A tsunami striking the U.S. mainland might seem far-fetched, but scientists say preparation is crucial because it will happen — it’s just a matter of when.
/ July 12, 2017
An ocean wave pulls away from the shore and then, as expected, it moves toward land again. But it keeps moving farther and farther inland. The water pushes over unsuspecting beachgoers, backyards and entire cities with startling speed. It leaves a wake of destruction in Indonesia that includes an estimated 230,000 deaths.
Several years later, a similar scene unfolds in Japan when ocean water flows onto land to submerge cars, homes and even a nuclear power plant that never again will return to functionality. That time, the flood waters claim approximately 16,000 lives.
The mind-boggling force of a tsunami is a horrifying spectacle, as the world witnessed in 2004 and 2011. Those disasters ingrained heart-wrenching images of water-borne tragedy into people’s minds around the world. For many Americans, though, such images depict a rare occurrence in far-off countries and not a phenomenon in the continental United States. But the reality is that a tsunami could happen here, and it would be equally devastating.
In fact, disaster researchers say preparation is essential because a tsunami will strike a U.S. coast — it’s just a matter of when.
Generally speaking, the formation of a highly damaging tsunami relies on an earthquake occurring along the ocean floor, with that earthquake reaching at least magnitude 7, but more likely magnitude 8 or 9. Other triggers include a massive landslide or a meteor strike.
Many factors play into a tsunami’s severity, but scientists study models for two main kinds of events: distant and local. A distant tsunami originates from a source that’s at least 620 miles — or more than three tsunami travel hours — away. Local tsunamis affect land within 62 miles of the trigger point and take less than an hour to reach shore.
A distant tsunami only involves a water event, but most models for local tsunamis first show a catastrophic earthquake capable of reverberating nearly all of the coast for many minutes. “If we have the 9.0 for up to five to 10 minutes, we’re talking about infrastructure not standing in almost any scenario,” said Oregon Office of Emergency Management Deputy Director Matt Marheine. It requires “a mass evacuation to get people off the coast and to a place where we can take care of them.”
The earthquake alone will take down structures from buildings to bridges, and the tsunami that follows can topple additional structures. Regardless of whether the tsunami is distant or local, the rushing water packs a punch. “Tsunami currents are a lot stronger than a typical ocean wave. That’s what really does a lot of damage to a lot of structures,” said U.S. Geological Survey Research Geophysicist Eric Geist. “A wave as low as … one-and-a-half feet could knock somebody down.”
Tsunamis typically involve multiple long waves coming ashore. They can remain relatively shallow or form a towering tidal wave higher than 50 feet tall. The waves are capable of washing miles past the coastline if terrain conditions are right.
But each disaster is different. Not all tsunamis exhibit a tidal wave and “sometimes the first wave isn’t the most damaging,” said Ryan Arba, earthquake and tsunami program branch chief at the California Governor’s Office of Emergency Services. The level of uncertainty with tsunamis means emergency managers should anticipate additional response planning compared to more predictable emergencies.
Just because a certain type of natural disaster traditionally doesn’t occur often in a particular area doesn’t equate to immunity. Tsunamis are quite rare, but not unprecedented, in the Atlantic Ocean.
Consider that Puerto Rico frequently experiences earthquakes. They’re relatively weak and have not produced a devastating tsunami in recent years, but that potential exists. Scientists note that the East Coast sometimes experiences “mini-tsunamis” from these earthquakes, but the waves are so small they don’t garner much attention outside the research sphere.
“If there was a very strong earthquake off of northern Puerto Rico … it would not be impossible that you could have a tsunami that would run from there to the southeastern United States,” said John Ebel, senior research scientist at Boston College’s Weston Observatory. However, the more probable East Coast tsunami scenario lies farther north because “we have more offshore earthquakes off the northeastern part of country than we do off the southeast,” he said.
Researchers also note an increase in the Northeast’s seismic activity. “From the middle of New Jersey due east into the Atlantic Ocean to the edge of continental shelf … [up] to the Northeast, we have had much more quake activity over last 40 years than we have in the past,” Ebel said. “On that basis alone, we would estimate there’s more probability of a tsunami due to a local earthquake.”
An earthquake or landslide near Africa or Europe also could cause a distant East Coast tsunami. That’s what happened in 1755 with an estimated magnitude 8.5 to 9 earthquake that originated in Lisbon, Portugal. But according to Geist, “What we really worry about are the big landslides off the U.S. East Coast that might generate a large tsunami. … That’s a very low probability, but a high-impact type of event.”
Although the East Coast stands to face significant losses in a tsunami, the West Coast threat receives more attention due to its higher probability. The West Coast and the Pacific Ocean experience frequent, strong seismic activity that could trigger a tsunami.
Researchers watch the Cascadia subduction zone particularly closely. The 620-mile ocean fault off the West Coast stretches from Northern California to Vancouver Island in Canada. Many scientists believe the region’s tectonic plates’ convergence and movement will cause an earthquake so strong it produces a local tsunami.
“This [Cascadia event] will be a big deal,” said Patrick Corcoran, coastal natural hazards specialist at Oregon State University.
Emergency managers consistently face the challenge that people aren’t interested in devoting significant time and effort to planning for rare events. Such is the case with devising tsunami plans in the United States. The inability to prevent or predict tsunamis turns emergency managers’ focus solely to disaster mitigation.
Local, state and federal offices often work collaboratively to assess an area’s tsunami risk, draw up a response plan and then educate the public about what to do during a tsunami warning. One notable collaboration is the National Weather Service’s TsunamiReady, a voluntary community recognition program promoting preparedness and collaboration.