This high tide is bound to wash away more than just your sand castle. A new study has found that bulges in Earth's crust — solid Earth tides — trigger about 1 percent of earthquakes.
As Earth and the moon grind through their gravitational ballet, our planet gets tugged hard near the equator. The force is so strong that as the moon passes overhead each day, it pulls Earth's surface up 11.8 inches.
Scientists have known about this effect for over a century and have speculated that it might cause earthquakes. Writing in the journal Earth and Planetary Science Letters, Laurent Metivier of Paris Diderot University in France and a team of researchers now claim they've found a distinct connection between solid Earth tides and earthquakes.
The team analyzed the largest set of earthquake data ever assembled, a global record of 442,412 quakes since 1973. In amongst Earth's tiny shivers and mega-tremors they discovered a daily cycle: earthquake probability was enhanced as the moon passed overhead, pulling against the bedrock and, for a few hours at a time, easing the stress that normally keeps faults locked.
The effect is most pronounced in smaller and shallower earthquakes, and harder to detect in tremors above magnitude 4.0.
"Theoretically it will impact big earthquakes too," Metivier said. "But the main problem is that there aren't enough big earthquakes to make a correlation."
John Vidale of the University of Washington said bigger quakes may be less sensitive to tidal forces because they occur on huge faults that can extend deep into the crust. Below about 12.4 miles depth, rocks are under such pressure that tidal forces barely affect them.
Overall, the triggering effect is much weaker than expected, though. Tectonic plates build stress slowly over centuries, but the stress tides exert on faults each day is far greater. If earthquakes only happened the moment a fault reached a critical "breaking point" level of stress, they would always occur right as the moon exerts its maximum tidal force on the fault -- earthquake high tide.
"It's clear that tidal stress is much faster, so you'd expect every earthquake to be triggered that way," Vidale said. "But this is not what we see. There's a response time to loading -- it takes days of pushing before the fault gives way. This give us a number that shows just how hard it is to start an earthquake."