Direct measurement of present-day tectonic movement and associated radon flux in Postojna Cave, Slovenia

Abstract

Micro-tectonic deformations have been monitored continuously in 3D in Postojna Cave, Slovenia with TM 71 extensometers since 2004. Two instruments, 260 m apart, were installed on the Dinaric oriented (NW-SE) fault zone that is situated about 1,000 m north of the inner zone of the regionally important Predjama Fault. Monitoring on both instruments has shown small tectonic movements (i.e., a general dextral horizontal movement of 0.05 mm in four years [Postojna 1] and extension of 0.03 mm in four years [Postojna 2]). Between the longer or shorter calm periods, eleven extremes have been recorded regarding characteristic changes in displacement. The largest short-term movement was a compression of 0.04 mm in seven days, detected in March 2005, which coincided with the 25 km distant Ilirska Bistrica earthquake (ML 5 3.9). About two months before the earthquake an extension of 0.05 mm occurred and one month before the earthquake the strain changed into a compression of 0.05 mm. The largest permanent peak was detected at the end of 2004. Along the y-axis (Postojna 1) there was a dextral horizontal movement of 0.075 mm in one month (November 10 to December 15, 2004). After the sinistral horizontal movement of 0.02 mm (December 15–27, 2004), the y-axis retained its permanent position on 0.05 mm, where it remained for more than a year. Regarding the extremes, ten earthquakes were selected that coincided with tectonic micro-displacements. In terms of speleogenesis, the monitored fault zone represents a stable cave environment. Because radon flux is known to change significantly during tectonic and seismic activities, radon air concentrations were monitored in parallel since 2006. During horizontal movements, either dextral or sinistral, radon pathways underground were partly closed, thus hindering radon migration and reducing its concentration in the cave air. Extension movements do not appear to have affected radon transport. Alternatively, the compression process (Postojna 2, February–August 2007) appears to have opened some new routes for radon transport, facilitating radon migration and increasing its concentration in air.