Secret earthquakes have been discovered in Montreal

As part of a citizen science project, about fifty volunteers helped detect minor earthquakes from their garden in Montreal.

Seismologist and McGill University professor Yajing Liu called on Montrealers to deploy 48 small seismometers in the greater metropolitan area. Buried a few centimeters underground, but through satellite communication, these electronic devices continuously record surrounding seismic activities during the month of December.

The scientist notes that Montreal is located in one of the three seismic zones in Quebec (i.e. Western Quebec, Charlevoix, and Basse-Saint-Laurent). However, only one stop Canadian National Seismic Network The RNSC has been installed on the island to detect seismic activity.

“There are very few earthquakes on the island,” adds Yajing Liu. I was wondering if it was because the island was “silent” or because there were not enough stations to monitor it. »

Citizen enthusiasm

About 80 people applied to participate in the project. Among the many applications, “we chose the distributors to get the best possible distribution,” the researcher emphasizes.

Yajing Liu, accompanied by McGill students, visited several backyards to bury earthquake sensors. why there? “On public land or in a park, a curious person might see part of the device sticking out and dig it up,” she answers.

Between noise and tremors

Recording seismic waves may seem simple, but seismometers pick up everything, including stray waves. “The trucks, the person walking, the wind, the river…we even capture the ambient sound,” says Yajing Liu.

She adds that there is always uncertainty associated with stone quarries near Montreal. “Rock explosions can cause waves similar to earthquakes,” she says.

How can we detect real earthquakes amidst this noise? That's the whole point of having so many seismometers.

In fact, an earthquake propagates across the Earth in two different waves. The first, the primary wave, moves quickly, shaking the ground horizontally. The second wave, the secondary wave, is slower, has a larger amplitude, and shakes the ground vertically. This difference is directly observed in the seismograms, which indicate two distinct peaks separated by a few seconds or minutes.

Once the tremor is identified, scientists analyze all the data to monitor whether the two waves were detected by all seismometers. The peaks will be similar, but more distant and delayed relative to each other as the seismometers are farther from the epicenter. With the precise location and time of seismometer recordings, scientists can monitor signals that indicate a potential earthquake.

The seismologist reports that a strong earthquake that occurred on December 3, 2023 was also detected in the Philippines. “It's 13,000 kilometers away, but we're able to record it at lower frequencies!” she says, with a smile on her lips.

Do better than national stations

Back in Montreal, the team is still analyzing and comparing its data Those of the RNSCSearching for earthquakes that are not visible to the eyes of national stations.

Locations of seismometers distributed by the McGill team.

Yajing Liu explains that it usually takes at least three seismic stations to identify the source of an earthquake and rule out extraneous noise. “If the earthquake is very weak, only one station may record it and we cannot be sure whether it is an earthquake or noise,” she says.

The first data collected as part of this project supports the idea that small earthquakes elude Montrealers and the official station. Even if the analysis continues, scientists estimate they will find a small number of earthquakes of less than 1.5 magnitude that went unnoticed during the month of December. But she adds that these are very small tremors; Earthquakes become perceptible to humans only with a magnitude of about 3 and in a radius of about thirty kilometers.