RADON MONITORING FOR EARTHQUAKE PREDICTION IN SOUTH CENTRAL TAIWAN

An integrated radon monitoring method is employed in Chuko fault zone in south central Taiwan for earthquake prediction.

Soil gas radon is monitored continuously with a solid-state detector and recorded with a data logger. The detector assembly is housed in a PVC pipe to reduce the influence of environmental factors. The fault zone is known to have deep source gases and sensitive to earthquake activities. It has spring upwelling and gases released were measured to have high ³He/⁴He ratios. The quantities of spring upwelling and natural gas releases are known to vary with earthquake activities. By placing the detector assembly housing in a structured soil gas-collecting ditch of much larger size, radon level is raised but fluctuation reduced. Data retrieval from a remote laboratory site at the National Taiwan University, from the end of October 2000 to the end of February 2001, showed that spike-like radon anomaly precursors occurred essentially before every major earthquake with a M_L > 4.0. The strong correlation between spike-like anomalies and major earthquakes suggests that these precursors are indeed seismic.

The precursors of the 15 major earthquakes during the recording period with a few service interruptions indicated that there are 0.49 to 7.40 days leading periods before the onset of earthquakes occurred within a 30 by 100-km radii ellipse. The trending of the long axis of the ellipse appears to reflect the direction of the current stress and the old structure trend of Taiwan. The new integrated approach in soil gas radon monitoring reveals that the spike-like anomalies are superior precursors and could be used to delineate time and the place of an earthquake. However, nature of basement geology could be another critical factor in determining these. Additional units of this type are being installed nearby with different basement geology for the testing.