June 2, 2003
Massive tsunami sweeps Atlantic Coast in asteroid
impact scenario for March 16, 2880
By Tim Stephens
If an asteroid crashes into the Earth, it is likely to splash down
somewhere in the oceans that cover 70 percent of the planet's surface.
Huge tsunami waves, spreading out from the impact site like the ripples
from a rock tossed into a pond, would inundate heavily populated coastal
A simulation of the tsunami waves is available online.
A computer simulation of an asteroid impact tsunami developed by UCSC
scientists shows waves as high as 400 feet sweeping onto the Atlantic
Coast of the United States.
The researchers based their simulation on a real asteroid known to
be on course for a close encounter with Earth eight centuries from now.
Steven Ward, a researcher at UCSC's Institute of Geophysics and Planetary
Physics, and Erik Asphaug, an associate professor of Earth sciences,
report their findings in the June issue of the Geophysical Journal
March 16, 2880, is the day the asteroid known as 1950 DA, a huge rock
two-thirds of a mile in diameter, is due to swing so close to Earth
it could slam into the Atlantic Ocean at 38,000 miles per hour. The
probability of a direct hit is pretty small, but over the long timescales
of Earth's history, asteroids this size and larger have periodically
hammered the planet, sometimes with calamitous effects. The so-called
K/T impact, for example, ended the age of the dinosaurs 65 million years
"From a geologic perspective, events like this have happened many
times in the past. Asteroids the size of 1950 DA have probably struck
the Earth about 600 times since the age of the dinosaurs," Ward
Ward and Asphaug's study is part of a general effort to conduct a rational
assessment of asteroid impact hazards. Asphaug, who organized a NASA-sponsored
scientific workshop on asteroids last year, noted that asteroid risks
are interesting because the probabilities are so small while the potential
consequences are enormous.
Furthermore, the laws of orbital mechanics make it possible for scientists
to predict an impact if they are able to detect the asteroid in advance.
"It's like knowing the exact time when Mount Shasta will erupt,"
"The way to deal with any natural hazard is to improve our knowledge
base, so we can turn the kind of human fear that gets played on in the
movies into something that we have a handle on."
Although the probability of an impact from 1950 DA is only about 0.3
percent, it is the only asteroid yet detected that scientists cannot
entirely dismiss as a threat. A team of scientists led by researchers
at NASA's Jet Propulsion Laboratory reported on the probability of 1950
DA crossing paths with the Earth in the April 5, 2002, issue of the
"It's a low threat, actually a bit lower than the threat of being
hit by an as-yet-undiscovered asteroid in the same size range over the
same period of time, but it provided a good representative scenario
for us to analyze," Asphaug said.
For the simulation, the researchers chose an impact site consistent
with the orientation of the Earth at the time of the predicted encounter:
in the Atlantic Ocean about 360 miles from the U.S. coast. Ward summarized
the results as follows:
The 60,000-megaton blast of the impact vaporizes the asteroid and blows
a cavity in the ocean 11 miles across and all the way down to the seafloor,
which is about 3 miles deep at that point. The blast even excavates
some of the seafloor.
Water then rushes back in to fill the cavity, and a ring of waves spreads
out in all directions. The impact creates tsunami waves of all frequencies
and wavelengths, with a peak wavelength about the same as the diameter
of the cavity. Because lower-frequency waves travel faster than waves
with higher frequencies, the initial impulse spreads out into a series
"In the movies they show one big wave, but you actually end up
with dozens of waves. The first ones to arrive are pretty small, and
they gradually increase in height, arriving at intervals of 3 or 4 minutes,"
The waves propagate all through the Atlantic Ocean and the Caribbean.
The waves decay as they travel, so coastal areas closest to the impact
get hit by the largest waves. Two hours after impact, 400-foot waves
reach beaches from Cape Cod to Cape Hatteras, and by four hours after
impact the entire East Coast has experienced waves at least 200 feet
high, Ward said. It takes 8 hours for the waves to reach Europe, where
they come ashore at heights of about 30 to 50 feet.
Computer simulations not only give scientists a better handle on the
potential hazards of asteroid impacts, they can also help researchers
interpret the geologic evidence of past events, Ward said. Geologists
have found evidence of past asteroid impact tsunamis in the form of
inland sediment deposits and disturbed sediment layers in the seafloor
that correlate with craters, meteorite fragments, and other impact evidence.
An important feature of Ward's simulation is that it enabled him to
calculate the speed of the water flows created by the tsunami at the
bottom of the ocean--more than 3 feet per second out to distances of
several hundred miles from the impact.
"That's like a raging river, so as these waves cross the ocean
they're going to stir up the seafloor, eroding sediments on the slopes
of seamounts, and we may be able to identify more places where this
has happened," Ward said.
He added that the waves may also destabilize undersea slopes, causing
landslides that could trigger secondary tsunamis. Ward has also done
computer simulations of tsunamis generated by submarine landslides.
He showed, for example, that the collapse of an unstable volcanic slope
in the Canary Islands could send a massive tsunami toward the U.S. East
A tsunami warning system has been established for the Pacific Ocean
involving an international effort to evaluate earthquakes for their
potential to generate tsunamis. Ward said that asteroid impact tsunamis
could also be incorporated into such a system.
"Tsunamis travel fast, but the ocean is very big, so even if a
small or moderate-sized asteroid comes out of nowhere you could still
have several hours of advance warning before the tsunami reaches land,"
he said. "We have a pretty good handle on the size of the waves
that would be generated if we can estimate the size of the asteroid."
Planetary scientists, meanwhile, are getting a better handle on the
risks of asteroid impacts. A NASA-led campaign to detect large asteroids
in near-Earth orbits is about half way toward its goal of detecting
90 percent of those larger than 1 kilometer in diameter (the size of
1950 DA) by 2008.
"Until we detect all the big ones and can predict their orbits,
we could be struck without warning," said Asphaug. "With the
ongoing search campaigns, we'll probably be able to sound the 'all clear'
by 2030 for 90 percent of the impacts that could trigger a global catastrophe."
Rogue comets visiting the inner solar system for the first time, however,
may never be detected very long in advance. Smaller asteroids that can
still cause major tsunami damage may also go undetected.
"Those are risks we may just have to live with," Asphaug
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