Megaquakes, which are subduction earthquakes with magnitudes of 8 or greater, occur about every 500 years on average along the Cascadia Subduction Zone in the Pacific Northwest. The earthquakes and related tsunamis can cause enormous damage. However, they may not be the most urgent seismic threat in the region, according to John Clague, a professor and expert on natural hazards at Simon Fraser University (SFU) in British Columbia.

Smaller-magnitude crustal earthquakes, which have a more compressed footprint but occur more frequently than megaquakes, are a significant concern that the public needs to be more aware of, he told Eos. Clague, who is SFU’s Canada Research Chair in Natural Hazards Research, spoke with Eos following his 19 October talk about great earthquakes, presented at the Geological Society of America’s annual meeting in Vancouver, British Columbia.

“It’s hard to convince the public”—about this concern regarding crustal earthquakes—“because they only think of the great ones. It’s partly our doing: We talk about these magnitude 9 earthquakes. But it’s got to be balanced by how rare they are. If you are interested in risk [and] what is the likelihood of damage to infrastructure, it is really carried by these more frequent [crustal] earthquakes,” he said.

Risk Is Tied to Frequency

Megaquakes along the Cascadia margin occur in clusters of three to five events, which then could be separated from other clusters by periods of up to 1000 years, Clague said. However, it is unclear how many events might be in a cluster and whether current conditions put us in a cluster or aseismic period.

“It makes a big difference in terms of the conditional probability of the next [megaquake]. If we are in a cluster, it jacks up the probability that we are close to the next one. If we are beginning an aseismic interval, then it really reduces the probability and almost, if you made the argument, says, ‘Well, let’s not worry about those; let’s worry about these local crustal earthquakes,’” Clague told Eos.

Those crustal earthquakes do present a significant threat, he said, noting that magnitude 7 earthquakes occur more frequently in the Pacific Northwest than the larger quakes do. Because the sources of crustal earthquakes can be closer to major population areas, even a magnitude 6—such as the quake that damaged Christchurch, New Zealand, in February 2011—can cause billions of dollars in damage. “There are not too many places”—in the Pacific Northwest—“you can put a magnitude 7 earthquake [where it] is not going to have a big impact,” he said.

Tsunami Hazards and Risks

Megaquakes not only bring about severe shaking but also can cause large tsunamis. For example, a tsunami recorded in Japan in 1700 was the result of a magnitude 8+ earthquake west of Vancouver Island, Canada, on 26 January of that year. Local tsunamis that are caused by megaquakes in the Pacific Northwest can also wreak havoc on the west coast of North America, including Oregon, California, and the western coast of Vancouver Island.

Vancouver, which is located on an inlet off the Strait of Georgia off British Columbia’s mainland, might not feel the full brunt of a subduction earthquake and a resulting tsunami, Clague said. “Any earthquake is going to produce an attenuation of ground motions as you move away from the source. If that earthquake were to occur right now, we would expect [in Vancouver] a kind of filtered ground motion signal, where you would get long-­period, low-­amplitude waves. It would feel like you were at sea.” He said the ground motions would produce an undulating rolling, but they would not be “the kind of thing that drives you off your feet at that distance.”

Clague said that even a resulting 2-meter-high wave from a tsunami, especially during high tide, could be very damaging to the Vancouver metropolitan area—particularly to ­low-­lying areas such as Vancouver’s neighboring city of Richmond, the Vancouver airport, and other infrastructure and buildings.

More Science and More Public Action to Reduce Risk

Clague called for more support for scientists working to understand seismicity in the region. He said that some areas for research include further refinements in earthquake chronology and a better understanding of earthquake faults, the linkages between earthquakes along the Cascadia subduction zone and crustal earthquakes, and the impacts of earthquakes on infrastructure.

In addition, Clague specified the need for better public education about seismic risk and said governments have a responsibility to focus on upgrading critical infrastructure and on emergency preparedness. He noted British Columbia’s recent initiative to upgrade old schools that are vulnerable to collapse. “You can’t do everything. It’s too costly. But you can ensure that your hospitals, your airports, your schools, your bridges are going to be functional after an earthquake,” he said.

—Randy Showstack, Staff Writer

© 2014. American Geophysical Union. All rights reserved.

© 2014. American Geophysical Union. All rights reserved.