“Seeing” Continental Drift
By: Stefi Weisburd
Since the theory of plate tectonics
was proposed 20 years ago, earth
scientists have relied on geologic
evidence averaged over millions of
years to guess the boundaries and
relative motions among the earth's
dozen plates -- the floating pieces of
the planet's outer shell, in which
Image 1: Africa and South America before and after
continental drift
continents and oceanic crust are embedded.
Remarkable advances in technology are now enabling scientists to "see" the motions of the
Earth's plates on a yearly basis. In addition to confirming the geologic results concerning
the large-scale motion of the plates, these new measurements are being used to home in on
the detailed interactions between plates at their boundaries
The two relatively new techniques use outer-space as a frame of reference. Energy signals
from distant stars are monitored at different stations on the Earth. The differences in the
arrival times for these signals help determine the movement of the plates between the
monitoring stations. In Satellite Laser Ranging (SLR), movement of the plates is measured
by comparing how long it takes for laser light pulses to leave the ground, bounce off a
satellite and return to the ground stations. This is similar to the techniques used by naval
submarines called sonar. Sound waves created by the submarine bounce off of objects and
back to the submarine. These signals are used to determine where objects are under water
and help the submarine navigate.
Thomas Herring at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.,
and his co-workers, is working with five years of measurements between sites on the North
American and Eurasian plates. The researchers estimate that the two plates are moving
apart at 1.9 centimeters per year (plus-or-minus 0.5 cm/yr). This compares well with the 1.7
cm/yr (plus-or-minus 0.3 cm/yr) geologic value based largely on the spacing between
magnetic stripes that were naturally
imprinted on the Atlantic seafloor.
Measurements of the relative motion
between the North American and
Pacific plates are consistent with
the geologic value of about 5.6 cm/yr
(plus-or-minus 0.3 cm/yr). The
Image 2: Alternating positive and negative bands
on seafloor
Pacific plate is sliding past the North American plate in a Northwest direction. The
movement of the two plates is responsible for earthquakes along California's San Andreas
Fault. In fact, the San Andreas Fault is often cited as a classic example of a sliding plate
boundary (transform plate boundary). However, 10 years ago, SLR measurements began to
reveal that the San Andreas Fault was moving at only 3.5 cm/yr. This "San Andreas
discrepancy" leaves about 2 cm/yr of movement that is taking place somewhere else. "We
want to know where that movement is because that is what gives us earthquakes," says
Thomas Jordan at MIT.
These new techniques for measuring plate movement will continue to help test and evaluate
the long-term pattern of plate movement in California. The real excitement for these
studies is that these techniques may catch shorter-term changes in plates prior to
earthquakes, especially now that NASA's Crustal Dynamics Project plans to make more
frequent measurements at fewer stations. "We have a cautious optimism," says Jordan,
"that these networks will lead to new insights to the rupture process of really big
earthquakes."