Changes in magnetic and telluric fields in a seismically active region of Eeastern Canada: Preliminary results of earthquake prediction studies

A project for detecting time-dependent electrical resistivity changes in crustal rocks by measuring short-period variations of the magnetic and telluric fields has been in operation since October, 1974 in a seismically active region northeast of Quebec City, Canada. Significant changes in amplitude and direction of telluric fields have been observed for variations ranging from 5 to 120 min. A change with a duration of a year or so seems to be a predominant feature. Superposed on it, a more rapid change with a duration of a few months is seen in late spring and early summer. Although the observed changes show a history similar to a premonitory change as expected from a model of dilatancy with fluid diffusion, they are unlikely to have any bearing on a dilatancy process in the crust since no earthquakes of magnitude greater than 3 associated with the changes have taken place in the survey area. It is pointed out, however, that the magnetotelluric method provides a useful means of detecting changes in electrical resistivity in the shallow crust. Thus the possibility exists of detecting resistivity changes associated with shallow earthquakes.