A possible seismomagnetic precursor of the El Mayor-Cucapah earthquake (Mw 7.2) of April 4, 2010, Baja California, México

we describe a possible seismomagnetic precursor to the mw 7.2 earthquake of april 4, 2010, with epicenter nearby the sierras el mayor and cucapah in ne baja california, méxico, (el mayor-cucapah earthquake). we used total-field magnetic intensity data recorded on a temporal magnetic base station on the roof of the earth sciences division of cicese, in ensenada, baja california, méxico, and magnetic observatories at tucson, arizona, and fresno, california, usa. these data were recorded at a sampling rate of one minute from march 26 and until april 19, 2010. a comparison of these data let us verify that during the measurement period the three stations recorded similar variations of the geomagnetic field. our analysis is based upon a statistical measure that we call index of magnetic variation, which allows us to detect and quantify anomalous variations of the magnetic field from data of a single station. based upon the magnetic field data, we suggest that the proposed index of magnetic variation identifies a possible seismomagnetic event, which begins 40 minutes before the earthquake and has duration of about 3 hours, as well as anomalous behavior of the geomagnetic field caused by a geomagnetic storm beginning on april 5.