Mystery tremors
detected deep within Earth
San Andreas Fault monitor picks up vibrations linked to plate
movement
By Ker Than
LiveScience
Updated: 9:45 p.m. ET April 13, 2006
Tremors deep inside the Earth are usually produced by magma flowing
beneath volcanoes, but a new study suggests they can also be
produced by the shifting and sliding of tectonic plates.
Scientists have recorded vibrations from underground tremors at a
geologic observatory along the San Andreas Fault, an 800-mile
(1,280-kilometer) scar in the earth that runs through California.
The fault marks the boundary between the Pacific Tectonic Plate and
the North American Plate.
Tectonic plates are large pieces of Earth's crust that bump and
grind like chunks of sea ice floating atop the ocean. Earth's
surface is made up of about 10 major tectonic plates and many more
minor ones.
Tremors are sustained vibrations that occur deep inside the
Earth.
"Unlike the sharp jolt of an earthquake, tremors within Earth's
crust emerge slowly, rumbling for longer periods of time,"
explained Kaye Shedlock, the program director for Earthscope at the
National Science Foundation.
EarthScope is a project investigating the structure and evolution
of the North American continent and the physical processes
controlling earthquakes and volcanic eruptions.
Normally, tremors are produced by the movement of magma in cracks
and other channels beneath volcanoes.
But there are no volcanoes located near the Earthscope San Andreas
Fault Observatory at Depth in Parkfield, Calif., where the new
tremors were recorded.
These are the first recordings of non-volcanic tremors deep inside
the Earth. They were recorded in deep boreholes that were drilled
down to a depth of about 2 miles (3.2 kilometers).
Instead of volcanoes, the scientists think the subterranean
rumblings might be caused by processes similar to those that
produce tremors near the Cascadia Subduction Zone, an active fault
that runs from mid-Vancouver Island to northern California.
Those tremors are caused by the sliding of the undersea Juan de
Fuca tectonic plate beneath the North American plate.
The two plates making up the San Andreas Fault are different from
those in the Cascadia Subduction Zone, however, in that they slide
past another, much like two cars moving very slowly in opposite
directions on a freeway, in what scientists call a "slip."
"Right now we have no recorded slip associated with the tremors, so
we haven't been able to see the other part," Earthscope facility
project director Greg van der Vink told LiveScience.
Earthscope researchers hope to link the two events definitively by
installing instruments called laser strainmeters inside the
borehole, which are capable of measuring slips as the tremors
happen.