Search

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.

Various and extensive geophysical monitoring began in 1974 in the Charlevoix Seismic Zone (CSZ). The historical record and a major earthquake in 1925 characterize this zone as the likely site of a future damaging earthquake (i.e. M= 5.5 or larger) and the aim of the monitoring was to develop the capability of predicting such earthquakes. The parameters studied included: microseismicity, seismic travel times, electrical impedance, vertical movement, horizontal movement, tilt, gravity change, and strain through water well level changes. Although no clear precursor was detected in the months before the largest seismic event that occurred during the observation period ( M= 5.0), the multiparameter experiment provided new insights into the structure and the mechanics of this active region.