Attention, moon-fearing humans: Your long-held suspicion that the full moon can interfere with life on Earth seems to have a scientific basis after all. No, the moon still doesn't make you crazy, horny or murderous. But it can trigger large earthquakes. One of many buildings cracked wide open by the 2010 Maule earthquake in Chile. Image: Wikimedia
Since the 19th century, scientists have debated whether the gravitational tug of the Moon and the Sun influence seismic activity here on Earth. In recent years, evidence has been mounting that the ocean's twice-daily high tides can result in small, slow-moving tremors. But to date, this relationship has been restricted to specific environments and parts of the world, such as the the San Andreas fault in California.
Now, we have the first strong evidence for a global relationship between the larger, monthly lunar cycle and powerful rumbles within the Earth. Examining three separate seismic databases, a team of geologists at the University of Tokyo has found an overlap between some of the largest quakes in recent memory (magnitude 5.5 or above), and periods of high tidal stress, when the Sun and Moon are aligned to yield the strongest gravitational tug on the Earth. Those events include the 2004 Sumatra quake (magnitude 9.3), the 2010 Maule quake in Chile (magnitude 8.8) and the 2011 Fukushima quake in Japan (magnitude 9.0).
While the details of how large earthquakes form and evolve haven't been fully worked out, the process is thought to involve a number of small ruptures cascading to form a really big one. Based on the new analysis, it would seem that such a cascade is more likely at the full or new moon, when the Sun and Moon are exerting the maximum amount of strain on the Earth. The authors also found that the fraction of large earthquakes compared with small earthquakes increased as tidal stress increased.
"[The study] suggests that the small additional encouragement from the tides can actually make an earthquake grow a bit larger than it would have otherwise," Nicholas van der Elst of the US Geological Survey told Gizmodo. "This is surprising, considering how small the tidal stress is, compared to the stresses generated by the ongoing earthquake."
Don't expect this to be the final word on the matter — many factors are involved in triggering earthquakes, and within the geologic community, there's a wide spectrum of viewpoints on the importance of the Moon's role. Still, the new results could be valuable to those in the earthquake prediction business, because, as the authors note, they "suggest that the final earthquake size can be estimated probabilistically".
More generally, the discovery could lead to new paradigms for understanding Earth's most epic shakeups.
"If the observation can be confirmed," van der Elst said, "this sort of tidal triggering could teach us something pretty fundamental about how earthquakes start, grow, and ultimately run their course."