Publication Date:
2022-05-25
Description:
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution December 1997
Description:
A new tomographic technique is employed to investigate the structure and dynamics
of the Pacific upper mantle. We invert band-center travel times of ScS reverberations and
frequency-dependent travel times of direct S phases, upper-mantle guided waves such as
SS and SSS, and the R1 and G1 surface waves for the 2D composite structure in the plane
of two Pacific corridors. The frequency-dependent travel times of the turning and surface
waves are measured from all three components of ground motion as phase delays relative
to a radially-anisotropic, spherically-symmetric oceanic mantle model, and their 2D
Fréchet kernels are constructed by a coupled-mode algorithm. The travel times of the
primary ScSn and sScSn phases and their first-order reverberations from the 410 and 660
discontinuities are measured as individual phases and the 2D Fréchet kernels for these
band-limited signals are calculated using the paraxial ray approximation. The model
parameters include shear-speed variations throughout the mantle, perturbations to radial
shear-wave anisotropy in the uppermost mantle, and the topography of the 410 and 660
discontinuities.
We construct vertical tomograms through two mantle corridors: one between the
Tonga subduction zone and Oahu, Hawaii, which traverses the central Pacific Ocean;
and the other between the Ryukyu subduction zone and Oahu, which samples the
northern Philippine Sea, the western Pacific, and the entire Hawaiian swell. Tests
demonstrate that the data sets for the two corridors resolve the lateral structure in the
upper mantle with a scale length of a few hundreds kilometers and greater but that the
resolving power decreases rapidly in the lower mantle. The model for the Tonga-Hawaii
corridor reveals several interesting features, the most significant being a regular pattern of
high and low shear velocities in the upper mantle between Tonga and Hawaii. These
variations, which are well resolved by the data set, have a horizontal wavelength of 1500
km, a vertical dimension of 700 km, and an amplitude of about 3%, and they show a
strong positive correlation with seafloor topography and geoid-height variations along
this corridor. The geoid highs correspond to a series of northwest-trending swells
associated with the major hotspots of the Society, Marquesas, and Hawaiian Islands.
Where these swells cross the corridor, they are underlain by high shear velocities
throughout the uppermost mantle, so it is unlikely that their topography is supported by
thermal buoyancy. This result is substantiated by the model from the Ryukyu-Hawaii
corridor, which exhibits a prominent, fast region that extends beneath the entire Hawaiian
swell. This anomaly, which resides in the uppermost 200-300 km of the mantle, is also
positively correlated with the undulations of the Hawaiian-swell height. The other
dominant features in the Ryukyu-Hawaii model include the high-velocity subducting
slabs beneath the Ryukyu and Izu-Bonin seismic zones, which extend throughout the
entire upper mantle; a very low-velocity in the uppermost 160 km of the mantle beneath
the northern Philippine Sea, which is ascribed to the presence of extra water in this
region; and a pronounced minimum in the amount of radial anisotropy near Hawaii,
which is also seen along the Tonga-Hawaii corridor.
A joint inversion of the data from the two corridors reveals the same anomaly pattern
and clearly demonstrates that the swells in the Central Pacific are underlain by fast
velocities. It is therefore implied that the topography of the swells in the central Pacific is
supported by a chemical buoyancy mechanism which is generated by basaltic volcanism
and the formation of its low-density peridotitic residuum. While the basaltic depletion
mechanism can produce high shear velocities in the uppermost 200 km, it cannot explain
the depth extent of the fast anomalies beneath the swells which, along Tonga-Hawaii
corridor, extend well into the transition zone. It is therefore hypothesized that the central
Pacific is underlain by a system of convective rolls that are confined above the 660-km
discontinuity. It is likely that these rolls are predominantly oriented in the direction of
plate motion (like "Richter rolls ") but the limited depth of the fast anomaly beneath the
Hawaiian swell (200-300 km) suggests that their pattern is probably more complicated.
Nevertheless, this convection pattern appears to be strongly correlated with the locations
of the Tahitian, Marquesan, and Hawaiian hotspots, which raises interesting questions for
Morgan's hypothesis that these hotspots are the surface manifestations of deep-mantle
plumes.
Description:
This research was supported by the National Science Foundation under grant EAR-
9628351 and by the Defense Special Weapons Agency under grant DSW A-F49620-95-1-
0051.
Keywords:
Seismic tomography
;
Seismology
;
Upwelling
;
Ocean waves
Repository Name:
Woods Hole Open Access Server
Type:
Thesis
Format:
application/pdf
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