Kyoto University researchers proposed a physical mechanism by which solar flares might influence earthquakes. The mechanism is capacitive coupling. The ionosphere — the electrically charged layer of the upper atmosphere — acts as one plate of a capacitor. The Earth's crust acts as the other. During major solar flares, the ionosphere's charge distribution changes rapidly, with total electron content shifting by tens of TEC units. Because the system is capacitively connected, these charge variations do not stay overhead. They propagate downward as electric field changes into the crust.
In intact rock, this would be negligible. But faults are not intact rock. They contain networks of tiny voids, fractures, and fluid-filled pores. The electric field concentrates at the tips and boundaries of these voids the way stress concentrates at crack tips in mechanical systems. The researchers calculated that ionospheric disturbances associated with major solar flares could generate electrostatic pressures of several megapascals within these crustal voids — a small but non-trivial fraction of the stresses that hold faults locked.
The claim is not that solar flares cause earthquakes. The claim is narrower and more interesting: a fault that is already within a few megapascals of failure — critically stressed, loaded by tectonic forces over decades or centuries — might be pushed past its threshold by a pressure source that originates 100 kilometers overhead. The earthquake was going to happen. The question is whether the ionosphere determined when.
The mechanism is controversial for good reason. The correlation between solar activity and seismicity has been proposed and rejected multiple times over the past century. The correlations are weak, the datasets are noisy, and the prior probability of a real connection — given the enormous energy mismatch between ionospheric disturbances and tectonic stresses — has always seemed low. What the Kyoto model adds is not better correlation but a physical pathway: the capacitive coupling, the field intensification in voids, the quantitative estimate of the resulting pressure.
The structural point is about thresholds. A system near a critical threshold can be triggered by forces that are negligible in absolute terms. The tectonic stress loading a fault might be hundreds of megapascals. A few additional megapascals from ionospheric coupling is less than one percent of the total. But if the fault is already at 99.5 percent of its failure stress, one percent is enough. The triggering force does not need to be large relative to the system. It needs to be large relative to the remaining margin.
This is why the mechanism is hard to test. It would only operate on faults that are critically stressed, and we cannot measure how close a specific fault is to failure. The prediction is not “solar flares cause earthquakes” but “solar flares occasionally determine the timing of earthquakes that were imminent anyway.” The second statement is weaker, more specific, and much harder to falsify — because you would need to know which earthquakes were imminent, which is precisely the information we lack.