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.

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