Latitudinal variation rate of geomagnetic cutoff rigidity in the active Chilean convergent margin

Abstract

Abstract. We present a different view of secular variation of the Earth's magnetic field, through the variations in the threshold rigidity known as the variation rate of geomagnetic cutoff rigidity (VRc). As the geomagnetic cutoff rigidity (Rc) lets us differentiate between charged particle trajectories arriving at the Earth and the Earth's magnetic field, we used the VRc to look for internal variations in the latter, close to the 70 • south meridian. Due to the fact that the empirical data of total magnetic field BF and vertical magnetic field Bz obtained at Putre (OP) and Los Cerrillos (OLC) stations are consistent with the displacement of the South Atlantic magnetic anomaly (SAMA), we detected that the VRc does not fully correlate to SAMA in central Chile. Besides, the lower section of VRc seems to correlate perfectly with important geological features, like the flat slab in the active Chilean convergent margin. Based on this, we next focused our attention on the empirical variations of the vertical component of the magnetic field Bz, recorded in OP prior to the Maule earthquake in 2010, which occurred in the middle of the Chilean flat slab. We found a jump in Bz values and main frequencies from 3.510 to 5.860 µHz, in the second derivative of Bz, which corresponds to similar magnetic behavior found by other research groups, but at lower frequency ranges. Then, we extended this analysis to other relevant subduction seismic events, like Sumatra in 2004 and Tohoku in 2011, using data from the Guam station. Similar records and the main frequencies before each event were found. Thus, these results seem to show that magnetic anomalies recorded on different timescales, as VRc (decades) and Bz (days), may correlate with some geological events, as the lithosphereatmosphere-ionosphere coupling (LAIC).

A review and upgrade of the lithospheric dynamics in context of the seismo-electromagnetic theory

“…The yellow arrow is the direction of the electric current J . This size of microcracks could be the one that allows us to explain the measurements of magnetic variations of Cordaro et al (2018) and Marchetti and Akhoondzadeh (2018) and the suggestion of Kelley et al (2017).…”

“…It is also necessary to determine the order of magnitude of the microcrack dimensions within the lithosphere. Cordaro et al (2018) show disturbances in the magnetic field prior to the 2010 Maule earthquake (36 • 17 24.0 S 73 • 14 20.4 W). If we consider the OSO station (40 • 20 24 S, 73 • 05 24.0 W), we can note that it is ∼ 450 km from the epicenter of the 2010 Maule earthquake (the closest magnetic station to earthquake).…”

“…A number of investigations attempting to relate the magnetic field to seismic events have emerged over the past few years, (e.g., Park, 1996;Surkov et al, 2003;Johnston et al, 2006;Balasis and Mandea, 2007;Sgrigna et al, 2007;Saradjian and Akhoondzadeh, 2011;Varotsos et al, 2011;De Santis, 2014, 2017Donner et al, 2015;Schekotov and Hayakawa, 2015;Daneshvar and Freund, 2017;Cordaro et al, 2018Cordaro et al, , 2019Marchetti and Akhoondzadeh, 2018;Pulinets et al, 2018; among others). However, there is still no unified causal mechanism that is widely accepted and that may account for the physics of all these observations prior to or during the occurrence of an earthquake (Hough, 2010), although the laboratory evidence shows the possibility of an increase in the conductivity of rocks when subjected to stress changes, through either microcracks or chemical imperfections (Freund, 2003;Anastasiadis et al, 2004;Cartwright-Taylor et al, 2014).…”

“…4. One of the most important groups of measurements corresponds to the recording of ultra-low-frequency (ULF) magnetic signals, i.e., frequencies below 1 Hz, as many researchers have found such anomalous frequencies prior to or during earthquakes, mainly close to millihertz and microhertz (Fenoglio et al, 1995;Sorokin and Pokhotelov, 2010;Schekotov and Hayakawa, 2015;De Santis et al, 2017;Cordaro et al, 2018Cordaro et al, , 2019Marchetti and Akhoondzadeh, 2018; among others), although according to Vallianatos and Tzanis (2003) the magnetic field oscillation frequencies that could be related to earthquakes have a range of at least 3 orders of magnitude, so that kilohertz variations measured by other groups could also be included (Rozhnoi et al, 2008;Büyüksaraç et al, 2015;Potirakis et al, 2018a; among others).…”

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

“…The first significant property in the secular variation of the geomagnetic field is determined by the shielding it exercises on the cosmic ray particles which reach the Earth, called the geomagnetic cutoff rigidity. It is defined as the product of the force of the magnetic field and the curvature radius of the incident particle rg, which relates the geomagnetic latitude with the classification of the particle trajectories and could be related to geological features in the Chilean margin [Pomerantz, 1971, Shea and Smart, 2001, Smart and Shea, 2005, Cordaro et al, 2018, Cordaro et al, 2019.…”