Fragmented Structure of Seafloor Faults May Dampen Effects of Earthquakes

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July 12, 2007

Many earthquakes in the deep ocean are much smaller in
magnitude than expected. Geophysicists from the Woods Hole Oceanographic
Institution (WHOI) have found new evidence that the fragmented structure of
seafloor faults, along with previously unrecognized volcanic activity, may be dampening the
effects of these quakes.

Examining data from 19 locations in the Atlantic, Pacific,
and Indian oceans, researchers led by graduate student Patricia Gregg
have
found that “transform” faults are not developing or behaving as
theories of plate tectonics say they should. Rather than stretching as
long, continuous fault lines
across the seafloor, the faults are often segmented and
show
signs of recent or ongoing volcanism. Both phenomena appear to prevent
earthquakes from spreading across the seafloor, thus reducing their
magnitude
and impact.

Gregg, a doctoral candidate in the MIT/WHOI Joint Program in
Oceanography and Oceanographic Engineering, conducted the study with
seismologist Jian Lin and geophysicists Mark Behn and Laurent Montesi, all from
the WHOI Department of Geology and Geophysics. Their findings were published in
the July 12 issue of the journal Nature.

Oceanic transform faults cut across the mid-ocean ridge
system, the 40,000-mile-long mountainous seam in Earth’s crust that marks the
edges of the planet’s tectonic plates. Along some plate boundaries, such as the
Mid-Atlantic Ridge, new crust is formed. In other regions, such as the western
Pacific, old crust is driven back down into the Earth.

If you imagine the mid-ocean ridge as the seams on a
baseball, then transform faults are the red stitches, lying mostly
perpendicular to the ridge. These faults help accommodate the motion and
geometry of Earth’s tectonic plates, cracking at the edges as the different pieces
of rocky crust slip past each other.

The largest earthquakes at mid-ocean ridges tend to occur at
transform faults. Yet while studying seafloor faults along the fast-spreading East
Pacific Rise, Gregg and colleagues found that earthquakes were not as large in
magnitude or resonating as much energy as they ought to, given the length of
these faults.

The researchers decided to examine gravity data collected
over three decades by ships and satellites, along with bathymetry maps
of the seafloor. Conventional wisdom has held that transform faults
should contain rocks that are colder,
denser, and
heavier than the new crust being formed at the mid-ocean ridge. Such
colder and
more brittle rocks should have a “positive gravity anomaly”; that is,
the
faults should exert a stronger gravitational pull than surrounding
seafloor
region. By contrast, the mid-ocean ridge should have a lesser gravity
field, because the crust (which is lighter than underlying mantle
rocks) is thicker along the
ridge and the newer, molten rock is less dense.

But when Gregg examined gravity measurements from the East
Pacific Rise and other fast-slipping transform faults, she was surprised to find
that the faults were not exerting extra
gravitational pull. On the contrary, many seemed to have lighter rock within
and beneath the faults.

“A lot of the classic characteristics of transform faults
didn’t make sense in light of what we were seeing,” said Gregg. “What we found
was the complete opposite of the predictions.”

The researchers believe that many of the transform fault
lines on the ocean floor are not as continuous as they first appear from
low-resolution maps. Instead these fault lines are fragmented into smaller
pieces. Such fragmented structure makes the length of any given earthquake rupture
on the seafloor shorter—giving the earthquake less distance to travel along the
surface.

It is also possible that magma, or molten rock, from inside
the earth is rising up beneath the faults. Earthquakes stem from the buildup of
friction between brittle rock in Earth’s plates and faults. Hot rock is more
ductile and malleable, dampening the strains and jolts as the crust rubs
together and serving as a sort of geological lubricant.

“What we learn about these faults and earthquakes underwater
could help us understand land-based faults such as the San Andreas in California or the Great Rift in eastern Africa,”
said Lin, a WHOI senior scientist and expert on seafloor earthquakes. “In areas
where you have strike-slip faults, you might have smaller earthquakes when
there is more magma and warmer, softer rock under the fault area.”

The findings by Gregg, Lin, and colleagues may also have
implications for understanding the theory of plate tectonics, which says that
new crust is only formed at mid-ocean ridges. By traditional definitions, no
crust can be created or destroyed at a transform fault. The new study raises
the possibility that new crust may be forming along these faults and fractures at
fast-spreading ridges such as the East Pacific Rise.

“Our understanding of how transform faults behave must be
reevaluated,” said Gregg. “There is a discrepancy that needs to be addressed.”

Funding for this
research was provided by the NSF Graduate Research Fellowship Program, the WHOI
Deep Ocean Exploration Institute, the NSF Ocean Sciences Directorate, and the
Andrew W. Mellon Foundation Awards for Innovative Research.

The Woods Hole Oceanographic Institution is a private,
independent organization in Falmouth,
Mass., dedicated to marine
research, engineering, and higher education. Established in 1930 on a
recommendation from the National Academy of Sciences, its primary mission is to
understand the oceans and their interaction with the Earth as a whole, and to
communicate a basic understanding of the ocean’s role in the changing global environment.