Gamma Ray and Radon Anomalies in Northern Taiwan as a Possible Preearthquake Indicator around the Plate Boundary

Figure 4

The temporal variations of (a) radon and gamma rays from July 1, 2014, to June 1, 2015. Blue and red lines indicate the 24-hour running averages of radon and gamma ray data, respectively; grey lines represent the raw data. Horizontal dashed lines indicate the annual averages of the observed results, and the anomalous threshold value (±1σ) is included with the yellow bar. The magnitude and depth of earthquakes are plotted in (b) and (c), respectively. The circle symbols in different colors indicate that the epicenter distance of the earthquake to YMSG station was <30 km (black), 30 to 100 km (red), 100 to 180 km (green), and >180 km (yellow). Red and blue arrows indicate the specific seismic events. The accumulated number of earthquakes at different distances is also shown as a histogram (d). One specific period of time marked by the dashed rectangle is enlarged in Figure 5.