HR: 13:55h
AN: V33D-02    [Abstracts]
TI: A Global Lithosphere-Asthenosphere Boundary?
AU: * Rychert, C A
EM: crychert@ucsd.edu
AF: Scripps Institution of Oceanography, University of California, San Diego 9500 Gilman Drive, La Jolla, CA 92093-0225, United States
AU: Shearer, P M
EM: pshearer@ucsd.edu
AF: Scripps Institution of Oceanography, University of California, San Diego 9500 Gilman Drive, La Jolla, CA 92093-0225, United States
AB: The notion that a rigid lithosphere moves over a weaker asthenosphere is widely accepted. Indeed, a seismically fast lid above a slower asthenosphere is observed regionally and globally by surface-wave studies. Furthermore, recent P-to-S (Ps) and S-to-P (Sp) studies have imaged the boundary on a regional scale at depths that are in general agreement with surface-wave studies. The Ps and Sp conversions provide high-resolution imaging of the lithosphere-asthenosphere boundary, which, where modeled, suggests that the boundary is sharp, occurring over depths of 11 km or less. Yet a global map of this fundamental boundary has yet to be determined since its depth and other properties likely vary among tectonic environments. We use scattered wave energy recorded at permanent seismic stations from 1990-2004 to map sub-Moho interfaces that might be associated with the lithosphere-asthenosphere boundary. Stacks of high-quality Ps data binned by epicentral distance at some individual stations suggest the presence of one or two negative discontinuities (velocity decreases with depth) at depths ranging from 60 to 110 km, arriving after the Moho Ps phase and before the first reverberated Moho phase. Sp results also generally confirm the presence of these discontinuities. These single-station results do not cover a global swath, but they do suggest that a discontinuity at these depths is a persistent feature for which regional or global imaging may be possible. Therefore, we have processed the entire IRIS FARM dataset to look for common global and regional features. In data binned by epicentral distance, direct conversions from the Moho and the 410- and 660-km discontinuities are clearly visible. The second crustal reverberation (Ppss) is also visible, whereas the first crustal multiple (Ppps) is not apparent. In addition, energy that is not related to crustal multiples is seen at 60 to 110 km depth, and is consistent with the existence of a velocity drop at these depths. The interference of this negative-polarity phase with the first crustal multiple, which has positive polarity, may explain the unexpected absence of this crustal phase. Data that are collapsed into a single trace by stacking along predicted Ps travel-time curves show a similar pattern. Direct conversions from the Moho, 410, and 660 are all apparent. The crustal reverberations are muted, as expected given their moveout. As in the single-station results, a significant anomaly is seen corresponding to Ps conversions from 60 to 110 km depth. Sp stacks also support the existence of a discontinuity in this depth range. Synthetic modeling of Ps indicates that this phase is too broad to be explained by a sharp boundary (0-5 km) at a single depth. However, a broader boundary, or more likely, a boundary that varies in depth with respect to location may better explain the character of this phase.
DE: 7203 Body waves
DE: 7208 Mantle (1212, 1213, 8124)
DE: 7218 Lithosphere (1236)
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting