The 8.8-magnitude earthquake in the Pacific Ocean triggered tsunami warnings on different continents, but the water didn’t end up causing mass damage. So, how did one of the largest earthquakes on record not produce some of the most damaging tsunamis in history?
How earthquakes trigger tsunamis
The answer lies in the way earthquakes cause tsunamis in the first place.
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“There’s not a direct linear relationship between the size of the earthquake and the size of the tsunami, it’s what makes modeling and prediction so very difficult,” Chip Fletcher, Dean of the School of Ocean and Earth Science Technology at the University of Hawai’i at Manoa, told Straight Arrow News.
Fletcher explained how the depth of a quake within the Earth’s crust will have a large impact on the formation of a tsunami.
“A deep earthquake, although it might be very powerful, is not likely to have the same capacity for disrupting the water column as a shallow earthquake,” Fletcher said.
While the depth matters, so does the movement of the quake.
“Is the sea floor actually rupturing up and down, which could then displace this water column and generate a wave or a series of waves? Or is the sea floor, maybe the earthquake, is doing a different type of motion that doesn’t have as much capacity to perturb the water column,” Fletcher said.
The displacement of water is one of the biggest factors in determining the size and impact of a tsunami.
“What matters, in the end, for how large the tsunami is, is how much water is displaced, how much water is pushed up. Typically for these type of events, and that that amount of water, is related to how much the fault actually slipped at the Earth’s surface,” Elizabeth Cochran, seismologist at the U.S. Geological Survey (USGS), told Straight Arrow News.
Local impacts
The size of the tsunami also may not cause the same amount of damage, no matter where it hits, whether it’s Alaska, Kamchatka or California. Certain ecological factors can mitigate the impact of the wave.
“If you have a barrier reef, or if you have an extensive shallow platform offshore of the coastline, as we do at a couple of locations in Hawaii, then the energy of that wave can be dissipated far offshore before it actually reaches where humans have built houses and roads,” Fletcher said.
Cochran echoed that sentiment, saying the height of the tsunami is also relative to the coastline it’s impacting. She referenced the 2011 tsunami that hit Crescent City, California, that killed one person.
“It doesn’t have to be a giant wall of water to have pretty severe life-threatening consequences,” Cochran said. “You can kind of think of a tsunami as a flooding river, right? It has a lot of power, even though it doesn’t sound like much to say it’s a couple of feet high. Consider, if you’re walking through a rushing river only a couple of feet deep, it could knock you off your feet, and in this case, there’s lots of debris and everything else that can be quite dangerous.”
Predicting a disaster
While the waves did not end up causing any reported deaths and limited damage, warnings came out around the world for people to evacuate.
The National Oceanic and Atmospheric Administration (NOAA) is the agency that issues those warnings. Along with the size of the quake, they use data from buoys across the ocean floor.
“NOAA will look at buoy information. There’s sensors in the oceans distributed around the, for example, the Pacific Ocean, that will look at what the wave heights are as that tsunami starts to travel outwards away from the earthquake,” Cochran said.
Because there is no direct correlation between the size of an earthquake and an ensuing tsunami, scientists can’t have an exact prediction on how big or severe a tsunami will be. Which is why NOAA takes the better safe than sorry approach.
“Whenever we’re doing these types of estimations of what the hazards likely to be, we certainly don’t want to underestimate what the hazard could be,” Cochran said. “You don’t want to leave people in areas that could be dangerous. And so, we’re going to look at what the probabilities are for different wave heights.”
Fletcher also referenced those buoys and said, while they can help predict the size of the waves, there’s more that can be done to come up with more accurate predictions. Several videos came out of the traffic and backup in Honolulu when the evacuation warnings came out.
“We still haven’t quite got this right,” Fletcher said. “There was gridlock in Downtown Honolulu and a lot of locations. I have a person in my office that lives about a mile from here, it took her two hours to get home. So, a lot of this gridlock occurred on the surface streets of very low-lying Honolulu in the area of the tsunami evacuation zone. And, you know, at the same time that the first wave was hitting our shoreline, you had people trapped in cars a couple of blocks from the coast. So, there’s still some elements to this that our leaders and managers of emergency warning systems need to figure out and think hard about.”
