HR: 0830h
AN: S31D-0789    [PDF]
TI: Seismic Scattering from Heterogeneity in the Continental Crust and Upper mantle
AU: * Thybo, H J
EM: thybo@geol.ku.dk
AF: Geological Institute, University of Copenhagen, Oster Voldgade 10, Copenhagen, DK-1350 Denmark
AU: Nielsen, L
EM: ln@geol.ku.dk
AF: Geological Institute, University of Copenhagen, Oster Voldgade 10, Copenhagen, DK-1350 Denmark
AB: We document pronounced seismic scattering from three heterogeneous zones in the continental crust and upper mantle under Eurasia: (1) The lower crust, (2) The mantle low-velocity zone, and (3) An interval around the top of the mantle transition zone. We model the scattering by use of viscoelastic, full wavefield, 2D finite-difference synthetic seismograms. The calculations are for large models of seismic velocity (2500 by 550 km), which include by heterogeneous layers, defined by random fluctuations of the elastic parameters and Q-values. Our primary data source is Peaceful Nuclear Explosion (PNE) seismic data sets collected in the former Soviet Union. High-frequency signals (up to 10 Hz with centre frequencies of 2-4 Hz) from the PNEs were recorded with a nominal receiver spacing of 10-15 km along profiles of up to 4000 km length. (1) Lower crustal scattering explains the characteristics of the teleseismic (or long-range) Pn wave at all frequencies in the data. This wave propagates as a whispering-gallery phase to more than 3000 km offset from the sources with apparent mantle velocity. It requires a large, positive vertical upper mantle velocity gradient. The model is consistent with outcrop and seismic observations. We reject a previously published model for the teleseismic Pn in terms of upper mantle scattering which only explains the wavefield characteristics at high frequency, but not at low frequency. It also leads to strong damping of later arrivals, in disagreement with observations. (2) Heterogeneity in the mantle low-velocity zone below the 8$\deg$ discontinuity at 100 km depth causes scattering of the main frequencies (2-4 Hz) of the seismic wavefield. The scattering is primarily observed directly behind the first arrivals in the 800-1400 km offset range. Pronounced delay of the first arrivals beyond ca. 800 km offset demonstrates the presence of the mantle low velocity zone. The best fit to observations is obtained by including an 80 km thick heterogeneous low-velocity zone below 100 km depth. The heterogeneity is represented by fluctuations described by a von Karman distribution function with a Hurst number of 0.5 and spatial correlation lengths of 5-10 km (horizontally) and 3-5 km (vertically). (3) Pronounced reflectivity before and after the `41'' reflection in the seismic sections constrains heterogeneity around the top of the mantle transition zone. The 2-4 Hz scattered signals may be explained by fluctuations in the 320-460 km depth interval with horizontal and vertical correlation lengths on the order of 20-40 km and 5-10 km, respectively. We present synthetic seismograms that include the effects from all three heterogeneous depth intervals. They explain most of the characteristics of the observed wavefield from the PNE sources.
DE: 7200 SEISMOLOGY
DE: 7203 Body wave propagation
DE: 7205 Continental crust (1242)
DE: 7218 Lithosphere and upper mantle
DE: 8122 Dynamics, gravity and tectonics
SC: Seismology [S]
MN: 2003 Fall Meeting