Original question asked by Phil - “Does the moon play a role in plate tectonics? Can its gravitational “tug” on the crust and the oceans trigger an earthquake or start earthquake storm?”

Earthquakes are usually caused by the release of potential energy built up by the friction between two bodies of rock. This can occur along fault zones or plate boundaries. The movement of the earth’s crust and tectonics plates is caused by heat-driven convection within the mantle. This heat comes from radioactive decay of elements. This motion is gravity assisted, known as slab push/pull, in which the mass of a descending slab will essentially drag the rest of the plate down with it, driving its motion. Even though these processes occur on a large spatial scale they are incredibly slow. So as you can see, traditional tectonic movement and earthquakes do not typically involve the moon.

However, that is not to say that it doesn’t have any effect. The gravitational pull of the moon is known to affect the ocean tides, and this gravitational pull also affects the rocky crust of the earth which is known as an earth tide. Both tides can have an effect on earthquakes and will be described individually. The moon and the sun both have a gravitational pull over the earth and an effect on the earth and ocean tides, however, due to the greater distal nature of the sun it is not considered with regards to these effects.

Ocean tides Source: National Geographic

Ocean tide: the gravitational pull of the moon and its orbit around the earth create the predictable swell in our oceans on a period of 12 hours (time interval between two high or two low tides). During particularly high tides or spring tides larger volumes of water are pushed against coastal areas. This can induce stresses in pre-existing fault zones inducing small magnitude earthquakes. In volcano-rich areas such as the Aleutian islands of Alaska, the larger volumes of water put pressure on magma chambers and can cause the magma to move. This is also likely to induce small earthquakes.

Earth tides _Source: _http://www.ipgp.fr/~lalmetiv/metivieretalepsl2009.pdf

Earth tides: As previously mentioned, an earth tide is a gravitational pull on the earth’s crust caused by our relative proximity to the moon. The location of the “swellings” are relatively predictable due to our understanding of the moon’s orbit. The sun has a lesser but similar effect, on account of it being further away. The pull of the moon relieves some of the stress acting along a fault plane, reducing the frictional forces acting on it. This increases the chance that the faults will slip and so an increase in the likelihood of a small magnitude earthquake following the earth tide.

References and further reading

Tidal triggering of earthquakes in the Northeast Pacific Ocean:

https://gji.oxfordjournals.org/content/179/2/1055

USGS FAQs - effect of the position of the moon and planets on plate tectonics:

https://www2.usgs.gov/faq/categories/9827/3354

Earth Tide (Wikipedia):

https://en.wikipedia.org/wiki/Earth_tide

Can Astronomical tidal forces trigger earthquakes?

http://www.scientificamerican.com/article/can-astronomical-tidal-forces-trigger-earthquakes/

Is earthquake activity related to the sun or moon?

http://www.earthquakes.bgs.ac.uk/research/earthquakeSunMoon.html

What causes tides to change in the ocean?

http://oceanlink.island.net/oinfo/tides/tides.html

How do tectonic plates move?

https://www.geolsoc.org.uk/Plate-Tectonics/Chap2-What-is-a-Plate/Plate-Movement