Researchers at two universities have used seismic
techniques and computer modeling to plumb some of the earth’s
deep mysteries — very, very deep, that is.
Scientists have known for at least two decades that certain seismic
waves travel faster through a few patches in the lowermost mantle,
but the reason for the unusual behavior has eluded them.
But now scientists at the University of Michigan and Yale University
are shedding new light on the phenomenon.
Earlier research offered two possible explanations for the anistrophy
observed in areas where subduction zones plunge to meet the core-mantle
boundary, some 700 kilometers below the earth’s surface.
Seismic shear waves could be split into two components either by
layering of materials with different chemical compositions, or by
alignment of mineral grains through which the seismic waves pass.
The new research, first reported earlier this year in Nature
magazine, indicates mineral grain alignment is the best explanation
for the anomalies roughly underlying Alaska and the Caribbean, according
to one of the paper’s authors, Peter van Keken, assistant professor
of geological sciences at the University of Michigan.
"We enhanced this picture (earlier research using seismic tomography),"
van Keken said.
The researchers used multiple computer models to test their theory,
van Keken said, "and they all seem to work."
While anistrophy is well-understood by explorationists working
in the earth’s crust, "anistrophy in the lower mantle is rather
special," he said.
The solid, flowing mantle is mostly isotropic with a few exceptions.
"The main thing we did was find an explanation consistent with
observations (of earthquake data) and computer modeling," van Keken
"The computer modeling shows results consistent with Newton’s
laws," he added, "which is always nice."
'A Confusing Place'
Van Keken said the odd behavior occurs in what amounts to "slab
graveyards," where materials from the crust and upper mantle plunge
toward the earth’s iron core.
The paper’s lead author, University of Michigan doctoral.
student Allen McNamara, said the "strange" seismic behavior is "not
in the air anymore." The research, which included work by Yale’s
Shun Karato, helps show how the mantle is flowing.
Karato served as the team’s specialist in deformation mechanisms
while McNamara and van Keken did the numerical modeling.
"The lower mantle is a confusing place," McNamara said, with high
temperature contrasts at the thermal boundaries.
Van Keken said he would like to see further testing of the models,
perhaps with ray tracing, to "close the loop back to seismology."
The findings "open the door to further work," McNamara said.
Although the experiments appear to explain some of the oddities
observed in the lower-mantle, another unusual patch deep beneath
the central Pacific, where seismic velocity is lower than average,
remains a mystery.
"We cannot explain it with the current model," van Keken said.
McNamara said the Pacific patch is an upwelling rather than a subduction
zone, and "you expect to see hot mantle welling up."
Mineral grain alignment does not appear to explain the behavior
there, so other causes — such as chemistry or melting — must
be explored, McNamara said.
While the deep mantle research does not have direct applications
for exploration, there are analogs in the near surface, van Keken
said, and enhancing understanding of the earth’s dynamics can
aid studies in all areas.
Much of the numerical modeling can be used in different areas of
study, McNamara said:
"The same equations control the processes."
"And the research trains students in complicated systems," van
Keken added. "It helps prepare them for difficult jobs — perhaps
in the petroleum industry."