Bevis, M.G., 1982, Hypocentral trend surface analysis: regional and fine structure of selected intermediate depth Benioff zones: Ithaca, New York, Cornell University, Ph.D. dissertation, 113 p.
Trend surfaces, constructed from spherical surface harmonic expansions, are fitted to sets of hypocenters via solution of the discrete linear inverse problem, in order to characterize both regional and local structure of selected Benioff zones. Hypocentral trend surfaces predict depth to the 'middle' of a Benioff zone, using (theta) and (phi) as predictors. Consideration of a trend surface cannot be divorced from consideration of the associated distribution of residuals. Hypocentral trend surfaces are generated for three high quality data sets obtained by local seismic networks in Honshu (Japan), Cook Inlet (Alaska), and S. Peru. Hypocenters beneath Honshu generate a bimodal distribution of residuals about their trend surface. The two 'planes' of the double Benioff zone are separated by about 30 km. No double zones are detected beneath Cook Inlet or S. Peru. Local data indicate the flat slab - steep slab transition beneath S. Peru is achieved by contortion rather than tearing of the slab. The seismic zones beneath Cook Inlet and S. Peru have thicknesses of about 25 km. Regional trend surfaces are established with teleseismic data from South and Middle America, and Tonga-Kermadec. We find no evidence for disjunctive platelets in these zones. Our data suggests subducted slabs remain coherent at intermediate depths, and have seismic thicknesses of about 30 km. The Benioff zone beneath S.W. Mexico flattens eastward and becomes aseismic before reaching 96°W. The Central American Benioff zone is relatively steep. Slabs beneath N. Chile and C. America are in downdip tension. The geometry of subducted lithosphere beneath Central and South America apparently controls the distribution of Quaternary volcanism in the overlying plate. There is no general association between flat slabs and magmatic gaps however. The flat slab - steep slab transition in Middle America (≈96°W) may relate to the difference in age of oceanic lithosphere either side of the Tehuantepec Ridge. The N. America-Caribbean plate boundary in Central America has no perceptible influence on the geometry of the underlying Benioff zone. Very strong delections of subducted Nazca plate beneath Ecuador and S. Peru are associated with consumption of the Carnegie and Nazca Ridges.
Theses and Dissertations
