Temporal variation of gamma rays as a possible precursor of earthquake in the Longitudinal Valley of eastern Taiwan

A gamma-ray station has been established recently along the Longitudinal Valley in eastern Taiwan, which is a suture zone with NNE-striking on the boundary between Eurasia plate and Philippine Sea plate. Stresses arise in the crust in accordance with crustal deformations due to plate motions and geodynamic processes. Gamma-ray emission associated with the radioactive decay of radon, which is discharged into surface from deep source area, is continuously recorded in the station. Fluctuations of gamma-ray data are found to show a clear inverse correlation with atmospheric temperature. Based on the linear regression method, possible temperature effects can be removed and the corrected data can be obtained. Anomalous variations in the gamma-ray counting rate may appear as precursors a few days before some local earthquakes in eastern Taiwan. It is noted that there are two significant increases in the gamma-ray counting rate during the periods from March 20 to 26 and from April 22 to May 1, 2014. These increases are observed about one week before two earthquake swarms from March 26 to 27 and from May 3 to 5, 2014. Furthermore, a progressive anomaly with a continuous increase (∼8%) in gamma-ray count was recorded two weeks before the Fanglin Earthquake (ML = 5.9, May 21, 2014) occurring in the Longitudinal Valley area. A continuous monitoring on the gamma-ray data can improve our understanding of the possible relationship between the observed gamma-ray variations and the earthquakes in the study area.