Earthquakes typically occur when the rocks of Earth’s tectonic plates break as a result of tectonic stress building up over time. However, worldwide seismicity does not follow a random distribution. Statistical research in the past suggests that large, powerful earthquakes commonly occur in groups. And though many researchers have done statistical studies to try to determine the cause, invoking various effects, like the gravitational pull by the Moon, the Sun or even other planets, no compelling theories have yet been proposed. A new study published in the journal Scientific Reviews argues that there is a statistically significant correlation of earthquake clusters with powerful eruptions on the Sun, even if it is still unclear how the two phenomena are connected.

The researchers used data from NASA-ESA’s Solar and Heliospheric Observatory (SOHO) satellite to reconstruct the solar activity of the past 20 years. Increased solar activity correlates with solar eruptions when large amounts of positively charged particles (protons) are emitted from the Sun. By comparing the SOHO data with the historical record of strong earthquakes, the scientists noticed more earthquakes occurred when the number and velocities of incoming solar protons increased. Specifically, when protons peaked, there was a spike in quakes above magnitude 5.6 for the next 24 hours.

The sun is constantly releasing vast amounts of energy and particles in the form of the solar wind. The most powerful eruptions also release electrically charged particles, like ions and electrons. When they reach Earth, these charged particles can interact with Earth’s magnetic field. The new research suggests that particles from very powerful eruptions, specifically the positively charged ions, might also affect the rocks of Earth’s crust.

Increasing tectonic deformation of rocks can cause an electric current, as electrically charged particles are “squeezed out” of the crystalline structure of the single mineral grains. Based on this observation, the new study suggests that by inverting this process, charged particles coming from the Sun may cause the mineral grains to expand, increasing the tectonic stress until the rocks break.

Other scientists remain cautious about such claims, criticizing how the data was selected. The study looked only at earthquakes over a magnitude of 5.6. Earthquakes with a magnitude over 5 happen in general every month, over a magnitude of 6 every 4 to 6 months, and stronger quakes are even rarer. Twenty years might not enough time to get a reliable set of data. For example, a 2013 paper analyzing 100 years of sunspot activity and geomagnetic data found no evidence of a connection between the Sun’s activity and Earth’s seismic activity.