Analysis of geomagnetic measurements prior the Maule (2010), Iquique (2014) and Illapel (2015) earthquakes, in the Pacific Ocean sector of the Southern Hemisphere

Abstract. It has been possible to detect variations in the vertical component of the geomagnetic field (Bz) through its first and second derivate in a range of frequencies (microHz); these seem to be roughly related with some major seismic subduction events. We studied the period 2010–2015, analysing the daily values of magnetic records over periods close to the last three significant events that occurred through the Chilean margin, i. e., along a boundary between convergent plates that is characterized by the occurrence of seismic events of magnitude greater than Mw8. These are the events of Iquique 2014, Illapel 2015 and Maule 2010, all at different latitudes, on different dates and characterized by different types of margin (erosive or accretionary). Certain similarities were found in the associated magnetic field variations: 1) Variation in the radial or z component of the geomagnetic field and its first and second temporal derivative, modelled as a small jump, and small oscillations in the second derivative, generating a frequency band between 1c / 48.9 hours and 1c / 79.13 Hrs. 2) A variable time lapse of between 30 and 120 days; and 3) The seismic event. Furthermore, when analysing spectrograms for the second temporal derivate of the radial component, different behaviour is found related to its spectral density. This takes the form of an increase in ultra-low frequencies (0.01–0.4 mHz) between the start of the magnetic jump and the seismic event. These frequencies are lower than those found during the last years by research groups that related magnetic field and earthquakes, furthermore the concept of time lapse close to 30 days is in agreement with those research groups. The previous analyses may not be so robust, this is why additionally a new method is used with stations closer to the events and time periods of two years. We analysed the daily cumulative number of anomalous behaviour in z component of magnetic field on ground based magnetometers. The results show an increase in the number of magnetic anomalies prior to the occurrence of the three earthquakes. The behavior of the anomalies is similar to those presented by other authors for other earthquakes with similar methods in ionosphere. All this magnetic features might recover seismic information of the events and could be related with Lithosphere-Atmosphere-Ionosphere Coupling.