No abstract

The November 25, 1988 Saguenay, Québec earthquake (mb 5.9) was preceded by a foreshock 62 hours earlier (mb 4.4) and followed by over 50 aftershocks, of which only two have been larger than magnitude 3. The unusually large 29 km depth of the main shock is well determined using a field network deployed after the foreshock. Nearly all of the aftershock activity is shallower than the main shock, suggesting that the rupture propagated upwards. The event produced unexpectedly large amounts of high frequency energy, as evidenced by the value of mbLg (6.5) and by high accelerations observed at distances ranging from 40 to 800 km. Focal mechanisms for the foreshock, main shock and largest aftershock indicate thrust faulting. The felt area exceeds 3.5 million km2 and closely approaches that of the 1925 Charlevoix earthquake. The location of the Saguenay event in a region with little historic seismicity, its depth, and its large high-frequency generation, pose several problems for existing models of seismic zoning and hazard assessment for Eastern North America.

Very often after a strongly felt earthquake, the media and the general public consider seismologists to be the source of all information, particularly in regions where earthquakes occur infrequently. Widely felt earthquakes can have a strong social impact (“the post-earthquake vibrations among the public”) whenever the population is unprepared. Since 1935, eastern Canadian seismologists have been involved in four cases of post-earthquake communications with social impact. Recently, seismologists of the Geological Survey of Canada developed a communications strategy to help people cope with post-earthquake stress, particularly that component of stress arising from an insufficient knowledge of earthquakes. A communications plan is prepared before the emergency response period; a high priority is given to those persons who felt the earthquake most strongly; basic information on earthquake phenomena and preparedness is provided; and finally, seismologists make themselves both known and available before and after the earthquake. With this strategy, seismologists can help to relieve unnecessary anxiety and to promote positive preparation.