Detection of ULF geomagnetic signals associated with seismic events in Central Mexico using Discrete Wavelet Transform

Abstract

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.

Detection of electromagnetic anomalies of three earthquakes in Mexico with an improved statistical method

“…The application of a DWT filter using different levels in order to obtain frequency components that correspond to associated seismic anomalies shows that there are not predetermined favorable frequency ranges -apparently it depends on the direction, the distance, and the magnitude of each EQ analyzed. This kind of ULF lead time seems to be consistent with a previous work by Chavez et al (2010). The epicentral distance of these three superficial EQs varies from 563 to 1974 km.…”

“…The characteristics of the EQs are presented in Table 1. The distance between the EQs and the geomagnetic observatory is consistent with that considered in a previous study (Chavez et al, 2010). In such study the perturbations associated with the magnitude of the EQs could be observed.…”

“…This type of analysis was selected due to its time-frequency decomposition capabilities. This methodology has been previously reported for co-seismic event detection in H , D, and Z components (Chavez et al, 2010). For this study, six DWT levels were analyzed using a wavelet mother function Daubechies 1 (DB1), but we present only the D6 for comparative reasons (see Fig.…”

“…1). Finally, the results obtained in this paper would be compared with earlier results obtained with the wavelet method (Chavez et al, 2010). …”

Hardware-software system for simulating and analyzing earthquakes applied to civil structures

“…Another kind of signal frequently used to excite a structure is ambient vibration such as traffic and wind. As well as synthetic signals, ambient vibration is used to obtain the modal parameters of a structure (Brownjohn et al, 2003;Ren et al, 2004). From the above stated, it is necessary to have a simulator capable of synthesizing or generating the three types of signals, so as to excite any kind of structure under different conditions.…”