Automated radon monitoring at a hard‐rock site in the southern California transverse ranges

Data are presented from 20 months of near‐real‐time (three samples per day) radon monitoring at a hard‐rock site in the Transverse Ranges of southern California. An annual cycle is evident in the data which is attributed to thermoelastic strains in the vicinity of the borehole site. Between April 1, 1977, and October 31, 1978, there were 11 earthquakes with magnitudes ≥2.0 within 25 km of the monitoring site. Three of these events appeared to be preceded by precursory signals, four were preceded by ‘possible’ precursory signals, and four were not preceded by any apparent precursors. Before the 4.6 M Malibu earthquake of January 1, 1979, a ‘possible’ precursory signal sequence of 40–50 days' duration was observed.