The colored circles on the large map indicate the complex spatial rupture pattern as a function of time during the Sumatra earthquake in April 2012. The white star indicates the epicenter of the magnitude-8.6 mainshock. The area shaded in darker red in the inset indicates the location of the area of study. Credit: Caltech/Meng et al. |
The
powerful magnitude-8.6 earthquake that shook Sumatra on April 11, 2012,
was a seismic standout for many reasons, not the least of which is that
it was larger than scientists thought an earthquake of its type could
ever be. Now, researchers from the California Institute of Technology
(Caltech) report on their findings from the first high-resolution
observations of the underwater temblor, they point out that the
earthquake was also unusually complex—rupturing along multiple faults
that lie at nearly right angles to one another, as though racing through
a maze.
The
new details provide fresh insights into the possibility of ruptures
involving multiple faults occurring elsewhere—something that could be
important for earthquake-hazard assessment along California’s San
Andreas fault, which itself is made up of many different segments and is
intersected by a number of other faults at right angles.
“Our
results indicate that the earthquake rupture followed an exceptionally
tortuous path, breaking multiple segments of a previously unrecognized
network of perpendicular faults,” says Jean-Paul Ampuero, an assistant
professor of seismology at Caltech and one of the authors of the report,
which appears online today in Science
Express. “This earthquake provided a rare opportunity to investigate
the physics of such extreme events and to probe the mechanical
properties of Earth’s materials deep beneath the oceans.”
Most
mega-earthquakes occur at the boundaries between tectonic plates, as
one plate sinks beneath another. The 2012 Sumatra earthquake is the
largest earthquake ever documented that occurred away from such a
boundary—a so-called intraplate quake. It is also the largest that has
taken place on a strike-slip fault—the type of fault where the land on
either side is pushing horizontally past the other.
The
earthquake happened far offshore, beneath the Indian Ocean, where there
are no geophysical monitoring sensors in place. Therefore, the
researchers used ground-motion recordings gathered by networks of
sensors in Europe and Japan, and an advanced source-imaging technique
developed in Caltech’s Seismological Laboratory as well as the Tectonics
Observatory to piece together a picture of the earthquake’s rupture
process.
Lingsen
Meng, the paper’s lead author and a graduate student in Ampuero’s
group, explains that technique by comparing it with how, when standing
in a room with your eyes closed, you can often still sense when someone
speaking is walking across the room.
“That’s
because your ears measure the delays between arriving sounds,” Meng
says. “Our technique uses a similar idea. We measure the delays between
different seismic sensors that are recording the seismic movements at
set locations.”
Researchers
can then use that information to determine the location of a rupture at
different times during an earthquake. Recent developments of the method
are akin to tracking multiple moving speakers in a cocktail party.
Using
this technique, the researchers determined that the three-minute-long
Sumatra earthquake involved at least three different fault planes, with a
rupture propagating in both directions, jumping to a perpendicular
fault plane, and then branching to another.
“Based
on our previous understanding, you wouldn’t predict that the rupture
would take these bends, which were almost right angles,” says Victor
Tsai, an assistant professor of geophysics at Caltech and a coauthor on
the new paper.
The earthquake ruptured along multiple faults. Dotted lines indicate interpreted fault planes. Colored arrows indicate the direction of rupture. Credit: Caltech/Meng et al. |
The
team also determined that the rupture reached unusual depths for this
type of earthquake—diving as deep as 60 km in places and delving beneath
the Earth’s crust into the upper mantle. This is surprising given that,
at such depths, pressure and temperature increase, making the rock more
ductile and less apt to fail. It has therefore been thought that if a
stress were applied to such rocks, they would not react as abruptly as
more brittle materials in the crust would. However, given the maze-like
rupture pattern of the earthquake, the researchers believe another
mechanism might be in play.
“One
possible explanation for the complicated rupture is there might have
been reduced friction as a result of interactions between water and the
deep oceanic rocks,” says Tsai. “And,” he says, “if there wasn’t much
friction on these faults, then it’s possible that they would slip this
way under certain stress conditions.”
Adding
to the list of the quake’s surprising qualities, the researchers
pinpointed the rupture to a region of the seafloor where seismologists
had previously considered such large earthquakes unlikely based on the
geometry of identified faults. When they compared the location they had
determined using source-imaging with high-resolution sonar data of the
topography of the seafloor, the team found that the earthquake did not
involve what they call “the usual suspect faults.”
“This
part of the oceanic plate has fracture zones and other structures
inherited from when the seafloor formed here, over 50 million years
ago,” says Joann Stock, professor of geology at Caltech and another
coauthor on the paper. “However, surprisingly, this earthquake just
ruptured across these features, as if the older structure didn’t matter
at all.”
Meng
emphasizes that it is important to learn such details from previous
earthquakes in order to improve earthquake-hazard assessment. After all,
he says, “If other earthquake ruptures are able to go this deep or to
connect as many fault segments as this earthquake did, they might also
be very large and cause significant damage.”
Along
with Meng, Ampuero, Tsai, and Stock, additional Caltech coauthors on
the paper, “An earthquake in a maze: compressional rupture branching
during the April 11 2012 M8.6 Sumatra earthquake,” are postdoctoral
scholar Zacharie Duputel and graduate student Yingdi Luo. The work was
supported by the National Science Foundation, the Gordon and Betty Moore
Foundation, and the Southern California Earthquake Center, which is
funded by the National Science Foundation and the United States
Geological Survey.