Giant tsunamis as result of asteroid impact

Lets talk about a nightmare. 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.

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.

March 16, 2880, is the day the asteroid known as 1950 DA, a huge rock two-thirds of a mile in diameter (1.1–1.4 km) , 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.

Asteroid risks are interesting because the probabilities are so small while the potential consequences are enormous.
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. 
“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,” says an UCSC scientist.

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,”
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 ( 120 meters) 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. It takes 8 hours for the waves to reach Europe, where they come ashore at heights of about 30 to 50 feet (6 to 15 meters).

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 the simulation is that it enabled  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,”

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) .
“Until we detect all the big ones and can predict their orbits, we could be struck without warning,” said Asphaug, a UCSC scientist. “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.