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 limestone-cave in Korea for one year. Unusually large ²²⁰Rn peaks were observed only in February 2011, preceding the 2011 M9.0 Tohoku-Oki Earthquake, Japan, while large ²²²Rn peaks were observed in both February 2011 and the summer. Based on our analyses, we suggest that the anomalous peaks of ²²²Rn and ²²⁰Rn activities observed in February were precursory signals related to the Tohoku-Oki Earthquake. Thus, the ²²⁰Rn-²²²Rn combined isotope pair method can present new opportunities for earthquake forecasting if the technique is extensively employed in earthquake monitoring networks around the world.