Spectral-decomposition techniques for the identification of radon anomalies temporally associated with earthquakes occurring in the UK in 2002 and 2008

During the second half of 2002, the University of Northampton Radon Research Group operated two continuous hourly-sampling radon detectors 2.25 km apart in the English East Midlands. This period included the Dudley earthquake (M(L)=5, 22 September 2002). Also, at various periods during 2008 the Group has operated other pairs of continuous hourly-sampling radon detectors similar distances apart in the same region. One such period included the Market Rasen earthquake (M(L)=5.2, 27 February 2008). Windowed cross-correlation of the paired time-series was used to identify simultaneous short-duration anomalies. In the 2002 data, only two periods of significant cross-correlation were observed, each corresponding temporally to a UK earthquake, one to the Dudley earthquake and the other to a smaller earthquake in the English Channel (M(L)=3, 26 August 2002). In the 2008 data, cross-correlation initially revealed little evidence of simultaneous short-duration anomalies but cross-correlation of data de-noised and de-trended using Empirical Mode Decomposition (EMD) revealed clear simultaneous short-duration anomalies which correspond temporally to the Market Rasen earthquake.