I have previously cited two research groups from last year that linked lunar cycles to the triggering of earthquakes.
Ide, Satoshi, Suguru Yabe, and Yoshiyuki Tanaka. "Earthquake potential revealed by tidal influence on earthquake size-frequency statistics." Nature Geoscience 9.11 (2016): 834-837.
van der Elst, Nicholas J., et al. "Fortnightly modulation of San Andreas tremor and low-frequency earthquakes." Proceedings of the National Academy of Sciences (2016): 201524316.
Delorey, Andrew A., Nicholas J. van der Elst, and Paul A. Johnson. "Tidal triggering of earthquakes suggests poroelastic behavior on the San Andreas Fault." Earth and Planetary Science Letters 460 (2017): 164-170.
Yet, there is another research paper that is mind-boggling in terms of establishing correlation that predated this work by 5 years:
Kolvankar, Vinayak G. "Sun, Moon and Earthquakes." New Concepts in Global Tectonics Newsletter 60 (2011): 50-66. PDF
This paper is an obscure newsletter with no citations according to Google Scholar, but makes strong claims with a comprehensive spatio-temporal data analysis to back it up:
"During this study we also noticed that the Sun position in terms of universal time (GMT) has some links to the Earthquake-Moon distance (EMD) together with Sun-Earth-Moon (SEM) angle. In this paper we explored all the features of this relationship. It is astonishing to see that over 98% of worldwide earthquakes faithfully follow the straight-line relationship between the Sun position or GMT timings with (EMD+SEM). This proves beyond any doubt that the vast majority (98% ) of worldwide earthquakes are governed by the Sun and Moon. Even the smaller earthquakes in the magnitude range of 2-3 faithfully follow this relationship. It is also seen that numerous aftershocks, which follow any major earthquake, faithfully follow straight-line curves, generated by the plot for (EMD+SEM) vs GMT timings. For all the plots providing earthquake data, for 00 hours GMT, the earthquake commences at the mean position of the longitude range (of the area under study) on the X axis (EMD+SEM).It is seen that for plots for longitude ranges close to +/- 1800, the earthquake plot occupies the central strip running diagonally from the origin of the plot. For all other plots, for 00 hours GMT, the earthquake commences at the mean longitude of the area under study, for the Sun’s position opposite the earthquake region (1800 out of phase).Basically it is seen that all these earthquakes are triggered by the Earth tides caused by the positions of the Sun and the Moon and this process seems to be the primary triggering mechanism for all worldwide earthquakes. This includes deep-focus earthquakes, which are affected by Earth tides as shown by the earlier study (Kolvankar et al., 2010)."
Kolvankar compiled sets of what he calls "earthquake plots" which isolate the timing of the lunar+solar orbital path with frequencies of earthquakes (via a scatter density plot) in specific locations.
There's nothing really mathematically complex about this result. It's simply the result of data mining and then plotting a pairwise relationship in a discovered pattern. I am not going to check his results because I don't have access to the data he analyzed, but I can plot something similar based on another well-known physical behavior. Below is the locii of points that tracks the zenith point of the sun in the sky.
Perhaps too obvious, but the zenith occurs at GMT high-noon (12H) only at the longitude corresponding to Greenwich, UK, and is offset elsewhere. This has physical significance in that this is the point at which sunlight is strongest overhead. There is no deviation in the points from a straight line, as the earth's rotation is deterministic (the data is artificially created for the sake of argument).
For earthquakes, the relationship is different, but can be interpreted in the similar geographic fashion -- only instead involving the sun and the moon.
Kolvankar stumbled on to the slightly more complex spatio-temporal relationship as he describes in the Acknowledgements section of the paper:
"... the author had some discussion about the access database management of over five hundred thousand earthquakes. Mr Rahul Kesarkar demonstrated various aspects of the access database software. During this time, we became aware of the relationship between the GMT timings and the EMD + SEM. I wholeheartedly thank Mr Rahul Kesarkar for his efforts, which brought the relationship between these two parameters to our notice."
The mechanism behind this is straightforward to understand. Earthquakes are triggered after stress builds up over time and the strain is released at a fault. All that is happening with the lunar tidal force is that a small excursion is provided at a time prior to that at which it would have randomly occurred. That happens to coincide with a specific lunisolar alignment at a set latitude-longitude. Kolvankar has a figure describing this configuration:
From the later PNAS article, the following figure shows how the stress is elevated temporarily at a fortnightly cycle:
The PNAS people (van der Els et al) somehow missed the Kolvankar paper. I had found it a while ago when I discussed the topic here and later here at the Azimuth Project forum. Since that time I lost track of it, but easily found it again with a Google search.
Now, it may be true that the lunar trigger to earthquakes is not enough to have any practicality for prediction. There is still a significant stochastic element in determining the range of time that an earthquake will get triggered. That is still largely unknowable, except in a probabilistic sense.
Yet, there is no threshold trigger in the ocean as it responds gradually to lunar and solar tidal forcing. That means that ocean tides are highly predictable and can be predicted far in the advance, just like the recent solar eclipse. That's a true deterministic behavior, in contrast to the stochastic nature of earthquakes.
So now you can understand why the finding of a lunar forcing for ENSO and El Nino events is also a breakthrough. Similar to diurnal and semidurnal ocean tides, the gradual fortnightly and monthly cycling of the moon's orbit creates a deterministic outcome suitable for prediction years and decades in advance of the ENSO event.
What's sad about reporting on this kind of research is the amount of push-back one gets. As I described at Azimuth, I was banned from commenting at Physics Forums in 2015 because I had the gall to suggest that the moon may be causing more physical behaviors than the familiar ocean tidal effect and solar eclipses. The former high-school student that runs Physics Forums is anti-research and mostly runs the site so he can do homework for kids that are too lazy to do it themselves. My mistake for believing that something labelled Physics Forums might actually be interested in discussing real physics.
But it does point out an interesting situation. That is: millions of people will flock to see the moon causing a solar eclipse, yet a single guy in India named Kolvankar figured out the pattern behind the moon triggering earthquakes and he gets roundly ignored. Such is the wacky world of cutting-edge scientific research with its own unwritten rules of discourse. Perhaps not this bad, but as George Monbiot said years ago: "Tell people something they know already, and they will thank you for it. Tell them something new, and they will hate you for it."
More likely is that the new information will simply be ignored because it did not follow the established protocol for disseminating results. Apparently it's better to wait for someone with credentials to report the findings and then jump on the bandwagon when it becomes consensus. Everything else is conspiracy talk -- which perhaps is not hard to understand given the popularity of the crackpots at WUWT and Tallbloke's Talkshop.
To preface his earlier quote, Monbiot also gave this advice:
"So the task of the environmental journalist is not just to highlight damage to the environment. It is not just to challenge some of humankind’s most fundamental perceptions. It is to challenge humanity itself. I hope I am not putting you off."