Do tides trigger earthquakes? Possibly another reason to respect the moon

Tsunamis - a devastating effect of an earthquake. Although there are some tsunami warning systems and seismologists can sometimes predict when earthquakes will occur, tsunamis continue to catch us off guard (see below, the Tohaku earthquake and tsunami in Japan). Scientists are continually looking for better ways to predict earthquakes around the world. Turns out they might be able to use the phases of the moon.

 ( Image Credit:  U.S. Navy photo by Petty Officer 3rd Class Kevin B. Gray) 

(Image Credit: U.S. Navy photo by Petty Officer 3rd Class Kevin B. Gray) 

Last year, a paper in the journal Physics and Chemistry of the Earth suggested that the moon may be responsible for triggering earthquakes. As I looked through this paper, I though to myself, "now this is interesting, but do I believe it?" The answer is I don't know. So I'd like to take this opportunity to walk through this study using a critical lens.

a conjunction or opposition, especially of the moon with the sun

Why did I immediately jump onto the critical train? This topic of astronomical syzygy in conjunction with earthquakes was immensely popular back in the 1800s. By the way, syzygy is used here to talk about the idea that scientists thought when the Earth, Moon, and Sun were all aligned (for instance, in that order during new moon), the tidal forces should be strongest, causing earthquakes and other phenomenon to happen. Scientists were obsessed but were never able to find a definitive link between earthquakes and the phases of the moon. So what has changed?

 The Hellenic subduction zone in Greece. ( Image Credit : Wikimedia Commons, Mikenorton)

The Hellenic subduction zone in Greece. (Image Credit: Wikimedia Commons, Mikenorton)

The scientists studied earthquakes along the Hellenic Arc fault in Greece and found a correlation between the frequency of earthquakes and the tidal lunar monthly variations. 

On interesting argument in the paper is that while the tidal stresses are much less than stress from the earthquake itself, the tidal stresses are actually comparable to or greater than the tectonic stress accumulation in a fault that precedes an earthquake.

But what is the amount of force that a syzygy can actually exert on the Earth? And how does it exert this force? 

 A tide schematic of the tidal effects of the Moon on the Earth with force vectors. ( Image Credit:  Wikimedia Commons)

A tide schematic of the tidal effects of the Moon on the Earth with force vectors. (Image Credit: Wikimedia Commons)

Let's say the alignment is Earth, Moon, and Sun. The Sun is much more massive than the moon, but it exerts a very small force on the Earth because it is very far away and you'll remember that Newton told us that gravitational force decreases with the square of distance. So although the moon is smaller, it has a bigger effect. However, this doesn't mean we can just ignore the Sun altogether. In fact, this study out of Greece found that one of the time periods of greatest correlation was when all three celestial bodies were lined up.

Okay, so the moon is pulling on water on the Earth's surface, which causes high tide on the side of the Earth towards the moon. But why is there high tide on the opposite side? The Earth is actually being pulled away from the water! There's a way to explain this with vectors, but a good way to think about it is that the water on the far side is experiencing slightly less pull than the crust on the far side because it's slightly farther away. Low tides happen where there is no pull from the moon or pulling away by the body of the Earth because the water is occupied elsewhere. 

So with our new understanding of tides, I'd like to return to the study at hand. Why haven't there been correlations found for all types of earthquakes that occur all over the earth? It turns out that although the force of the moon is also pulling on the crust it may have the biggest effect on the oceans. So during high tide, as water piles up, the weight of the water may stress underwater faults more so than land faults, leading to the discovery of this correlation appearing over an underwater fault zone. Also, like I said earlier, the force from tides may be similar to that of building up stresses in the fault already, so the tides may act as the final straw that broke the camel's back. Perhaps the fault stresses don't always need this extra push.

Regardless, the studies that have been finding correlations are all relatively new, so while it's an exciting idea that we might be able to use the forces of the moon to predict when certain earthquakes may occur, there's still much to be learned on this topic.