Despite what you might have read elsewhere, no, the San Andreas Fault is probably not going to suddenly slip and trigger the "Big One". Yes, it’s been shaking a little curiously recently, but it’s an active fault zone, so seismic activity is always expected to occur.
As picked up by the United States Geological Survey (USGS), a 4.6M quake shook California’s Monterey County nine days ago, and there have been 134 additional, weaker tremors around 5 kilometers (3 miles) of the original quake’s epicenter.
These tremors are occurring on what is known as a “creeping section” of the San Andreas Fault. This means that it’s always slipping, and seismic activity is run-of-the-mill here. Admittedly, a 4.6M event is unusually energetic, but this can easily be explained by a temporary snag that built up more stress than usual. The fault section is now returning to a state of normality.
No-one at the USGS is expecting that this is the precursor to the Big One in California. “It's a reminder that we live in earthquake country, I suppose,” Ole Kaven, USGS seismologist, told SFGate.
Yes, there is a 95-99 percent chance that there will be a 6.7M-level rupture in the next 30 years. There’s also a one-in-three chance of a 7.5M quake occurring in the next three decades, but this latest quake isn’t evidence that either of them is on its way.
It’s perfectly understandable that the worryingly quiet San Andreas Fault network, running through the heavily populated state of California, gets all of the attention from the media. A recent study, however, pointed out that the Cascadia Subduction Zone (CSZ) running off the coast of Oregon, Washington and western Canada may be the one to watch out for.
Although it’s impossible to tell what will rupture in a major manner first, the CSZ hasn’t experienced a subduction-related earthquake since 1700. Not only has it been building up stress ever since, but its geological architecture ensures that when it does slip, it will almost certainly be incredibly energetic.
The type of plate boundary ensures that, whatever happens in California, it’ll almost certainly never match up to the might of the CSZ.
The world’s most powerful earthquakes occur when one tectonic plate sudden slips beneath another – something that isn’t possible on the San Andreas Fault line, but is on the CSZ. The CSZ is also certain to trigger a tsunami when it juts forwards.
These Californian shakes may sound worrying, but if you’re really worried about the Big One, then you may need to consider that it’s not just San Francisco and Los Angeles in the firing line, but Seattle and Vancouver too.