Variations of soil–gas composition around the active Chihshang Fault in a plate suture zone, eastern Taiwan

The active Chihshang Fault in the Longitudinal Valley of eastern Taiwan is part of the suture boundary between the Eurasian plate and Philippine Sea plate. Soil–gases collected systematically across the Chihshang Fault reveal anomalies at specific positions that coincide with the geological, geophysical and topographical characteristics of faults/fractures in studied area. An automatic soil radon monitoring station was set up at the fault zone, at the same place as existing monitoring wells and creep meter monitoring. Some anomalous high soil radon concentrations were observed, usually a few days before local earthquakes. However, other anomalies could not be related to any earthquake event, and may be related to surface creeping of the fault as revealed from the creep meter monitoring in the studied area. In addition, heavy rainfall induced significant increase of soil radon concentrations which may be related to the rapid surface creep that was triggered by the heavy rain in a short time.