In a new study published in Nature Geoscience yesterday, Japanese researchers have found that many of the largest earthquakes occurred near the time of maximum tidal strain — or during new and full moons when the Sun, Moon, and Earth align.
The possibility that tidal stress can trigger earthquakes is long debated. In particular, a clear causal relationship between small earthquakes and the phase of tidal stress is elusive. However, tectonic tremors deep within subduction zones are highly sensitive to tidal stress levels, with tremor rate increasing at an exponential rate with rising tidal stress, the authors said in the study's abstract.
"Thus, slow deformation and the possibility of earthquakes at subduction plate boundaries may be enhanced during periods of large tidal stress. Here we calculate the tidal stress history, and specifically the amplitude of tidal stress, on a fault plane in the two weeks before large earthquakes globally, based on data from the global, Japanese, and Californian earthquake catalogs. We find that very large earthquakes, including the 2004 Sumatran, 2010 Maule earthquake in Chile and the 2011 Tohoku-Oki earthquake in Japan, tend to occur near the time of maximum tidal stress amplitude."
Although this tendency is not obvious for small earthquakes, the authors have also found that the fraction of large earthquakes increases (the b-value of the Gutenberg–Richter relation decreases) as the amplitude of tidal shear stress increases.
"The relationship is also reasonable, considering the well-known relationship between stress and the b-value. This suggests that the probability of a tiny rock failure expanding to a gigantic rupture increases with increasing tidal stress levels. We conclude that large earthquakes are more probable during periods of high tidal stress."
For the 15 days leading up to each quake, Satoshi Ide, a seismologist at the University of Tokyo, and his colleagues assigned a number representing the relative tidal stress on that day, with 15 representing the highest, and found that large quakes such as those that hit Chile and Tohoku-Oki occurred near the time of maximum tidal strain - or during new and full moons when the Sun, Moon and Earth align, Nature explained in a news release published September 12. For more than 10 000 earthquakes of around magnitude 5.5, the researchers found, an earthquake that began during a time of high tidal stress was more likely to grow to magnitude 8 or above.
Honn Kao, a seismologist at the Geological Survey of Canada and Natural Resources Canada in Sidney said that this is a very innovative way to address this long-debated issue. “It gives us some sense into the possible relationship between tidal stress and the occurrence of big earthquakes. Perhaps the minuscule added strain of tides could be the final factor that nudges a geological fault into rupturing."
There are too many factors that contribute to triggering an earthquake to untangle what role tides might have, such as how stress transfers within the ground to cause a geological fault to move, and the current study will not be the final word on the matter, Kao said.
John Vidale, a seismologist at the University of Washington in Seattle who helped to debunk some of the more tenuous tide–earthquake claims, said that the results are plausible.
"They've done a very careful job," Vidale said. However, the discovery does not affect how societies should prepare for possible earthquakes. "Even if slightly enhanced by the tides, the probability of a quake happening on any particular day in an earthquake-prone region remains very low. It’s too small to take some actions,” he says.
Ide is now looking at an additional list of earthquakes that occur where plates with oceanic crust plunge beneath the continental crust, to see if the pattern holds up there as well.
The Moon will reach its next full phase on September 16, 2016. On the same day, a penumbral lunar eclipse will occur. During this type of eclipse, the Moon will darken slightly but not completely. It will be visible throughout most of eastern Europe, eastern Africa, Asia, and western Australia.
- "Earthquake potential revealed by tidal influence on earthquake size–frequency statistics" - Satoshi Ide, Suguru Yabe and Yoshiyuki Tanaka - Nature Geoscience - September 12, 2016 - DOI: 10.1038/ngeo2796
Featured image: Planetary positions on September 16, 2016. Credit: Solar System Scope