Multifractal detrended fluctuation analysis of soil radon in the Kachchh Region of Gujarat, India: A case study of earthquake precursors

In this paper, the scaling properties of earthquake-induced soil radon emissions in the Kachchh region of Gujarat, India, were demonstrated through the application of multifractal detrended fluctuation analysis (MFDFA). The analysis considered soil radon data recorded at four stations, namely Rampar, Rapar, Pragpar and Paddhar, over a period of one year (January 1, 2020 to December 31, 2020). At these stations, soil radon along with soil pressure and temperature are recorded continuously at 10-minute intervals. In the first step, periodic oscillations such as diurnal and semi-diurnal periodicities of the raw soil radon data were removed by applying empirical mode decomposition (EMD). The aperiodic soil radon at all measuring stations was used as input for the MFDFA analysis. In order to obtain the exact source of the multifractality, the MFDFA is also applied to the shuffled and surrogate data of the aperiodic soil radon time series at all measuring stations. It can be observed that the fluctuation function (Log (Fq(s))) for all measuring stations increases linearly with increasing scale value(s). In contrast, the generalized Hurst exponent (H(q)) of the shuffled radon values is about 0.5, the H(q) value of the surrogate and observed radon series is greater than 0.5 at all measuring stations. The scaling exponent τqof shuffled Radon shows an approximately linear trend, while surrogate and soil observed radon show a non-linear trend. At all stations, the shuffled series shows a narrower spectrum compared to the surrogate and observed soil radon time series. This shows that observed soil radon has a long-range correlation. Three local earthquakes of June 14, 2020 (M=5.3), August 28, 2020 (M=4.1) and November 1, 2020 (M=4.1) near monitoring stations are considered for analysis in this study. The multifractal spectrum of soil radon is also examined during the seismically quiet and disturbed period. A broader spectrum is obtained during the disturbed period compared to the quiet period. This may be due to heterogeneity characterized by anomalous emission of soil radon gas before the occurrence of earthquakes during the monitoring period.