HR: 0830h
AN: S31D-0788    [PDF]
TI: Transition Zone Heterogeneity Imaged on Long Range Explosion Data
AU: * Ross, A R
EM: ross@seis.geol.ku.dk
AF: Geological Institute, University of Copenhagen, Oester Voldgade 10, Copenhagen, DK-1350 Denmark
AU: Thybo, H
EM: ht@seis.geol.ku.dk
AF: Geological Institute, University of Copenhagen, Oester Voldgade 10, Copenhagen, DK-1350 Denmark
AU: Nielsen, L
EM: ln@seis.geol.ku.dk
AF: Geological Institute, University of Copenhagen, Oester Voldgade 10, Copenhagen, DK-1350 Denmark
AU: Perchuc, E
EM: per@igf.edu.pl
AF: Institute of Geophysics, Polish Academy of Sciences, ul. Ksecia Janusza 64, Warsaw, PL-01452 Poland
AB: Standard models of the upper mantle transition zone do not explain the complexity of arrivals observed on long range explosion profiles collected in Siberia and North America. We observe reverberative arrivals on high-resolution, nuclear (Peaceful Nuclear Explosions - PNEs) and conventional explosion seismic profiles in two offset ranges which have moveouts and traveltimes which cannot be explained by reflected or refracted arrivals from a simple transition zone with boundaries at 410 km and 660 km. The phases we observe are also visible on short period recordings of PNEs made outside the former Soviet Union, notably on NORSAR and Grafenberg array data. In addition to anomalous secondary arrivals, first arrival traveltime misfits suggest anomalous velocities above and in the transition zone. Additional complexity is required to explain several features of the long-range profiles: 1) a reverberative arrival between the first arrivals and the reflected arrival from the 410 discontinuity in the offset range 1500 to 2500 km; 2) a coda after the reflected 410 discontinuity arrival in the same offset range as above; 3) a previously unreported reverberative arrival with a long 10 to 15 second coda observed after the first arrivals in the offset range 2500 to 3500 km and 4) increased amplitudes close to the first arrivals in the offset range 2600 to 3000 km. To fit these observations we need a 520km discontinuity and two separate zones of small-scale heterogeneity : one from 320 km down to 410 km and one below 410 km down to at least 460 km and possibly as deep as 520 km. Both 2-D visco-elastic waveform modeling and reflectivity modeling suggest that local heterogeneity in the form of seismic scatterers with typical scale lengths of 20 km horizontally and 5 km vertically and velocity fluctuations of 0.2 km/s can explain the character of the arrivals. The cause of the heterogeneity may be local variations in composition interacting with the several phase transformations expected in the transition zone or the transformation of pyroxenes into the garnet mineral majorite. A further possibility is locally high iron content in the lower upper mantle leading to small scale heterogeneities.
DE: 7203 Body wave propagation
DE: 7218 Lithosphere and upper mantle
DE: 7219 Nuclear explosion seismology
SC: Seismology [S]
MN: 2003 Fall Meeting