The Japanese word tsunami originally meant "harbor wave." Due to shifts in linguistic norms, its meaning has gotten more specific. Tsunamis are now defined as a series of large waves caused by sudden disturbances on the ocean floor. And when we say "large," we're not kidding around. The biggest waves can be 100 feet (30 meters) tall, 60 miles (100 kilometers) wide, and travel fast enough to cross an entire ocean within a single day.
Underwater volcanic eruptions can set off killer tsunamis, and so can landslides or — on rare occasions — meteorite impacts. But by far, earthquakes are the most common cause. To trigger a tsunami, you'll need a big one measuring 7 or higher on the Richter Scale.
So how can we explain tsunami-like waves that appeared in Lake Michigan in April 2018? Magnitude 7 earthquakes are almost unheard of in this part of North America and you won't find any active volcanoes under the Great Lakes. We can also rule out major landslides and big meteorites as potential culprits because none have been reported there lately.
Yet on April 13, 2018, two tsunami-like waves ripped across Lake Michigan's midsection. An animated video simulation of their progress was recently released by the Great Lakes Environmental Research Laboratory, an organization run by the National Oceanic and Atmospheric Administration (NOAA).
Near the coastal city of Ludington, Michigan, the water level rose by 13.9 inches (35.3 centimeters) in 42 minutes. This sudden influx completely submerged one of the breakwaters near the city. A photographer who captured the dramatic scene noted that the water level returned to normal 10 minutes later. Twenty-four miles (39 kilometers) to the north, in the city of Manistee, Michigan, some docks were damaged and a Riverwalk was flooded by the abrupt water fluctuations.
NOAA has retroactively classified the April 13 waves as meteotsunamis. These are tsunami-like events that can be quite destructive. But whereas real tsunamis are made by seismic activity or other geologic processes, meteotsunamis have weather-related origins.
When the latter appear, thunderstorms are usually responsible. Changes in wind and air pressure accompany the storms. The disturbances may generate sizable waves when a storm passes over some body of water. Those waves are liable to travel far away from the storm, meaning that meteotsunamis can appear without warning even on clear, sunny days.
Just like regular tsunamis, meteotsunamis are especially dangerous in bays and harbors. The shallow water causes the waves to slow down, which makes them increase in height. If the water enters a narrow inlet and starts sloshing around, the waves can get even taller.
The ones observed on Lake Michigan did not result in any human deaths. However, the lake has seen fatal meteotsunamis before: In 1954, a 10-foot (3-meter) wave of this sort killed seven fishermen at a Chicago pier. A world-wide nuisance, meteotsunamis have also taken lives in places like Japan's Nagasaki Bay. They've been documented crashing onto every continent except Antarctica.