A ferry boat rests inland amidst destroyed houses after a 9.0 earthquake and subsequent tsunami struck Japan March 11. (Photo by Lance Cpl. Garry Welch / U.S. Marine Corps) |
The
magnitude 9 earthquake and resulting tsunami that struck Japan on March
11 were like a one-two punch – first violently shaking, then swamping
the islands – causing tens of thousands of deaths and hundreds of
billions of dollars in damage. Now Stanford researchers have discovered
the catastrophe was caused by a sequence of unusual geologic events
never before seen so clearly.
“It
was not appreciated before this earthquake that this size of earthquake
was possible on this plate boundary,” said Stanford geophysicist Greg
Beroza. “It was thought that typical earthquakes were much smaller.”
The earthquake occurred in a subduction zone, where one great tectonic
plate is being forced down under another tectonic plate and into the
Earth’s interior along an active fault.
The
fault on which the Tohoku-Oki earthquake took place slopes down from
the ocean floor toward the west. It first ruptured mainly westward from
its epicenter – 32 kilometers (about 20 miles) below the seafloor –
toward Japan, shaking the island of Honshu violently for 40 seconds.
Surprisingly,
the fault then ruptured eastward from the epicenter, up toward the
ocean floor along the sloping fault plane for about 30 or 35 seconds.
As
the rupture neared the seafloor, the movement of the fault grew
rapidly, violently deforming the seafloor sediments sitting on top of
the fault plane, punching the overlying water upward and triggering the
tsunami.
“When
the rupture approached the seafloor, it exploded into tremendously
large slip,” said Beroza. “It displaced the seafloor dramatically.
“This
amplification of slip near the surface was predicted in computer
simulations of earthquake rupture, but this is the first time we have
clearly seen it occur in a real earthquake.”
Rupture begins at the epicenter and quickly shoots to the west for the first 40 seconds of the earthquake. Rupture then rockets eastward, as westward slip slows. After 75 seconds, eastward slip slows and westward slip picks up a little before it all dies down. (Refresh page to repeat animation.) Courtesy of Greg Beroza |
“The
depth of the water column there is also greater than elsewhere,” Beroza
said. “That, together with the slip being greatest where the fault
meets the ocean floor, led to the tsunami being outlandishly big.”
Beroza is one of the authors of a paper detailing the research, published online last week in Science Express.
“Now
that this slip amplification has been observed in the Tohoku-Oki
earthquake, what we need to figure out is whether similar earthquakes –
and large tsunamis – could happen in other subduction zones around the
world,” he said.
Beroza
said the sort of “two-faced” rupture seen in the Tohoku-Oki earthquake
has not been seen in other subduction zones, but that could be a
function of the limited amount of data available for analyzing other
earthquakes.
There
is a denser network of seismometers in Japan than any other place in
the world, he said. The sensors provided researchers with much more
detailed data than is normally available after an earthquake, enabling
them to discern the different phases of the March 11 temblor with much
greater resolution than usual.
Prior
to the Tohoku-Oki earthquake, Beroza and Shuo Ma, who is now an
assistant professor at San Diego State University, had been working on
computer simulations of what might happen during an earthquake in just
such a setting. Their simulations had generated similar “overshoot” of
sediments overlying the upper part of the fault plane.
Following
the Japanese earthquake, aftershocks as large as magnitude 6.5 slipped
in the opposite direction to the main shock. This is a symptom of what
is called “extreme dynamic overshoot” of the upper fault plane, Beroza
said, with the overextended sediments on top of the fault plane slipping
during the aftershocks back in the direction they came from.
1) Rupture of the fault plane begins at the epicenter. 2) Rupture travels westward, down the fault plane towards Honshu. The island suffers violent shaking for 40 seconds. 3) The upward sloping east side of the fault plane begins to rupture, continuing for 30 to 35 seconds. The sediments overlying the east side expand up the fault plane in response to the force of the rupture. 4) The water above the sediments is pushed into an unstable dome that then flows out in all directions as a tsunami. Illustration by Anna Cobb |
“We
didn’t really expect this to happen because we believe there is
friction acting on the fault” that would prevent any rebound, he said.
“Our interpretation is that it slipped so much that it sort of overdid
it. And in adjusting during the aftershock sequence, it went back a
bit.
“We don’t see these bizarre aftershocks on parts of the fault where the slip is less,” he said.
The
damage from the March 11 earthquake was so extensive in part simply
because the earthquake was so large. But the way it ruptured on the
fault plane, in two stages, made the devastation greater than it might
have been otherwise, Beroza said.
The
deeper part of the fault plane, which sloped downward to the west, was
bounded by dense, hard rock on each side. The rock transmitted the
seismic waves very efficiently, maximizing the amount of shaking felt on
the island of Honshu.
The
shallower part of the fault surface, which slopes upward to the east
and surfaces at the Japan Trench – where the overlying plate is warped
downward by the motion of the descending plate – had massive slip.
Unfortunately, this slip was ideally situated to efficiently generate
the gigantic tsunami, with devastating consequences.
Other
coauthors of the Science Express paper are Annemarie Baltay, a graduate
student in geophysics at Stanford, and Satoshi Ide, an associate
professor of Earth and planetary science at the University of Tokyo.
Funding for the research was contributed by the Japanese Society for the Promotion of Science.
