The fault that produced the monster 9.0-magnitude earthquake off the
northwest coast of Japan in March 2011, triggering a tsunami and killing more
than 18,000 people, was formed in a layer of Pacific clay that is extremely thin
and particularly slippery, says new deep-sea research.
This clay is found throughout the northwestern Pacific Ocean, said Christie
Rowe, assistant professor with the department of earth and planetary sciences at
McGill University in Montreal.
“The risk of rare, very large earthquakes and tsunamis from Japan, Russia and
across the Aleutians may be greater than we knew before 2011,” she said in an
interview.
The thin, slippery clay lubricated the fault and made the earthquake grow
larger toward the ocean floor, which increased the potential for causing a large
tsunami, said Rowe.
It doesn’t mean there will be more earthquakes, but the clay may help an
earthquake get bigger once it starts.
Rowe is co-author of one of three papers published in the journal
Science on Friday. The papers are based on a months-long drilling
expedition that scientists undertook last year in the Pacific, which entailed
multiple stints on Chikyu, a deep-sea drilling vessel built by the Japanese
about a decade ago for sea-floor studies.
During the expedition, researchers also discovered that when the sea floor
east of Sendai, Japan, cracked open, a portion of the earth heaved upward — in
some places as much as an unprecedented 50 metres.
It’s the biggest slip known in an earthquake. (Slip is the total displacement
between two tectonic plates during an earthquake.)
Another surprise was that friction on this slippery fault was remarkably low
when the earthquake hit, said Emily Brodsky, a geophysicist at the University of
California, Santa Cruz, and a co-author of another of the papers. The low
resistance, she said, helps explain the gigantic slip during the
earthquake.
In Japan, earthquakes are not unusual. The archipelago is located in an area
where several continental and oceanic plates meet, causing frequent seismic
activity.
There have been more than 2,800 earthquakes in Japan since the big one in
March 2011.
But that one was the strongest to ever hit Japan and one of the five largest
earthquakes registered anywhere since modern seismological record-keeping began
in 1900.
The earthquake
and the tsunami that followed destroyed more than 125,000 buildings and
triggered a meltdown at the Fukushima nuclear reactor. Almost three years later,
the cleanup is still ongoing.
One major reason scientists undertook this expedition was to figure out the
friction so they could understand how much the fault could potentially move in
the future.
“You can only measure friction after an earthquake,” Brodsky said. “You take
measurements, including temperature of the fault. Measuring heat is a good way
to measure friction.”
Now that scientists know this fault is slippery, it raises the question of
whether other faults with similar clay content could correlate with particularly
large slips, said Brodsky.
It also raises the question of whether another earthquake on that fault could
be as strong or even worse.
Brodsky said that area in Japan will likely have another 9.0-magnitude
earthquake eventually, but not in the near future. “It does appear that all the
stress on the fault was released in this event (earthquake), so it is very
unlikely that you would have another one right away at this location.”
The expedition that pulled the veil from the monstrous earthquake was in
itself remarkable.
Scientists on Chikyu drilled across the fault and installed a temperature
observatory in one of three boreholes almost seven kilometres under the seabed.
“We not only made a hole, we pulled rocks out of it, we put pipes into the
hole, we put instruments into the hole, we took temperature measurements,” said
Brodsky. “It was a big challenge.”
The ship carried eight kilometres of pipe in 11-metre sections. Once it left
port, the drilling crew fitted the pieces together and, at the drill site,
lowered it into the ocean through a hole in the centre of the ship.
It worked well except during storms, said Rowe.
That’s when the iron pipe became a pendulum that could swing around and
damage the ship. “When big swells came in, we had to disassemble the pipe, piece
by piece, from the top, to reduce the amount of drag. When the storm passed, we
built it again so we could continue drilling.”
It was also challenging to locate the drill bit when it was “dangling on . .
. kilometres of pipe, which sways under the boat and is pulled by currents,”
said Rowe.
“It was an extraordinary feat of engineering and navigation.”
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