Big Quakes Signal Changes Coming To Earth's Crust
On
April 11 of this year, an extraordinary cluster of earthquakes struck
off Sumatra. The
largest shock, magnitude 8.7,
produced stronger ground-shaking than any earthquake ever recorded.
And it surprised seismologists by triggering more than a dozen
moderate earthquakes around the world.
NPR,
27
September, 2012
The
quakes are also a sign of big changes to come in the Earth's crust.
But
chances are you don't remember the April 11 quake off Sumatra. That's
because it was far offshore, so nobody got severely shaken, and it
didn't produce a killer tsunami. But the event didn't simply fade
away for Ross Stein at the U.S. Geological Survey in Menlo Park,
Calif.
"I
was having dinner with my wife at a restaurant a few hours after this
earthquake had occurred, and I kept getting paged," Stein says,
"and I would look and see that a large earthquake had occurred
somewhere around the world. And it just seemed very, very odd that we
were having this run of earthquakes of magnitude 5 and larger in this
short space of time."
Stein's
pager didn't stop after dinner. It kept going off for nearly a week,
because moderate quakes kept popping up all around the world.
As
Stein and his colleagues report
in Nature magazine,
the big quake triggered 16 moderate quakes. That defies conventional
wisdom. Big quakes trigger big aftershocks locally, but the distant
quakes they trigger are typically tiny.
It
turns out this was no ordinary quake.
"The
shaking produced by this earthquake is larger than any other
earthquake recorded, even though we have earthquakes such as
the magnitude-9.2
event in 2004,
which is 10 times larger than this one," Stein says.
The
energy from this quake was all released in about two minutes,
compared with eight minutes for the quake that triggered the deadly
Indian Ocean tsunami in 2004. And while most quakes of this size move
the ground up and down — which can cause tsunamis — ground
movement during the April quake was side to side.
"This
is the largest strike-slip, or San Andreas-like earthquake, ever
recorded," Stein says.
The
shock sent a series of long seismic waves around the world, shaking
the ground horizontally and ringing the planet like a bell. Those are
called Love
waves,
and Stein suspects they were strong enough to trip quakes on faults
that were pretty close to failing to begin with — not creating new
hazards, just changing the timing of quakes.
And
the story here isn't just about distant quakes. Seismologists
studying the April 11 quakes themselves were also in for a surprise.
"The
main shock, the 8.7 event, ruptured on four different faults, which
is really unusual," says Thorne Lay at the University of
California, Santa Cruz. He and his colleagues describe this in
a separate
paper in Nature.
Lay's team found that the quake zigzagged through a hash mark of
faults. And the quakes themselves were not where anybody expected
them to be.
"They
didn't happen where we typically observe very big earthquakes,"
Lay says. "It was not on the boundaries between tectonic plates,
where plates are rubbing against each other."
Instead,
Lay thinks we're witnessing the gradual evolution of a new boundary
between tectonic plates.
Here's
the big picture. Australia and India sit on the same vast chunk of
rock, along with a large swath of the seafloor. This makes up one
tectonic plate, "and it's all been moving together northward
pretty much coherently for many tens of millions of years," he
says.
The
western half of that plate has run up against Asia. That slow-motion
collision is pushing up the huge Himalayan mountain chain. But the
other half of that plate, to the east, is still moving north,
unimpeded.
"One
part got stuck," Lay says. "The other part kept going. And
that sort of wrenched the plate."
That
wrenching spot is where the April 11 events took place. Over millions
of years, Lay figures, a new plate boundary will form there.
"This
event is a very tiny step in that evolution. But it's geologically
fascinating to actually be able to record it in our lifetimes and see
that process going on," he says.
The
new plate boundary will appear only after thousands of more quakes
like the ones in April tear up the rock and shift the stresses that
are constantly reshaping our planet.
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