Some Evidence Of A Correlation Between Solar Activity and Large Earthquakes Worldwide

There has been some very tentative work on cosmic rays and volcanoes (Explosive volcanic eruptions triggered by cosmic rays: Volcano as a bubble chamber—ScienceDirect, 2011) but because that paper is pretty much an outlier/one-off, we've never linked to it.

This next paper on the solar proton flux and earthquakes has a little more heft to it but it is still early days.
From the journal Nature, July 13, 2020:

Abstract

Large earthquakes occurring worldwide have long been recognized to be non Poisson distributed, so involving some large scale correlation mechanism, which could be internal or external to the Earth. Till now, no statistically significant correlation of the global seismicity with one of the possible mechanisms has been demonstrated yet. In this paper, we analyze 20 years of proton density and velocity data, as recorded by the SOHO satellite, and the worldwide seismicity in the corresponding period, as reported by the ISC-GEM catalogue. We found clear correlation between proton density and the occurrence of large earthquakes (M > 5.6), with a time shift of one day. The significance of such correlation is very high, with probability to be wrong lower than 10^–5. The correlation increases with the magnitude threshold of the seismic catalogue. A tentative model explaining such a correlation is also proposed, in terms of the reverse piezoelectric effect induced by the applied electric field related to the proton density. This result opens new perspectives in seismological interpretations, as well as in earthquake forecast.

Introduction
Worldwide seismicity does not follow a Poisson distribution¹, not even locally². Many authors have proposed specific statistical distributions to describe such a non-poissonian behavior^3,4,5,6,7 but none of these is really satisfactory, probably because the underlying physical process has not been really understood. Many authors have hypothesized that a tidal component may show up in earthquake activity (e.g.^8,9) but generalized evidence has never been proven. Quite recently, some authors¹⁰ suggested that earthquake occurrence might be linked to earth rotation speed variations. There is also a smaller number of researchers that studied possible links among solar activity, electro-magnetic storms and earthquakes (e.g.^{11,12,13,14,15,16}). The first idea that sunspots could influence the earthquake occurrence dates back 1853, and is due to the great solar astronomer Wolf¹⁷. Since then, a number of scientists has reported some kind of relationship between solar activity and earthquake occurrence^16,18,19; or among global seismicity and geomagnetic variation^15,20 or magnetic storms^21,22. Also, some mechanisms have been proposed to justify such correlations: small changes induced by Sun-Earth coupling in the Earth’s rotation speed²³; eddy electric currents induced in faults, heating them and reducing shear strength²⁴; or piezoelectric increase in fault stress caused by induced currents²⁵. However, none of these studies allowed achieving a statistically significant conclusion about the likelihood of such mechanisms. On the contrary, ²⁶ argued that there is no convincing argument, statistically grounded, demonstrating solar-terrestrial interaction favoring earthquake occurrence. However, the large interest nowadays for possible interactions between earthquake occurrence and extra-terrestrial (mainly solar) activity, is testified for instance by the Project CSES-LIMADOU, a Chinese–Italian cooperation aimed to launch a satellite to study from space the possible influence of solar activity and ionospheric modifications on the seismicity²⁷. In this paper, we will definitively establish the existence of a correlation between solar activity and global seismicity, using a long data set and rigorous statistical analysis. Once such a correlation is demonstrated, we propose a tentative, at the moment qualitative, mechanism of possible sun–earthquakes interaction.

Statistical assessment of the correlation solar activity–earthquakes
Since our aim was to verify the existence of a link between solar activity and earthquakes, we considered two data sets: worldwide earthquakes, and SOHO satellite proton measurements....

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Related: here's Dr. Jeffrey Love of the US Geological Survey back in 2013 at the American Geophysical Union's Geophysical Research Letters: Insignificant solar‐terrestrial triggering of earthquakes

We last visited Dr. Love, who I kept thinking of as the Love Doctor, in April 2020's:
Sunspots and Agricultural Production (William Herschel does a driveby)