Why Didn’t Last Night’s Alaskan Quake Send A Tsunami To LA?

When it comes to natural hazards in Los Angeles, tsunamis are not at the top of the risk list.

However, there’s a reason why last night’s 8.2 magnitude earthquake in Alaska, had experts watching for a tsunami along California’s West Coast.

It struck along a part of the Aleutian subduction zone — where the Pacific Plate is being swallowed up by the North American Plate — that’s oriented in such a way that energy from quakes there is funneled directly towards us.

And we did see some action. For example, a 1.3-foot tsunami surge was recorded north of Los Angeles in Port San Luis on Thursday afternoon. No damage has been recorded.

So why didn’t we see dangerous waves materialize?

“We got kind of lucky,” said seismologist Lucy Jones, founder of the Lucy Jones Center for Science and Society, who worked on a US Geological Survey project that modeled nearly this exact same scenario back in 2013.

In their work, researchers found that a quake in that same spot could potentially cause damaging 4-foot waves to reach the Port of Los Angeles. Those waves, they said, could inundate low-lying areas like Balboa Island in Newport Beach and Mission Bay in San Diego with water.

The difference between last night’s quake and the one in their scenario is that it was much, much weaker.

“8.2 can cause a tsunami that does a lot of damage locally, but it really isn’t moving enough water to get big amounts of water moving across the ocean,” said Jones. Their modeled quake was a 9.1 magnitude temblor, roughly 22 times stronger than what we saw last night.

It’s not until magnitude 8.8 or 9.0 that you start to see far off tsunami damage, said Jones. The Great Alaskan quake of 1964, which caused damage in the Port of L.A., was a 9.2

Why The Tsunami Risk For SoCal Is Now Lower

The other reason why we didn’t see tsunami activity down here is because the rupture didn’t make its way all of the way to the sea floor, meaning much less water was displaced than could've been.

The 2011, 9.1 magnitude Tohoku quake in Japan was further away from California than last night's quake, yet it caused damage in Santa Cruz and Crescent City. Not only was it substantially bigger, but there was approximately 70 meters of displacement, meaning a lot of water had to go somewhere.

A sign with an arrow pointed upward reads: Tsunami evacuation routes in Seal Beach.

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Megan Garvey

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LAist

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“Now that we’ve had this earthquake it’s less likely we’ll get a really really big one out of this location," said Jones. "We’ve actually reduced the likelihood of a really bad tsunami for Los Angeles by this event happening.”

If you’re curious, about just how far inland we can expect water to go if we do get hit by a tsunami, you can check out this handy inundation map put out by the California Geological Survey. The good news is that much of our infrastructure along the coast is protected from inundation. But if you're lounging on the beach and you get a tsunami warning, it's probably best to pack up and head inland.