Solar Activity Triggers Microearthquakes
Several scientists have suggested connections between solar activity, geomagnetic activity, and earthquake frequency. Singh has also found strong correlations between geomagnetic activity (definitely sun-triggered) and microearthquakes. He discovered first that the great solar storm of August 1972 was accompanied by large surges of both geomagnetic activity and microearthquakes. Following this lead, he studied records between 1963 and 1969, again finding strong correlations.
(Singh, Surendra; "Geomagnetic Activity and Microearthquakes," Seismological Society of America, Bulletin, 68:1533, 1978.)
Comment. While one can conceive of ways in which the streams of electrically charged solar plasma can modulate geomagnetic activity, the coupling of solar plasma variations to microearthquakes is more obscure.