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.



NOAA’s National Tsunami Hazard Mitigation Program cooperates with U.S. communities of all sizes to assist with emergency management activities to reduce the impact of tsunamis, such as probabilistic modeling, educating the public and monitoring conditions following a tsunami. The program also provides funding for community response plans.



Education campaigns include efforts to inform schools, businesses and hotels about evacuation routes and procedures. Many municipalities’ leaders walk evacuation routes to ensure evacuation route signs are in place and clear, and to note any areas for improvement. Some California beaches are among those that installed tsunami kiosks with extensive information about evacuating inundation zones, which especially helps visiting tourists. But despite all the measures already in place, “We can always do more and reach more people,” Arba said. “We continue to support and expand wherever possible.”



According to Norfolk, Va.’s Director of Emergency Preparedness and Response, Jim Redick, tsunami responses are “all about how we’re able to alert and notify the community.”



Some communities incorporate sirens as part of their tsunami notification plan, but that is falling somewhat out of favor in the United States, as it is with certain municipalities that use tornado sirens. Part of the problem is the cost of siren installation and maintenance, but there’s more to the issue. “The second thing is confusion,” Redick said.



A blaring siren indicates different things to different populations and isn’t necessarily specific enough to prompt people to take immediate, appropriate action. Norfolk, a TsunamiReady community, houses the world’s largest U.S. naval station, and nearby residents therefore might believe a siren indicates a military emergency. But in another part of the city a siren signals an incident at the nuclear power plant. “Rather than trying to continuously educate the community about what [a siren] means, it’s just easier to contact them directly and tell them what the threat is,” Redick said.



Modern communication eases that direct contact through means including notification emails or texts sent to system subscribers. Another tactic involves pushing wireless emergency alerts to every mobile device within range of a regional cell tower. “That allows us to reach people we never would have been able to before,” such as tourists who haven’t signed up for a community’s emergency alerts, Redick said.



Even though the United States has not experienced a devastating tsunami in modern times and therefore doesn’t have experiences to draw from, emergency managers can learn from other countries’ successes rather than starting from scratch.



Oregon communities have instituted the Tsunami Blue Line program, an idea borrowed from a similar project in New Zealand that involves painting blue lines on the pavement to indicate the minimum point citizens should evacuate past during a tsunami warning. It’s a visible, easily understood reminder to residents: “Don’t slow down until you pass this line because a 50-foot wave will make it here,” Marheine said.



Japanese communities instruct residents to be self-reliant and to immediately execute personal evacuation plans during a tsunami warning, instead of relying on instructions from government officials, Corcoran said. “In Japan, although 16,000 tragically died [in 2011], there were 200,000 in the inundation zones at the time. So 90 percent of the Japanese evacuated effectively,” he said. “People can do this.”



Citizens must understand the importance of staying at higher ground until authorities issue the all-clear. “People in the past have had injuries or loss of life because the first wave comes in and people go back [home], and then another wave comes,” Arba said.



Assessing and communicating the ongoing threat takes cooperation among all levels of government, but “we encourage the public to listen to their local officials,” Arba said. “They are the ones who would have the [best information], through working with [state officials] and the National Weather Service.”



Emergency managers need to anticipate the worst after a tsunami, Arba said, meaning they should plan for infrastructure assessments, re-establishing ingress and egress, “power outages, logistical needs, food and shelter, all of those things we would plan for in a large event.” Those are all short-term issues that don’t consider factors like long-term sheltering.



Self-reliance is necessary during water disasters more than some other emergencies because floods render useless traditional tactics for aiding citizens in crisis. For example, floods prevent first responders from being able to drive vehicles to a rescue scene.



The Oregon Office of Emergency Management instructs people to plan for two weeks of isolation and self-sufficiency following a tsunami. “We’re going to be on our own,” Corcoran said. “The bigger it is, the longer and more isolated we’re going to be.”



That’s concerning considering “the number of people needing emergency services is going to be extremely high,” Marheine said. “First, figure out a way to get fuel and capability back into the system because most of the lifelines into those communities will be destroyed.” After that, establish “points of distribution and triage. … Set up these lily pads of resources that can help as many people as possible, and then start branching out from there.”



As is the case with other disaster and emergency situations, mitigating a tsunami’s effects primarily centers on advance preparation and getting resident buy-in through continuous engagement. “Public education is a great challenge, to try to reach everyone,” Marheine said. “We’re proud of our efforts to not only develop an understanding of [tsunamis], but also that the citizens are taking a role in preparedness.”

