Tectonic Summary

Magnitude 5.2 ONTARIO-QUEBEC BORDER REGION, CANADA
Wednesday, June 23, 2010 at 17:41:41 UTC

EARTHQUAKES IN THE WESTERN QUEBEC SEISMIC ZONE
Western Quebec Seismic Zone People in the large Western Quebec seismic zone have felt small earthquakes and suffered damage from larger ones for three centuries. The two largest damaging earthquakes occurred in 1935 (magnitude 6.1) at the northwestern end of the seismic zone, and in 1732 (magnitude 6.2) 450 km (280 mi) away at the southeastern end of the zone where it caused significant damage in Montreal. Earthquakes cause damage in the zone about once a decade. Smaller earthquakes are felt three or four 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. Most of the bedrock in the Western Quebec seismic zone was formed as several generations of mountains rose and were eroded down again over the last billion or so years.

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 Western Quebec seismic zone is far from the nearest plate boundaries, which are in the center of the Atlantic Ocean and in the Caribbean Sea. The seismic zone is laced with known faults but numerous smaller or deeply buried faults remain undetected. Even the known faults are poorly located at earthquake depths. Accordingly, few, if any, earthquakes in the seismic zone can be linked to named faults. It is difficult to determine if a known fault is still active and could slip and cause an earthquake. As in most other areas east of the Rockies, the best guide to earthquake hazards in the Western Quebec seismic zone is the earthquakes themselves.

Preliminary Earthquake Report
U.S. Geological Survey, National Earthquake Information Center
World Data Center for Seismology, Denver