M 2.9 - 14 km NW of Saint-Pacôme, Canada

Earthquakes in the Charlevoix-Kamouraska Seismic Zone

The Charlevoix-Kamouraska seismic zone straddles the St. Lawrence River in southeastern Quebec. People in the seismic zone have felt small earthquakes and suffered damage from larger ones for three and a half centuries. The zone is one of the most seismically active in North America east of the Rocky Mountains. The first and largest known damaging earthquake (magnitude about 7) in the seismic zone occurred in 1663. Several others have caused damage since then, most notably in 1925 (magnitude 6.2), and the most recent damage from an earthquake in the seismic zone was in 1979 (magnitude 4.8). Earthquakes cause damage in the seismic zone every few decades. Smaller earthquakes are felt roughly two or three times a year.

Earthquakes east of the Rocky Mountains, although less frequent than in the west, are typically felt over a much broader region. East of the Rockies, an earthquake can be felt over an area as much as ten times larger than a similar magnitude earthquake on the west coast. A magnitude 4.0 eastern earthquake typically can be felt at many places as far as 100 km (60 mi) from where it occurred, and it infrequently causes damage near its source. A magnitude 5.5 eastern earthquake usually can be felt as far as 500 km (300 mi) from where it occurred, and sometimes causes damage as far away as 40 km (25 mi).

Faults

Earthquakes everywhere occur on faults within bedrock, usually miles deep. Various plate motions formed most bedrock and faults beneath the seismic zone over the last billion years. Ancient continents rifted apart to form oceans, and land masses collided to raise mountains that were then eroded down. The Charlevoix-Kamouraska seismic zone straddles the boundary between billion-year-old rocks of ancient North America on the northwest, and younger rocks of the Appalachian Mountains on the southeast.

At well-studied plate boundaries like the San Andreas fault system in California, often scientists can determine the name of the specific fault that is responsible for an earthquake. In contrast, east of the Rocky Mountains this is rarely the case. The Charlevoix-Kamouraska seismic zone is far from the nearest plate boundary, which is in the center of the Atlantic Ocean. Current thinking is that larger earthquakes in the seismic zone may occur on large faults that parallel the St. Lawrence River, whereas most of the smaller earthquakes may occur in highly fractured rock near the large faults.