What people report ‘feeling’ in earthquakes offer valuable information

There is a distinct difference between magnitude and intensity when it comes to earthquakes, and what people actually “feel” can provide valuable insight to seismologists, Hadi Ghofrani, a seismologist and research assistant at the University of Western Ontario, noted during a luncheon Thursday in Toronto.

Ghofrani quipped during the luncheon, hosted by the Property Casualty Underwriters Club (PCUC), that seismologists consider people to be good seismometers.

Citing the magnitude 5 (M5) earthquake in Val-des-Bois, Quebec in 2010, he told attendees the event’s Did you feel it? report – just one of the many such reports available on Natural Resources Canada’s website – includes responses from approximately 59,000 people who filled out electronic forms.

People “can be in random regions” where seismometers cannot be placed, Ghofrani pointed out. “So their reports are very important because we can find the correlations between the felt intensities and what was the reality.”

Understanding earthquake intensity could be important with regard to potential damage at and away from the quake epicenter, and could possibly point the way to prevention and mitigation measures.

The earth’s crust is made up of major plates that are moving relative to each other, Ghofrani said. Although the movement is slow, it “will cause stress to build up like when you bend a pen or a ruler,” he explained to luncheon attendees. “When this stress gets higher than the friction between two blocks, you’ll have a sudden break, which we’ll call an earthquake.”

When seismologists refer to magnitude, they are talking about the size of the earthquake at its starting point, while the intensity “refers to the observed effects of an earthquake at the surface,” Ghofrani noted during his presentation.

Magnitude does not change with distance, but intensity does (geology and local site conditions also influence intensity). “So we could have the same event, with the same magnitude, but experience it differently in different locations,” he said.

To illustrate how intensity changes with distance from an earthquake epicenter, Ghofrani explained that intensity is measured using the Modified Mercalli Intensity (MMI) Scale (information posted on Natural Resources Canada’s website defines MMI 1 as not felt, or, except rarely under especially favourable circumstances, while MMI 12 is defined as total damage, with practically all works of construction damaged greatly or destroyed).

Using a theoretical example of an M6.4 earthquake, at 18 km depth, 85 km from Ottawa, Ghofrani reported the following:

• at 25 km from the epicenter, about seven to 20 seconds after the event, intensity would be MMI 8 or MMI 9, meaning the quake would be felt by everyone in the area and there may be damage to buildings;
• at 50 km, about 15 to 20 seconds after the quake, the intensity is reduced to MMI 7, with people feeling the quake;
• at 85 km in Ottawa, the intensity is about MMI 5, with people realizing something has happened, but no damage to buildings; and
• the intensity becomes smaller and smaller as the wave travels to Montreal (about 200 km from the epicenter) and then to Boston (about 500 km away).

By the time the wave reaches Washington, “nobody cares,” Ghofrani quipped.

With regard to magnitude, Ghofrani said that a quake’s magnitude depends on the size of the reactivated fault surface. Ground shaking (amplitude of ground movement) increases by a factor of 10 with an increase of one order of magnitude, he explained to luncheon attendees.

To illustrate, Ghofrani noted that in terms of ground shaking, an M6 earthquake is about 10 times that of an M5 quake, while an M9 earthquake is about 100 larger than an M7 quake.

But the most important thing is the energy released, which increases by a factor of about 32, he emphasized. “If we compare M6 and M5, M6 is 30 times the strength of M5. And it’s around 1,000 times stronger than M4.”

The March 2011 earthquake in Japan, for example, was magnitude 9.1, releasing 600 million times the energy released by the Little Boy atomic bomb at Hiroshima, equal to enough energy to power the entire United States for 100 years. Ground shaking lasted for about five minutes.

Almost all earthquakes are weaker than M5 in eastern Canada, which is below the threshold for causing damage to engineering facilities, Ghofrani pointed out.

Most earthquakes are M2.5 or less and are not felt, while M4 can be felt over larger areas and the threshold for making damage is around M5, he said.

Eastern Canada has had some larger quakes, including the M7 event in Charlevoix in 1663, and the M7.2 event in Grand Banks in 1929.

More recently, there has been the M5 quake in Val-des-Bois, Quebec in 2010, and an M3.9 quake in Montreal in 2012. And on Friday, Earthquakes Canada reported a 4.4-magnitude quake about 37 km north of Cap-Chat in the Lower St. Lawrence seismic zone.

Ghofrani noted that during the Val-des-Bois quake, the seismometer at the top of the Peace Tower on Parliament Hill showed larger amplifications than the one in the basement. “This example has clearly shown that buildings can resonate with the shaking and amplify the ground motions.”

In addition, citing the Christchurch quakes in 2010 and 2011, he noted that most of the building damages to masonry related to the façade of the buildings. “This is important because we do have equivalent masonry buildings in Ottawa,” he said, pointing to the James Street and the Byward Market areas.

Most large-magnitude earthquakes are happening at subduction regions, when two plates are subducting beneath each other, Ghofrani explained. “We do expect to see a magnitude 9 megathrust earthquake similar to the 2011 Tohoku earthquake along Cascadia subduction zone.”

The Cascadia subduction zone, which includes Vancouver, is one of the few places in the world with all three types of major earthquakes, Ghofrani told attendees. The Pacific plate is diverging from Juan de Fuca plate; the Juan de Fuca plate is subducting beneath the North American plate; and immediately north of this area is the Queen Charlotte fault, an active transform fault in which the plates are moving sideways in relation to one another.

“This could be the major reason for causing megathrust earthquake,” which have a return period of about 500 years, Ghofrani said. One such quake occurred in the region in 1700.

Ghofrani made a number of general observations about earthquakes in Canada:

• seismologists expect a quake disaster will occur in a Canadian city;
• earthquake effects on buildings are somewhat predictable;
• building codes provisions can mitigate some losses, although not all;
• the consequences for recovery are somewhat unpredictable; and
• preparing for earthquakes can help reduce losses.