As well as helping to piece together details about the end of the dinosaurs, the researchers said the findings offer insight into late Cretaceous geology.
“It was a global tsunami,” said Molly Range, a scientist at the University of Michigan and the study’s corresponding researcher. “Everyone saw that.”
Following the asteroid impact, there would be extreme water level rises in two phases, the team found: the rim wave and the subsequent tsunami waves.
“If you’ve just dropped a pebble in a puddle, there’s that initial splash; it’s the rim wave,” Range said.
Those rim waves could have reached an inconceivable height of a mile – and that’s before the tsunami really kicked in, the paper estimates.
“Then you see a wedge effect with the water being repelled symmetrically [from the impact site]”Range said, noting that asteroid Chicxulub struck in the Gulf of Mexico, just north of what is currently the Yucatán Peninsula.
After the first 10 minutes after impact, all airborne debris associated with the asteroid stopped falling into the gulf and displacing water.
“It had calmed down enough and the crater had formed,” Range said. This is around the time the tsunami started crossing the ocean at the speed of a commercial airliner.
“The continents looked a little different,” Range said. “Most of the east coast of North America and the north coast of Africa easily saw waves over 8 meters. There was no land between North America and the South America, so the wave went to the Pacific.
Range compared the episode to the infamous 2004 Sumatran tsunami which followed a magnitude 9.2 earthquake on the west coast of North Sumatra. More than 200,000 people died.
The megatsunami more than 60 million years ago had 30,000 times more energy than what happened in 2004, Range said.
To simulate the megatsunami, the team of scientists used a hydrocode, a three-dimensional computer program that models the behavior of fluids. Hydrocode programs work by numerically breaking down the system into a series of small Lego-like blocks and then calculating the forces acting on them in three dimensions.
The researchers relied on previous research and assumed the meteor was 8.7 miles in diameter and had a density of around 165 pounds per cubic foot – roughly the weight of an average adult man crammed into a volume the size of a milk crate. This means that the entire asteroid probably weighed around two quadrillion pounds – that’s a 2 followed by 15 zeros.
After the hydrocode produced a simulation of the initial stages of the impact and the first 10 minutes of the tsunami, the modeling was entrusted to a pair of models developed by NOAA to manage tsunami propagation in the world’s oceans. The first was called MOM6.
“Initially, we started using the MOM6 model which is a general-purpose ocean model, not just a tsunami model,” Range said. The team was forced to make assumptions about bathymetry, or the shape and slope of the seafloor, as well as the depth of the ocean and the structure of the asteroid’s crater. This information, along with the tsunami waveform from the hydrocode model, was pumped into MOM6.
In addition to building a model, study researchers examined geological evidence to study the path and power of the tsunami.
Range co-author Ted Moore found evidence of major disturbances in sediment stratification on ocean shelves and coasts at more than 100 sites, supporting the results of the study’s model simulations.
Modeling predicted tsunami flow speeds of 20 centimeters per second along most of the world’s coasts, more than enough to disturb and erode sediment.
The researchers said the geological findings added confidence to their model simulations.
In the future, the team hopes to learn more about the extent of the flooding that accompanied the tsunami.
“We would like to look at flooding, which we haven’t done just with this work in progress,” Range said. “You really need to know the bathymetry and topography.”
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