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Amazing findings deep inside the Earth

Researcher uses earthquake data to investigate the planet’s interior

BY LÉO CHARBONNEAU | AUG 05 2008

Earthquakes are Alessandro Forte’s stock in trade. When one hits somewhere on the planet, the geophysicist at Université du Québec à Montréal uses the information generated from the trembler to further his research. And the bigger the quake, the more data it produces. Think Chengdu this past May.

Dr. Forte, who holds the Canada Research Chair in Dynamic Earth Modeling and is a member of the Canadian Institute for Advanced Research’s Earth System Evolution Program, says it is an “ironic and poignant” aspect of his research. “One has to mentally disassociate oneself from what earthquakes are doing on the surface relative to what information they are providing us about structures within the planet.”

It is these internal structures, and how they relate to geologic processes on the surface, that interest Dr. Forte. The ability of researchers to directly sample the Earth’s interior is limited to the first few kilometres of the Earth’s crust. Dr. Forte is interested in the region extending about 3,000 km below that, and that’s where seismology comes in.

Earthquakes of sizable magnitude are essentially an everyday occurrence, and measuring their activity has also become an everyday affair. “Each earthquake radiates seismic waves deep inside the planet that cross from one side to the other,” he says. “So if you have a network of very sensitive seismographs, these come into action to measure the minute oscillations and anomalies in these oscillations.” This information is then used to create computer models of the Earth’s interior. Conceptually, the process is not unlike that used by MRI machines to view structures within the body.

Dr. Forte recently made waves for his research concerning a seismically active zone in the Mississippi River Valley that has fascinated geologists for decades. Ground zero for these scientists is the town of New Madrid, Missouri, the epicentre of a series of three massive earthquakes that hit in the winter of 1811-12. “The shaking was so great, church bells in Boston rang about 1,000 kilometres away,” says Dr. Forte.

New Madrid is an enigma; it sits in what should be a seismically stable region. The Earth is covered with large tectonic plates and where these plates collide, earthquakes are generated. But New Madrid is smack in the middle of the North American Plate, far from the plate’s edges.

However, the Mississippi Valley is in fact an ancient fault line formed when a “supercontinent” was breaking up about 600 million years ago, says Dr. Forte. (This is well before the most recent supercontinent, Pangaea, began to split apart about 225 million years ago to form our current set of continents.) That earlier break-up, incidentally, also led to the faults that extend along the Ottawa, Saguenay and St. Lawrence River valleys.

“These ancient faults are essentially parts of the crust that were under enormous tension. They almost split open, but they didn’t,” Dr. Forte explains. “With time, eventually, they healed, shall we say. But there still remain zones of permanent weakness.”

These weaknesses are not in themselves sufficient to create earthquakes. There also need to be stressors acting on those weaknesses, but where from? “That is where our work now comes into play,” he says.

Looking deep under the crust, Dr. Forte and his collaborators found evidence of a huge “slab-like feature” about 600 km beneath the surface and extending down another 1,000 km or so. The slab is on its side and sink- ing straight into the Earth’s interior. “We knew immediately,” he says, that it could only be an ancient lithospheric plate.

This plate, they hypothesize, was once part of the ocean floor that collided with the West Coast of North America during the last 90 million years at what is now the San Andreas Fault. It then slid under a large part of North America, creating the uplift of the Rocky Mountains, and “today is plunging nearly vertically under the eastern half of North America, and the axis of plunge is literally right under the Mississippi River Valley.”

Using advanced computer modelling, the researchers calculated that this slab is creating a flow that is pulling the valley downwards. “Once we saw that, we thought, ‘Aha, there’s the smoking gun.’ We finally have the connection with the stresses that are creating the seismic activity.” The controversial findings were published in Geophysical Research Letters in early 2007.

“Many geologists protested at first, but their protests have become more muted with time,” says Dr. Forte. Additional support for the theory came with a 5.2-magnitude quake on the Richter scale that hit Mount Carmel, Illinois, on April 18. The ancient plate discovered by Dr. Forte extends north under Illinois.

The researcher is now using the same modelling techniques to see what enigmas lie beneath the surface closer to home. “There’s a history in the 20th century of large earthquakes in the Saguenay and St. Lawrence River Valley,” he notes. A very large earthquake in 1929 off the Grand Banks severed the undersea cables to Europe and generated a tsunami that hit Newfoundland. In 1988 a 5.9-magnitude quake hit Quebec’s Charlevoix region.

“Quebec has a seismic hazard potential which is almost as high as the West Coast of Canada,” says Dr. Forte. He and colleagues hope to secure funds from the Canada Foundation for Innovation to install a series of seismometers in Quebec over the next few years “to try to understand what forces are generating this significant seismic activity.”

Although his expertise does not extend to the consequences of an earthquake on the surface, he admits to being concerned about how even a relatively small event could trigger significant damage, “given the state of disrepair of many of the structures in Montreal.”

Asked if the region is due for an earthquake, he answers without hesitation: “Yes, there is absolutely no doubt that there will be other significant events happening in Eastern Canada … The question, as always, is when.” If some find that prospect unnerving, he says they could always move to the most seismically tranquil area of Canada – the Prairies.

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