Forsyth Donald W.

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Forsyth
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Donald W.
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  • Preprint
    Geophysical evidence from the MELT area for compositional controls on oceanic plates
    ( 2005-06-29) Evans, Rob L. ; Hirth, Greg ; Baba, Kiyoshi ; Forsyth, Donald W. ; Chave, Alan D. ; Mackie, Randall L.
    Magnetotelluric (MT) and seismic data, collected during the MELT experiment at the Southern East Pacific Rise (SEPR) constrain the distribution of melt beneath this mid-ocean-ridge spreading center and also the evolution of the oceanic lithosphere during its early cooling history. In this paper, we focus on structure imaged at distances ~100 to 350 km east of the ridge crest, corresponding to seafloor ages of ~1.3 to 4.5 Ma, where the seismic and electrical conductivity structure is nearly constant, independent of age. Beginning at a depth of about 60 km, there is a large increase in electrical conductivity and a change from isotropic to transversely anisotropic electrical structure with higher conductivity in the direction of fast propagation for seismic waves. Because conductive cooling models predict structure that increases in depth with age, extending to about 30 km at 4.5 Ma, we infer that the structure of young oceanic plates is instead controlled by a decrease in water content above 60 km induced by the melting process beneath the spreading center.
  • Article
    The Cascadia Initiative : a sea change In seismological studies of subduction zones
    (The Oceanography Society, 2014-06) Toomey, Douglas R. ; Allen, Richard M. ; Barclay, Andrew H. ; Bell, Samuel W. ; Bromirski, Peter D. ; Carlson, Richard L. ; Chen, Xiaowei ; Collins, John A. ; Dziak, Robert P. ; Evers, Brent ; Forsyth, Donald W. ; Gerstoft, Peter ; Hooft, Emilie E. E. ; Livelybrooks, Dean ; Lodewyk, Jessica A. ; Luther, Douglas S. ; McGuire, Jeffrey J. ; Schwartz, Susan Y. ; Tolstoy, Maya ; Trehu, Anne M. ; Weirathmueller, Michelle ; Wilcock, William S. D.
    Increasing public awareness that the Cascadia subduction zone in the Pacific Northwest is capable of great earthquakes (magnitude 9 and greater) motivates the Cascadia Initiative, an ambitious onshore/offshore seismic and geodetic experiment that takes advantage of an amphibious array to study questions ranging from megathrust earthquakes, to volcanic arc structure, to the formation, deformation and hydration of the Juan De Fuca and Gorda Plates. Here, we provide an overview of the Cascadia Initiative, including its primary science objectives, its experimental design and implementation, and a preview of how the resulting data are being used by a diverse and growing scientific community. The Cascadia Initiative also exemplifies how new technology and community-based experiments are opening up frontiers for marine science. The new technology—shielded ocean bottom seismometers—is allowing more routine investigation of the source zone of megathrust earthquakes, which almost exclusively lies offshore and in shallow water. The Cascadia Initiative offers opportunities and accompanying challenges to a rapidly expanding community of those who use ocean bottom seismic data.
  • Thesis
    Anisotropy and the structural evolution of the oceanic upper mantle
    (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 1973-09) Forsyth, Donald W.
    The dispersion of Love and Rayleigh waves in the period range 17-167 sec. is used to detect the change in the structure of the upper mantle as the age of the sea-floor increases away from the mid-ocean ridge. Using the single station method, the group and phase velocities of Rayleigh waves were measured for 78 paths in the east Pacific. The focal mechanisms of the source events were determined from P-wave first motion data and the azimuthal variation in Rayleigh wave amplitudes. In order to describe the observed Rayleigh wave dispersion, both a systematic increase in velocities with the age of the sea-floor and anisotropy of propagation are required. The maximum change in velocity with age is about 5%, with the contrast between age zones decreasing with increasing period. The greatest change occurs in the first few million years, due to the rapid cooling and solidification of the upper part of the lithosphere. In the 0-5 m.y. age zone, the average thickness of the lithosphere can be no greater than 30 km, including the water and crustal layers. Within 10 m.y. after formation, the lithosphere reaches a thickness of about 60 km. As the mantle continues to cool, the shear velocity within the lithosphere increases within the area of this study, no change occurs in the upper mantle deeper than about 80 km. Rayleigh waves travel fastest in the direction of spreading. The degree of anisotropy in Rayleigh wave propagation is frequency-dependent, reaching a maximum of 2.0 l 0.2 percent at a period of about 70 sec. Several models are constructed which can reproduce this frequencydependent anisotropy. The regional phase velocities of the fundamental and first higher Love modes have been simultaneously measured using a new technique. The squares of the difference between the observed phase and the predicted phase are sumed over 45 paths for a set of trial phase velocities. The trial velocities which give the minimum sum correspond to the average phase velocities of the fundamental and first higher modes. The Love wave data is inconsistent with the Rayleigh wave data unless SH velocity is higher than SV velocity within the uppermost 125 km of the mantle. Anisotropy deeper than 250 km is suggested, but not required, by the data.