[meteorite-list] Massive Tsunami Sweeps Atlantic Ocean In Asteroid Impact Scenario For March 16, 2880

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:25:45 2004
Message-ID: <200305272008.NAA21923_at_zagami.jpl.nasa.gov>

http://www.ucsc.edu/news_events/press_releases/text.asp?pid=355

May 27, 2003
Contact: Tim Stephens (831) 459-2495; stephens_at_ucsc.edu
UC Santa Cruz Press Release

Massive tsunami sweeps Atlantic Coast in asteroid impact scenario
for March 16, 2880

FOR IMMEDIATE RELEASE

SANTA CRUZ, CA--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 areas. A computer simulation of an asteroid impact tsunami developed
by scientists at the University of California, Santa Cruz, 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 the Institute of Geophysics and Planetary Physics at
UCSC, and Erik Asphaug, an associate professor of Earth sciences, report
their findings in the June issue of the Geophysical Journal International.

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 ago.

"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 said.

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," Asphaug
said. "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 journal
Science.

"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
of waves.

"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," Ward said.

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 Coast.

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 said.

_____

Notes for reporters (movies, images, additional information):

A movie of the tsunami simulation can be viewed at
http://es.ucsc.edu/~ward/1950-DA(5).mov.

Images can be downloaded from the web at
http://www.ucsc.edu/news_events/download/.

Reporters may contact Ward at (831) 459-2480 or ward_at_uplift.ucsc.edu, and
Asphaug at (831) 459-2260 or asphaug_at_es.ucsc.edu.

Additional information about the asteroid 1950 DA is available at:
http://neo.jpl.nasa.gov/1950da/.

The article by Ward and Asphaug in the Geophysical Journal International is
available online at:
http://es.ucsc.edu/~ward/papers/gji_final_35N.pdf.

                                    #####
Received on Tue 27 May 2003 04:08:24 PM PDT