Geomagnetic anomalies registered in the Iberian Peninsula before a time period of intense seismic activity

Abstract

Geomagnetic data registered by Geomagnetic Observatories of Almeria, Coimbra, San Fernando and Toledo, located in the Iberian Peninsula and Tenerife in Canary Islands were used to study geomagnetic field anomalies from August 1968 to March 1969. Data used were obtained in Annals of the Observatories as average hourly values of the magnetic declination and horizontal and vertical components of the magnetic field. Magnetic field disturbances were studied using average monthly values, average daily values and average hourly values. The anomalies were quantified using IGRF model values and monthly average hourly values of the Observatories data. During the time period of our study four earthquakes of magnitude equal or higher than 3 are registered in the seismic catalogue with two epicentres in the Gibraltar Region and two epicentres in the Gulf of Cadis. In the early hours of February 1969 an earthquake of high magnitude (M=8) was felt in Portugal, Spain and North-Western Morocco. Near two hours later, another earthquake of magnitude 6.5 was recorded. In that day 32 earthquakes were recorded in the region. In the second part of the work , a special attention is given to the earthquake of February 10th,1969, due to the location of this epicentre and the temporal proximity of February 28 th. The magnetic field in the region suffered three strong magnetic disturbances during the month of February in 1969. The earthquake of February 10th occurred just before the start of the second interval of intense magnetic perturbations. The third time interval of intense magnetic disturbances occurred before and during February 28 th day. The analysis made using hourly values to time intervals including the earthquake occurrence (five earthquakes) shows “equal values” of magnetic anomalies occurring in time intervals of three hours. Strong variations of the horizontal field occurred before the earthquakes of February 10th and February 28 th.