The long-asked question of whether the moon's gravitational pull can cause earthquakes is one step closer to being answered.
Researchers from the University of Tokyo studied data from the past 20 years, measuring high tides and how strong the moon's pull was on the Earth's crust at those times. They focused on the two weeks before a large earthquake hit. And they found that some of the biggest quakes in history happened when the Earth's crust was under the highest tidal stress.
Satoshi Ide and colleagues noticed the Dec. 26, 2004 Sumatran earthquake, most notable for its horrendous, deadly tsunami, occurred near the time of full moon and spring tide. So did the Feb. 27, 2010 temblor in Maule, Chile. These quakes both happened close to the peak of tidal stress, when the moon and sun teamed up to exert the greatest gravitational influence over Earth. The March 11, 2011 Tohoku-Oki earthquake in Japan, which caused that country’s devastating tsunami, occurred during the neap tide, but the tidal stress was high at that time.
Research published in July 2016 also suggests that the constant gravitational dance between the Earth, the sun and the moon is capable of triggering earthquakes along tectonic plates.
The discovery, published in the Proceedings of the National Academy of Sciences, is the result of years of work by a team from the U.S. Geological Survey to better understand and predict the nature of earthquakes. Using a collection of extremely sensitive seismometers, the team was able to monitor quakes along California's San Andreas fault as deep as 19 miles underground.
Imperceptible to humans, these low frequency quakes were found to align with the rise and fall of the daily tides. The largest outbursts occurred during the twice-monthly spring tides, when the sun and moon align and exert the most gravitational pull. If that force happens to coincide with the direction of a particular fault, it can induce earthquakes.
"It's kind of crazy, right? That the moon, when it's pulling in the same direction that the fault is slipping, causes the fault to slip more — and faster," lead author Nicholas van der Elst told Los Angeles Times. "What it shows is that the fault is super weak — much weaker than we would expect — given that there's 20 miles of rock sitting on top of it."
To get a sense of how a fault like this works, you can get a better sense of this "tortured landscape" as it's described in the video below, which also offers a close-up look inside the fault itself:
The tide-triggered quakes have enlightened researchers as to how faults like the San Andreas operate. Researchers now know there exists a transition zone between the common, smaller temblors on the deep end and the shallow, more damaging quakes in the upper crust. By better understanding the frequency of the deep earth vibrations, we may one day achieve the as-yet elusive goal of earthquake prediction.
"Every little thing we learn about the way faults work may ultimately contribute to a better understanding of the earthquake cycle and when and where big earthquakes are likely to happen," van der Elst explained to LiveScience. "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."
Editor's note: This story has been updated since it was originally published in July 2016.