A comparative study of geoelectric signals possibly associated with the occurrence of two M_s > 7 EQs in the South Pacific Coast of Mexico

During past decades, several studies have suggested the existence of possible seismic electric precursors associated with earthquakes of magnitude M > 7. However, additional analyses are needed to have more reliable evidence of pattern behavior prior to the occurrence of a big event. In this article we report analyses of self-potential ΔV records during approximately two years in three electro-seismic stations in Mexico located at Acapulco, Guerrero; Petatlan, Guerrero and Pinotepa Nacional, Oaxaca. On 18 April 2014 an M_s7.2 earthquake occurred near our Petatlan station. Our study shows two notable anomalies observed in the behavior of the Fourier power spectrum of ΔV for ultra low frequency ULF-range, and the transition of the α_l-exponent of the detrended fluctuation analysis of the ΔV time series from uncorrelated to correlated signals. These anomalies lasted approximately three and a half months before the main shock. We compare this electric pattern with another electric signal we reported associated with an M_s7.4 that occurred on 14 September 1995 at Guerrero state, Mexico. Our characterization of the anomalies observed in both signals point out similar features that enrich our knowledge about precursory phenomena linked to the occurrence of earthquakes of magnitude M > 7.