Geomagnetic variations possibly associated with the Pisco earthquake on 15 August 2007, Peru E. M. Takla a, b, ⁎, K. Yumoto a, c , J. Ishitsuka d , D. Rosales e , S. Dutra f , T. Uozumi c , S. Abe c a Department of Earth and Planetary Sciences, Kyushu University, Japan b National Research Institute of Astronomy and Geophysics, Egypt c Space Environment Research Center, Kyushu University, Japan d Geophysical Institute of Peru, Ancon Observatory, Peru e Geophysical Institute of Peru, Huancayo Observatory, Peru f National Space Research Institute, Brazil abstract article info Article history: Received 28 April 2011 Received in revised form 8 December 2011 Accepted 9 December 2011 Available online 22 December 2011 Keywords: Geomagnetic variations ULF signal Crustal stress Piezomagnetic effect On 15 August 2007, Pisco earthquake (magnitude 8.0) hit the central coast of Peru near the MAGDAS Ancon (ANC) station. Geomagnetic data from ANC and other reference stations have been analyzed to detect any signature related to this great earthquake. Results indicate the presence of annual geomagnetic variations in the vertical component at ANC and Huancayo (HUA) stations (in the vicinity of the epicenter of Pisco earthquake). These variations have a quasi-sinusoidal waveform with amplitudes of about 10 and 5 nT for ANC and HUA stations respectively. They appeared clearly during the period preceding the onset of the Pisco earthquake especially at ANC station. By using HUA, Eusebio (EUS) and Kourou (KOU) as reference sta- tions in the vicinity and away from the epicenter of Pisco earthquake, a clear disappearance of the diurnal variation of the vertical component was observed at ANC station during the day of earthquake. Moreover, the Pisco earthquake and another earthquake (on 29 March 2008) near ANC station were found to occur con- currently with the depressions in the polarization ratio (Z/H) of Pc 3 (10–45 s) amplitude. Such anomalous variations appear to be a result of changes in the crustal stress field and the lithospheric conductivity in the studied region. © 2011 Elsevier B.V. All rights reserved. 1. Introduction The main component of the total geomagnetic field has an internal origin due to the self-exciting dynamo process in the Earth's core, which can be approximated by a geocentric axial dipole (Merrill et al., 1996). In addition to the internal source, the geomagnetic field also has sources in the ionosphere, magnetosphere and the lith- osphere (Olsen et al., 2002). Several processes can generate variations in the Earth's magnetic field. These variations can be described as either external or internal with respect to the terrestrial surface. The geomagnetic variations caused by physical processes related to the solar wind and the magnetosphere are known as external variations, while the variations of internal origin are mainly associated with induced and remanent magnetization within the Earth's crust (Mandea and Purucker, 2005). Local crustal geomagnetic anomalies can be caused by magnetic minerals that respond to the change in the planetary field or react to the crustal stress field variations. Hence, the crustal geomagnetic variations are controlled in part by the mechanical properties of the Earth's lithosphere. As a result, these variations may contain signatures of tectonic processes (Krolevets and Sheremet'eva, 2004). So researchers are trying to use specific features of geomagnetic variations as precursors of earthquakes. Studies of the geomagnetic variations linked with tectonic activi- ties began intensively in the 1950s. Since then, the tectonomagnetic effects have been subject of many theoretical and experimental inves- tigations (Kapička et al., 1997; Nagata, 1969). Anomalous geomagnet- ic variations occurred in association with earthquakes are widely accepted and many examples are reported (Hattori, 2004; Hayakawa et al., 2007; Masci et al., 2009; Nishida et al., 2007; Takla et al., 2011; Yamazaki and Rikitake, 1970; Yumoto et al., 2009). In ad- dition, Liu et al. (2006), reported changes of the underground con- ductivities and currents during earthquakes preparation period; which in turn can cause anomalous geomagnetic variations. The purpose of the present study is to examine any signature of the Pisco earthquake during 2007 on the geomagnetic field measure- ments in the vicinity of its epicenter. To obtain our goal, high resolu- tion geomagnetic data (1-s) from the Circum-pan Pacific Magnetometer Network (CPMN) and MAGnetic Data Acquisition Sys- tem (MAGDAS) Project have been analyzed. The local geomagnetic field variations at Ancon (ANC) [compared with three reference sta- tions] suggest a possible association with the Pisco earthquake. Tectonophysics 524–525 (2012) 29–36 ⁎ Corresponding author. Tel.: + 81 80 39967646. E-mail address: takla@serc.kyushu-u.ac.jp (E.M. Takla). 0040-1951/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2011.12.008 Contents lists available at SciVerse ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto