Results from Prior NSF Support: William Menke. Award Number OCE-11965 Amount $304,958; Period 08/15/98-11/30/00, Title Active Seismic Imaging of Axial Volcano, PI's William Menke & Maya Tolstoy. The region of Axial Volcano, Juan de Fuca Ridge region provides an excellent opportunity to study the interplay between active "hot spot" and "mid-ocean ridge" magmatic systems. Important questions include how the two magma systems are fed; their magma and heat budgets; the degree of interconnectedness (or interaction) between them; their relationship to seismicity and geodetic strains; the role of each in plate-tectonic spreading and and crustal formation; and their effect on the geochemistry (e.g. mixing, fractionation) of erupted basalts. Information on the physical layout of the magma systems is critical to the study of each of these issues. The purpose of this research was to investigate these questions through the tomographic imaging of the region using seismic data from an active seismic airgun-to-obs experiment. The experiment was remarkably successful, both in the sense that voluminous high-quality data were obtained, and in the sense that very clear signals associated with magma were detected in that data. The key elements of the new three-dimensional compressional velocity model of the Axial and Coaxial magma systems are: 1. A Very Large Axial Magma Chamber; A smaller Coaxial Magma Chamber, unconnected with the one at Axial; 3. Several other small low velocityzones are possibly outlier magma chambers from Axial; 4. Strong thickening of the crust beneath Axial volcano. The crust thickens from about 6 km far from Axial to 8 km near Axial to 11 km beneath the summit (West 2001). Publications:Menke et al., Shallow crustal magma chamber beneath the axial high of the Coaxial Segment of Juan de Fuca Ridge at the "Source Site" of the 1993 eruption, Geology 30, 359-362, 2002.West, The deep structure of Axial Volcano, Ph.D. Thesis, Columbia University, 2001.West et al., Magma storage beneath Axial volcano on the Juan de Fuca mid-ocean ridge, Nature 25, 833-837, 2001. Results from Prior NSF Support: J. Gaherty, EAR-9814565, An Analysis of Upper Mantle Heterogeneity and Anisotropy in Western North America Using Recordings from Broadband Permanent and Temporary (PASSCAL) Seismic Stations; duration 1/99-12/00 (no cost to 12/01), $80,848. This grant funded several studies of the nature of seismic heterogeneity and anisotropy in the upper mantle. The primary effort has been a three-dimensional tomographic analysis of anisotropic heterogeneity beneath the California region [Gupta and Gaherty, 2000]. Models based on regional earthquakes show that radial anisotropy extends through the lithosphere with an average magnitude of about 2%, and the variability correlates with tectonics. Extension of this model to include azimuthal anisotropy required teleseismic data, which in turn required the development of a new array analysis [Freybourger et al., 2001]. We also investigated the lateral variation in upper-mantle anisotropy by characterizing radial anisotropy in distinct tectonic regimes. We found that models of tectonic and stable North America display over 6% variation in isotropic shear velocity in the upper 200 km, but have virtually identical radial anisotropy over path lengths of >2000 km [Hutko and Gaherty, 2000]. This implies that the significant contrast in thermal and mechanical properties does not correspond to a major difference in large-scale mantle fabric. Finally, we found that anisotropy associated with the northern EPR contrasts sharply with that along the Reykjanes Ridge, which most likely reflects hotspot-fueled buoyant upwelling beneath the ridge [Gaherty, 2001]. Freybourger, M., J.B. Gaherty, and T.H. Jordan, Upper-mantle structure of the Kaapvaal craton from surface-wave analysis, Geophys. Res. Lett., 28 , 2489-2853, 2001. Gaherty, J.B., Seismic evidence for hotspot-induced buoyant upwelling beneath the Reykjanes Ridge, Science, 293, 1645-1647, 2001. Gupta, P. and J.B. Gaherty, Upper-mantle anisotropy beneath California and the surrounding region, EOS Trans, 81, fall meeting supplement, 2000. (manuscript in preparation). Hutko, A. and J.B. Gaherty, Upper-mantle anisotropy beneath North America: Comparing active and stable tectonic regimes, EOS Trans, 81, spring meeting supplement, 2000. (manuscript in preparation). Results from Prior NSF Support: Vadim Levin (with Jeffrey Park) Grant EAR-9805206 ($248,117), Anisotropy and the Structural Geology of the Tectosphere (07/15/1998-07/01/2001). In this project we have used observations of shear-wave splitting and P-to-S mode conversions in teleseismic body waves to study the depth-dependent anisotropic structure of the lithosphere in a numberof locations. We have inferred tectonic mechanisms that are consistent with the structures implied by C-1data. Results obtained in stable continental regions (New England Appalachians and Arabian Shield) lead us to believe that continental lithospheric mantle preserves the rock texture from its last large-scale tectonic event. In studies of tectonically active regions (Kamchatka and Cascadian subduction zones) we were able to identify likely anisotropic signatures of ongoing geodynamic processes. The grant supported in part the M.Sc. thesis work of H. Yuan, and the following publications (in part or in full): Levin, V., and J. Park, 2000. Shear zones in the Proterozoic lithosphere of the Arabian Shield and the nature of the Hales discontinuity, Tectonophysics, v323, pp. 131-148. Levin, V., W. Menke and J. Park, 2000a. No regional anisotropic domains in the northeastern US Appalachians, JGR v105, pp. 19029-19042. Levin, V., J. Park, J. Lees, M. T. Brandon, V. Peyton, E. Gordeev, and A. Ozerov, 2002a. Crust and uppermantle of Kamchatka from teleseismic receiver functions, Tectonophysics, in press, 2002 Menke, W and V. Levin, A Waveform-based method for interpreting SKS splitting observations, with application to one and two layer anisotropic Earth models, submitted to GJI, 2002. Park, J., and V. Levin, Seismic anisotropy: Tracing plate dynamics in the mantle, Science, 296, 485-489, 2002. Peyton, V., V. Levin, J. Park, M. Brandon, J. Lees, E. Gordeev, A. Ozerov, Mantle Flow at a Slab Edge: Seismic Anisotropy in the Kamchatka Region, GRL, 28, pp 379-382, 2001. Yuan, H., J. Park and V. Levin, Skidmarks of trench-parallel terrane migration: Subduction-zone anisotropy structure under Corvallis, Oregon,