Earthquake Summary

Tectonic Summary

The June 23, 2010 Ontario-Quebec border region earthquake occurred at 1:42 pm local (eastern) time about 60 km (38 miles) north of Ottawa, Ontario, Canada's capital city. The preliminary estimate of magnitude (M) is 5.0, at a depth of roughly 19 km (12 miles). These estimates may change as more data becomes available.

This earthquake occurred near the southern edge of the Western Quebec Seismic Zone. Earthquakes within this zone are mostly small. They tend to cluster in a wide area that is slightly elongated northwest-southeast. Historically, earthquakes in the Western Quebec Seismic Zone have caused damage roughly once a decade. Three or four smaller events each year are felt in the region but are generally too small to cause damage. The largest earthquakes known in this part of Canada occurred in 1935 (M6.1), about 250 km (150 miles) to the northwest of todays event, and in 1732 (M6.2), about 150 km (100 miles) to the east. The 1732 earthquake caused significant damage in Montreal.

Earthquakes of the size of todays event are uncommon east of the Rockies, but many have occurred since the arrival of European settlers three centuries ago. In eastern North America and geologically similar regions worldwide, M5.0 to M5.5 earthquakes typically cause light to moderate damage out to a few tens of kilometers (miles) from the epicenter, depending on the number of people and type of buildings near the epicenter. Typically these earthquakes are felt hundreds of kilometers (miles) away. Earthquakes of this size and depth are unlikely to rupture the Earth's surface, although exceptions are known.

The main faults near this earthquake zone trend northwest. These faults form the Ottawa graben and were most active several hundred million years ago. Some of the faults of the graben have been reactivated one or more times since then. The initial focal mechanism of todays earthquake suggests reverse faulting on a fault trending southeast-northwest. However, the size and depth of this earthquake make it uncertain whether the causative fault can be identified.

 

EARTHQUAKES IN THE 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.

Earthquake Information for Canada