Continuous radon monitoring in soil gas towards earthquake precursory studies in basaltic region

The Koyna-Warna region, near the west coast of India, is well known for reservoir-triggered seismicity. The seismic activity in this region greatly increased following the construction of an artificial reservoir across the Koyna River during the 1960s. A destructive earthquake of M 6.3 occurred on December 10, 1967, and further 19 earthquakes of M>5 have been recorded during the preceding 40 years until 2007. The soil gas radon (222Rn) has been studied as an earthquake precursor by continuous monitoring (hourly) at two sites around the Warna reservoir. One site has a multi-sensor probe (installed at three different depths), together with a rainfall recording facility, and another probe is mounted on a hillock at Nivle. During the study period (2005–2007), a total of 11 earthquakes (including 2 aftershocks) of M 4–4.8 were recorded. Most of these events had recorded precursory radon signals. For a given earthquake, the 222Rn precursory signatures were recorded at one of the two sites only. Even multiple depth probes showed discordant behaviour in recording temporal Rn variation. Causes of non-concurrence in Rn recording between sites and probes, including the combined effect of site heterogeneity, focal depth, epicentral distance, earthquake magnitude, faults responsible for the earthquake, etc, are discussed.