Geomagnetic field precursors associated with earthquakes

An association between local geomagnetic field changes and earthquakes has been often  reported since the later part of  19th century. Such possible 'seism magnetic' effects are interpreted to be due to  the  stress-induced change  in the magnetization of rocks composing the earth ‘s crust. Observational trials to establish such association have suggested a strong correlation of the tectonic activity, such as earthquake occurrence or volcanism, with local anomalous areas where the secular change of geomagnetic field is larger than the normal one. Also the temporal fluctuation in the secular variation of the geomagnetic field corresponds to the maximum of seismic activity in the vicinity of observatory. In addition to this, the study of long-term behavior of induction vector of short period fluctuations suggests a sort of cyclic variation, the extremes of which appear to occur prior to large earthquakes. The changes in the transfer functions of such events tend to coincide with the occurrence of moderate and large earthquakes. The preliminary results of an analysis of geomagnetic data from Alibag indicate that seism magnetic effects of earthquakes in the Koyna region can be identified in the magnetic records of Alibag, which may provide a means of predicting earthquakes from Koyna region.