Temporal Variations of Gamma-Ray for Detecting Crustal Activity Changes in the Longitudinal Valley, Eastern Taiwan

Abstract:

A gamma-ray station was established along the Longitudinal Valley Fault, which is an extremely active high-angle thrust 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 is continuously recorded in the station, considered to be discharged into surface from deep source area. Fluctuations of gamma-ray data are found to show a clear inverse correction with atmospheric temperature. Based on the linear regression method, possible temperature effects can be removed and the corrected data can be obtained. Variations exceeding the normal trend can be used to identify events associated with geodynamic processes such as earthquake. These anomalies usually appeare as procursors a few days before the local earthquakes, which occur in eastern Taiwan. It is noted that there are two significant increases in the gamma-ray counting rate during the period of 20th to 25th March and the period of 22nd April to 1st May. These increases are observed one week before the earthquake swarm. Furthermore, a progressive anomalies were recorded two weeks before the Fanglin earthquake (M_L = 5.9, May 21, 2014) located in the Longitudinal Valley area. A continuous monitoring on the multiple parameters can improve our understanding of the relationship between the observed gamma-ray variations and the regional crustal stress/strain in the area.