Radon anomaly in the soil of Taal volcano, the Philippines: A likely precursor of the M 7.1 Mindoro earthquake (1994)

Abstract

The soil‐gas 222Rn concentration had been monitored almost continuously from June 1993 till November 1996 on Taal volcano, Luzon Island, the Philippines. During this measurement period, a singular Mb 7.1 earthquake occurred on November 15, 1994, between Luzon and Mindoro, 48 km south of the volcano. Twenty‐two days before the earthquake, an anomalous increase in soil‐gas radon (peak to background ratio = 6) was recorded, unique in the whole time series. The possible generation of this anomaly by typhoon Teresa, which struck Luzon Island a few days before, was ruled out one year later when super typhoon Angela, the most powerful storm to hit the Philippines in ten years, crossed Luzon Island along almost the same track without triggering a similar disturbance in the radon signal. Consequently, there is strong evidence that the Taal radon anomaly originated in stress accumulation preceding the Mindoro earthquake.