The Logan fault: an obstacle that is not a

entre-deux-ensembles-geologiques-soit(Quebec) “In the case of a third link between Quebec City and Lévis, one speaks of the risk that the fault may ask Logan if we dig a tunnel to the tip of the Island of Orleans. But in the East Energy Pipeline file, which cross the river at Portneuf, no one seems to care. The flaw does it end between the two? “Asks Jean-Philippe Cloutier, Lévis.

The flaw Logan has actually invited in several recent debates: not only the one surrounding the idea of ​​a tunnel between Quebec City and Lévis, but some have also invoked to justify the closure of the Gentilly-II. But it is true that one has not heard in the East Energy folder, which has still failed or controversy, or media coverage.

Why that? Your humble servant is not in the heads of environmental groups, politicians and pipeline builders, but one thing is certain: it is certainly not because the fault Logan stops between Portneuf and the island of Orleans …

There are 500 to 600 million years, while the ground today forming the southern Quebec was under water at the bottom of an ancient ocean called “Iapetus”, we read on the website (extraordinary) Planet Earth, the former geology professor at Laval University Pierre-André Bourque. For tens of millions of years of sediment accumulated at the bottom of the ocean, and are optionally processed is in sedimentary rocks, with time and the pressure of the water. Near the coast, these sediments (sand, mud, limestone flora and fauna remains) based on the continental shelf – in this case, the same geological formation as the Laurentians. Further, sediment were slightly different and were deposited on the oceanic crust, deeper.

Because of the plate tectonics, this phenomenon which derives gradually continents, cross the Iapetus Ocean eventually rise up out of the water. The shallow end, which was based on the base of the Laurentians, emerged largely unscathed in without being distorted: today is the “platform of the St. Lawrence,” or roughly the “lowlands” if preferred. The deepest part, it has been so distorted by its tectonic sedimentary rocks now form a chain of mountains: the Appalachians.

And in between is a long strip of highly fractured rocks: the fault Logan.

As suggested in this little history, and answer one of the questions Mr. Cloutier, no, the fault Logan does not stop in the region of Quebec, but squarely stretches south of Montreal to the end of Gaspésie.

So why did not he was further discussed in the Energy East issue?

In fact, says the researcher in Geological Engineering from Laval University Jacques Locat, a specialist in “geohazards” such as earthquakes, the real question is perhaps rather: why on earth do we still see the fault as Logan the source of earthquakes?

“It is a kind of urban legend, he said. It was very difficult to remove from the minds of people that the seismic activity in the region of Quebec has nothing to do with the Logan Fault. […] It is sure this is a flaw, so a broken rock zone, but it is completely inactive, so it poses no problem for construction. One can make tunnels in soil [in the sense of “relatively soft terrain,” as opposed to “rock solid”, Ed], so we can do in an area that is fractured, it just takes s ‘take it differently. But this is not a geotechnical and geomechanical particular problem of having to go through an area that was more or less broken. […] What we will do is just to inject the cement, and then we will drill in. I do not understand the fears that have been raised about it then. ”

Note that my colleague Annie Morin, in a folder on the tunnel released last year, had quoted two experts – one in geological engineering and another in civil engineering – who did not see, either, the fault Logan as a really problematic obstacle.

No earthquakes listed in the last centuries has been caused by the fault Logan insists Mr Locat. If there still is some seismic activity (although slight, let us say) in southern Quebec, he connects, it is mainly because of another “weakness” geological: the complex flaws in the St. Lawrence.

It’s that there is not always a river or even a furrow where the St. Lawrence flows today. So when it opened there very long, “flaws were formed, especially on the north shore, and it is usually these that cause earthquakes,” says Locat.

Now, is that St. Lawrence fault poses a danger for the construction of tunnel or pipeline? “It is simply that we know where the fault is to do the job properly. If you go to San Francisco [region crossed multiple flaws and often shaken by earthquakes, Ed], they make subways anyway, and they make bridges over faults. We just let people know it. And then we measure the activity levels of loopholes and built accordingly. […] It makes sections that cost a bit more expensive, but this is not an impediment. ”

Share
The Stopru