heterogeneity at the core-mantle boundary from the slowness of diffracted S profiles, in Structure and Dynamics of Earth’s Deep Interior, Geophys Monogr Ser, Vol. 46, edited by D.E. Smylie & R. Hide, pp. 55–63, AGU, Washington, D.C.Google Scholar
- Wysession, M.E. & Okal, E.A., 1989. Regional analysis of D” velocities from the ray parameters of diffracted P profiles, Geophys. Res. Lett., 16, 1417–1420.Google Scholar
- Wysession, M.E., Okal, E.A. & Bina C.R., 1992. The structure of the core-mantle boundary from diffracted waves, J Geophys Res, 97, 8749–8764.Google Scholar
- Xu, Y. & Koper, K.D., 2009. Detection of a ULVZ at the base of the mantle beneath the northwest Pacific, Geophys. Res. Lett., 36(L17301), doi:10.1029/2009GL039387.
- Zhao, D., 2004. Global tomographic images of mantle plumes and subducting slabs: insight into deep Earth dynamics, Phys. Earth Planet. Inter., 146, 3–34.Google Scholar