Abnormal increases in radon (222Rn, half-life= 3.82 days) activity have occasionally been
observed in underground environments before major earthquakes. However, 222Rn alone
could not be used to forecast earthquakes since it can also be increased due to diffusive
inputs over its lifetime. Here, we show that a very short-lived isotope, thoron (220Rn, half-
life= 55.6 s; mean life= 80 s), in a cave can record earthquake signals without interference
from other environmental effects. We monitored 220Rn together with 222Rn in air of a …