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.