Here's What You Need To Know About That Quake Swarm Near California And Oregon


Robin Andrews

Science & Policy Writer

Nothing to be alarmed about here. Bob Pool/Shutterstock

As has been widely reported in a huge variety of ways, an earthquake swarm has been detected off America's Pacific coast. These shakes off the coasts of Oregon and California state serve to remind us that this entire region is tectonically – seismically and volcanically – active, but they in no way suggest a massive quake, a “Big One”, is imminent.

An earthquake swarm describes a series of tremors, created by movement along a fault line, that occur closely in time and space. They’re collections of shakes that can mean very different things depending on when, where, and how they appear, and what other geological “symptoms” they happen alongside.


Earthquake swarms happen at Yellowstone, but that’s exactly what you’d expect to see above an active volcanic system, regardless of whether or not it’s a (hyped-up) supervolcano. Similarly, swarms along active fault lines, like those littering the shores of the western United States, aren’t too unusual either. So what’s the deal with these latest quakes?


As reported by Oregon Live using data from the United States Geological Survey (USGS), the 11 quakes ranged from 2.8 to 5.6M, and took place a fair way west of Crescent City, California, buried beneath the waves. They note Don Blakeman, a geophysicist at the National Earthquake Information Center, as saying that these sorts of quakes at these magnitudes are a common occurrence.

Those higher magnitudes can look scary, and often, the magnitude of quakes is highlighted by the press to suggest how supposedly terrifying they may be, but that’s misleading. Much depends on where they occur, and as ever, context is everything.

Quakes of similar magnitudes are rocking Kilauea’s summit fairly frequently, as magma drainage is triggering the collapse of the crater up there. They aren’t a threat, though, as they are taking place far from any human populations – the defining characteristic of whether something is a “hazard” or not.


In contrast, a 5.6M earthquake could indeed damage infrastructure and endanger human life if it occurs beneath a city. It would also have to be quite shallow, so much of the wave energy remains present and undissipated at the surface.

As noted by The Smithsonian, many other factors play into the story. The sediment type, for example, present beneath said city also matters. Below Mexico City, the sediments are unconsolidated and soggy, which means they can move very quickly if shaken enough – not ideal for the architecture above it.

So, magnitude aside, has this swarm got anything to do with the Big One? Nope.

As explained by geologist Erik Klemetti over at Discover, these were extensional earthquakes, the type that involve two tectonic plates – in this case, the Juan de Fuca and the Pacific plates – moving apart. Those that generate powerful tsunamis and tremors involve one plate descending under another in a subduction zone, or two plates sliding against each other, side by side.


The latter describes the San Andreas Fault; the former, the Cascadia Subduction Zone (CSZ). The swarm did not occur on either.

Those two are certainly worth worrying about. As time ticks on, there is a low but gradually increasing chance that one of them will slip in a major way and cause enormous damage to their prospective parts of the western seaboard.

The CSZ, in particular, hasn’t experienced a major subduction zone quake since 1700, when a huge quake and plate snap-back generated a tsunami that devastated parts of the coast and even went as far as Japan to wreak havoc. It became the stuff of legends for centuries until geoscientists eventually pieced the jigsaw together.

One day, it’s near-certain something similar will happen again – but not today, and perhaps not for centuries more. It’s impossible to say for sure when the CSZ will engage in such fury.


This earthquake swarm isn’t related to that, though. It’s just conventional, harmless tectonic shenanigans. Move along!


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