EARTH & ENVIRONMENT
Deep heat intensified mega-quake
Dried-out sediment magnified 2004 temblor’s destruction
B Y LAUREL HAMERS
Chemical transformations in minerals
deep beneath the seafloor could explain
why Indonesia’s 2004 mega-earthquake
was unexpectedly destructive, researchers report in the May 26 Science.
The magnitude 9. 2 quake and the tsunami that it triggered killed more than
250,000 people, flattened villages and
swept homes out to sea across Southeast
Asia. It was one of the deadliest earthquakes in recorded history.
“It raised a whole bunch of questions, because that wasn’t a place in the
world where we thought a magnitude 9
earthquake would occur,” says Brandon
Dugan, a geophysicist at the Colorado
School of Mines in Golden.
The thick but stable layer of sediment
where tectonic plates meet off the coast of
the Indonesian island of Sumatra should
have limited the power of an earthquake,
seismologists had predicted. But instead,
this quake was the third-strongest on
Dugan spent two months aboard a
research vessel with about 30 other scientists through the International Ocean
Discovery Program. The team drilled
down 1,500 meters below the seafloor in
two places off Sumatra, extracting narrow
cylinders of sediment. This sediment is
slowly moving toward the fault where
the 2004 quake occurred — a zone where
one massive tectonic plate slides over
another, pushing that plate downward.
Analyzing how sediment changes with
depth can provide a snapshot of the geologic processes at play near the fault zone.
Far beneath the seafloor, the researchers identified a sediment layer where the
water had a lower salinity than the water
in the sediment above or below. Since seawater seeping into the sediment would be
salty, the evidence of freshwater suggests
that the water must have instead been
released from minerals in the sediment.
For tens of millions of years, Dugan
proposes, minerals sat on the seafloor
taking in water — baking it into their
crystal structure. Then, more sediment
settled on top. It got toasty under such
a thick blanket of sediment, heating up
the minerals beneath. The temperature
increase triggered a chemical transfor-
mation within the sediment, pushing
water out of the mineral crystals and
into tiny pores between the grains.
The sediment sampled in this study
is still dehydrating. By the time any of it
reaches the plate boundary, Dugan says,
it’ll be buried under kilometers of more
sediment and will probably be completely dehydrated.
At first, the liberated water would have
softened the material, actually decreasing
the risk of a big earthquake by allowing it
to absorb more force, Dugan says. As the
sediment got closer to the fault over millions of years, though, the water flowed
away, leaving it brittle and unstable — the
perfect setup for a mega-quake.
The timing of this sediment dehydration process can make or break a quake.
Had the sediment near the fault been in a
softened state when the quake struck in
2004, the temblor might not have been
as deadly, Dugan says. But since enough
time had passed for it to become brittle,
the tectonic plates were able to rapidly
slip past each other for a much greater
distance during the quake. That massive
movement displaced the seafloor itself,
setting a tsunami into motion.
“It’s really the tsunamis from these
earthquakes that prove to be the deadliest,” says Roland Bürgmann, a seismologist at the University of California,
Berkeley who wasn’t part of the study.
And quakes that displace the seafloor are
far more likely to trigger tsunamis.
The findings could apply to other
faults with similarly thick sediment. But
more evidence is needed before applying
such analysis to faults beyond this one,
Bürgmann says. The Sumatran fault is
“only one data point. It doesn’t yet make
for a pattern.” s
black holes partner up. One theory is
that two neighboring stars each explode
and produce two black holes, which
then spiral inward. Another is that
black holes find one another within a
dense cluster of stars, as massive black
holes sink to the center of the clump
(SN Online: 6/19/16).
The new detection provides some
support for the star cluster theory: The
pattern of gravitational waves LIGO
observed hints that one of the black holes
might have been spinning in the opposite
direction from its orbit. Like a cosmic do-si-do, each black hole in a pair twirls on its
own axis as it spirals inward. Black holes
that pair up as stars are likely to have
their spins aligned with their orbits. But
if the black holes instead find one another
in the chaos of a star cluster, they could
spin any which way. The potentially mis-aligned black hole LIGO observed somewhat favors the star cluster scenario. The
measurement is “suggestive, but it’s not
definite,” says astrophysicist Avi Loeb of
Scientists will need more data to sort
out how black hole duos form, says physicist Emanuele Berti of the University
of Mississippi in Oxford. “Probably the
truth is somewhere in between.” Various
processes could contribute to the formation of black hole pairs, Berti says.
As with previous detections of gravitational waves, the scientists used their
measurements to test general relativity.
For example, while general relativity
predicts that gravitational waves travel
at the speed of light, some alternative
theories of gravity predict that gravitational waves of different energies travel
at different speeds. LIGO scientists
found no evidence of such an effect, vindicating Einstein once again.
Now, with three black hole mergers
under their belts, scientists are looking
forward to a future in which gravitational wave detections become routine.
The more gravitational waves scientists
detect, the better they can test their
theories. “There are already surprises
that make people stop and revisit some
old ideas,” Will says. “To me that’s very