I was thinking about one of my few forays over to Physics Forums, where I commented on quakes and lunar gravitational forces. This didn't go over so well, as indicated.

But now this:
[Spring tides trigger tremors deep on California’s San Andreas fault](http://www.sciencemag.org/news/2016/07/spring-tides-trigger-tremors-deep-california-s-san-andreas-fault)

which takes from this PNAS paper written by USGS scientists:

[Fortnightly modulation of San Andreas tremor and low-frequency earthquakes](http://www.pnas.org/content/early/2016/07/13/1524316113)

> "Significance -- The sun and moon exert a gravitational tug on Earth that stretches and compresses crustal rocks. This cyclic stressing can promote or inhibit fault slip, particularly at the deep roots of faults. The amplitude of the solid Earth tide varies over a fortnightly (2-wk) cycle, as the sun and moon change their relative positions in the sky. In this study, we show that deep, small earthquakes on the San Andreas Fault are most likely to occur during the waxing fortnightly tide—not when the tidal amplitude is highest, as might be expected, but when the tidal amplitude most exceeds its previous value. The response of faults to the tidal cycle opens a window into the workings of plate tectonics."

This is a chart representing the salient features of their argument. Stress builds up over time but the lunar tidal influence provides forcing excursions on ~two-week periods that can cause an earthquake.


What a difference a year makes. It might be OK to discuss this now.