Abstract.The geomagnetic observatory of Juriquilla Mexico, located at longitude -100.45 • and latitude 20.70 • , and 1946 m a.s.l., has been operational since June 2004 compiling geomagnetic field measurements with a three component fluxgate magnetometer. In this paper, the results of the analysis of these measurements in relation to important seismic activity in the period of 2007 to 2009 are presented. For this purpose, we used superposed epochs of Discrete Wavelet Transform of filtered signals for the three components of the geomagnetic field during relative seismic calm, and it was compared with seismic events of magnitudes greater than M s > 5.5, which have occurred in Mexico. The analysed epochs consisted of 18 h of observations for a dataset corresponding to 18 different earthquakes (EQs). The time series were processed for a period of 9 h prior to and 9 h after each seismic event. This data processing was compared with the same number of observations during a seismic calm. The proposed methodology proved to be an efficient tool to detect signals associated with seismic activity, especially when the seismic events occur in a distance (D) from the observatory to the EQ, such that the ratio D/ρ < 1.8 where ρ is the earthquake radius preparation zone. The methodology presented herein shows important anomalies in the Ultra Low Frequency Range (ULF; 0.005-1 Hz), primarily for 0.25 to 0.5 Hz. Furthermore, the time variance (σ 2 ) increases prior to, during and after the seismic event in relation to the coefficient D1 obtained, principally in the Bx (N-S) and By (E-W) geomagnetic components. Therefore, this paper proposes and develops a new methodology to extract the abnormal signals of the geomagnetic anomalies related to different stages of the EQs.

Part of Special Issue "Precursory phenomena, seismic hazard evaluation and seismo-tectonic electromagnetic effects" Abstract. Results of the ULF electromagnetic emission during the Izu, 2000 earthquake (EQ) swarm and Miyake volcano eruptions during the summer period of 2000 are presented and analyzed. The analysis of the obtained data has been performed in 3 main directions: traditional statistical analysis, i.e. analysis of time dynamics of spectral density, polarization ratios and their derivatives, Principal Component Analysis, and Fractal Analysis. The statistical characteristics were studied at different frequency sub-bands in a frequency range 10 −3 -0.3 Hz. These methods of data processing are described and the obtained results are illustrated and discussed. Some peculiarities of the obtained results such as the rise of the second principal component and the rise of the fractal index can be interpreted as possible shorttime precursors.

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Abstract.Results of ULF geomagnetic measurements at station Teoloyucan (Central Mexico, 99.11 35.735 W, 19.44 45.100 N, 2280 m height) in relation to seismic activity in the period 1998-2001 and their analysis are presented. Variations of spectral densities for horizontal and vertical components, polarization densities and spectrograms of magnetic field, their derivatives are analyzed as a part of traditional analysis in this study. Values of spectral density were calculated for 6 fixed frequencies f =1, 3, 10, 30, 100 and 300 mHz. Fractal characteristics of spectra were analyzed in the conception of SOC (Self-Organized Criticality). 2 nighttime intervals, 0-3 and 3-6 h by local time have been used to decrease the noise interference in row data. In order to exclude the intervals with a high geomagnetic activity from analysis we referred to Ap indices, calculated for corresponding time intervals. The contribution of seismic events to geomagnetic emission was estimated by seismic index k s =10 0.75Ms /10D, where Ms is the amplitude of the earthquake and D is the distance from its epicenter to the station.

Abstract.A method for detecting seismomagnetic signals is presented. This work presents evidence of geomagnetic perturbations associated with three earthquakes (EQs) that occurred in Mexico within a distance of ρ = (1.8) 10 0.45M , where M is the magnitude of the EQ, during the year 2010. An improved statistical analysis in relation with the polarization analysis (as the ratio of vertical magnetic field component to the horizontal one) has been developed. The variation index (S 4 ) shows important differences one day before and one day after the EQs (magnitudes considered are from 6 to 7.2).