This phenomenon has fascinated scientists for years. Today, it’s well established that “solid Earth tides”, as they’re called, can trigger tremors (very slight earthquakes that cause little to no damage) virtually anywhere that tectonic tremors are found. Scientists have also found evidence (though that evidence is scant) that solid-Earth tides can trigger more substantial earthquakes.
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Studying how these low-frequency earthquakes respond to the tides can reveal new information about the San Andreas and what it might mean for larger earthquakes, researchers say.
Basically, this means that the team has found a way to check how much movement is happening in the deeper sections of the fault, which could help them predict how much movement will happen towards its top.
In case you need a refresher, the San Andreas Fault – which stretches some 1,287 kilometres (800 miles) through California – is a geological region where the Pacific Plate and the North American Plate meet.
However, the tidal forces are incredibly weak in comparison with the forces that arise from the tectonic plate motions, and there are only a few confirmed examples of a connection between quakes and tides, mostly from deep faults underneath the edges of the oceans.
Tides, the rise and fall of the seas, are primarily caused by the gravitational tug of the sun and moon on Earth. These earth tides stress the faults deep inside the planet, making it easier or more hard for faults to slip depending on the tidal phase.
“The hope is that looking at low-frequency earthquakes that happen deep in the fault will ultimately shed light on how shallow parts of the fault accumulate stress”, van der Elst said. And in 2004, scientists found that tidally triggered earthquakes could occur on some faults where ocean plates dive under continental plates.
“It’s nearly like having a lot of little creep meters embedded in the fault”, Shelly said.
Researchers from the US Geological Survey (USGS) have linked seismic activity along California’s San Andreas fault to gravitational pull from the sun and moon. “When we don’t see them, we don’t know what’s happening; we don’t know whether it’s slipping silently or not slipping at all”.
“So if all of a sudden, we saw that the deep part of the fault was slipping a huge amount, it might be an indication that there was an increased chance of having an quake come at the shallower part of the fault”. “By actually measuring that, we get an estimate of what that stress budget is”. But they could be used as instruments in a “natural lab”, van der Elst says.
Essentially, scientists now have a way to measure the fault’s recharge time in certain locations.
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But this tidal effect, which usually happens twice a month during the highest tides that correlate with the new and full moons, won’t necessarily apply to the Big One we’ve all been waiting for, Van der Elst says. “And the southern San Andreas Fault, in particular, looks like it’s locked, loaded and ready to go”, Jordan said while addressing the audience at the 2016 National Earthquake Conference in Long Beach.
