Could catastrophic earthquakes hit the United States? Seismologists say that major cities are located near dangerous earthquake zones throughout the country, from the most infamous on the West Coast to potential time bombs in the Midwest and even on the Eastern Seaboard.
The Pacific Northwest
Stretching from northern Vancouver Island in Canada to northern California is the Cascadia subduction zone, where one giant plate of the Earth's surface is diving deep beneath another one.
"The very largest earthquakes all occur on subduction zones," said seismologist Geoffrey Abers at the Lamont-Doherty Earth Observatory of Columbia University in New York. "These are also the faults that make very large tsunamis that propagate across ocean basins to cause a lot of damage."
These "megathrust earthquakes" that threaten Seattle, Portland and Vancouver can be magnitude 9 or greater, geological records reveal. The area hasn't really seen any significant seismic activity since instruments began observations roughly a century ago, "but that's a fairly short period of time, and the system's accumulating strain," Abers said.
Other faults — cracks in the planet's surface — might lead to smaller earthquakes, "but if they're near population centers, they can cause a lot more damage," Abers noted. "The Seattle Fault runs right through downtown Seattle."
"Compared to Haiti, probably the most important difference between the U.S. are the building codes and building designs," Abers said. "The level of strong ground shaking we saw in Port-au-Prince was largely devastating because of the unreinforced masonry there and the shantytowns on unreinforced slopes. In the U.S., we'd always see some damage, but we'd expect to see less loss of life because the building codes are strong in California and somewhat strong in the Pacific Northwest."
The most well-known earthquake zone in the United States is the San Andreas Fault, where the Pacific Plate and the North American Plate are sliding past each other, running along heavily populated areas of California. Still, other fractures in the earth threaten the state as well, such as the Hayward Fault that lies mainly on the east side of San Francisco Bay, and previously unknown fault that caused the 1994 Northridge quake.
"The biggest earthquakes we have any evidence for in California are magnitude 8," Abers said. In California, the faults are very well studied, and the state is very well prepared.
That's not to say the risk is small.
Geologists expect the Los Angeles area will eventually be struck by an earthquake larger than any seen in recorded history.
Three of the largest earthquakes in North America recorded in history originated from the New Madrid fault system over the course of two months from 1811 to 1812. These magnitude 7 events shook with enough power to apparently force the Mississippi River to temporarily flow backward.
The quakes — the largest ones ever known in the center of the United States — have raised fears of a "big one" there sometime this century. The closest cities to the New Madrid fault system are Memphis and St. Louis.
"Fundamentally we don't have a good understanding of exactly how earthquakes in the middle of continents work," Abers said. "We understand a lot about tectonic plates at their boundaries — when it comes to the middle of plates, the thought is that somehow the movement of plates also builds up stresses in the middle of continents. If we can solve this, we could establish a better idea of what hazards earthquakes pose there."
A dense array of seismic instruments is now investigating the zone, and scientists are looking for evidence of ancient earthquakes to get a better sense of how it works.
The Eastern Seaboard
The ancient Ramapo Fault runs near New York City. It last experienced serious movement some 200 million years ago, "with mountains going up and volcanoes erupting in the area," Abers said. "It's thought it might be where stresses are building up today."
A number of other ancient faults go from Canada all the way at least to South Carolina. The largest earthquake in the northeast was probably the Cape Ann earthquake in 1755 off the coast of Massachusetts, which might have been a magnitude 5.9 quake.
"There's clearly much less activity on the Eastern Seaboard than in California or the Pacific Northwest, but the flip side of that is that the building codes aren't of the same standard," Abers said. "So even modest-sized earthquakes close to populated areas could be much more destructive. There are a lot of emergency preparedness groups in California, a lot of work on building codes and designs, but building to earthquake codes is expensive, and in the east there are older buildings built before people thought very hard about earthquakes. Buildings cause 80 percent of deaths in earthquakes from structure collapse."
"The ground in the East also propagates shaking much more efficiently, so earthquakes would affect a much bigger area," he explained. "In the west, the crust is much more busted up, so the seismic waves attenuate — the magnitude 6.7 Northridge earthquake felt in Southern California but not Northern California. In the east the ground has hardened for millions and millions of years, so I felt shaking in Boston from a magnitude 6.0 earthquake in central Quebec that was 500 miles from where I was sitting."
Alaska experiences the most earthquakes in the United States, more per year than the combined total of the rest of the country. The nation's largest recorded earthquakes have all happened there as well.
"The last big earthquake in 1964 there was the largest ever recorded in the United States at magnitude 9.2," Abers said. "It destroyed several towns and it heavily damaged Anchorage. The risk of tsunami is high for coastal areas as well."
The Alaskan-Aleutian megathrust is a subduction zone "much bigger than in Cascadia," Abers said. "The saving grace in Alaska is that the population density is much lower than in California and the Pacific Northwest, but the population of the state has increased dramatically, and there's infrastructure there such as the Alaska Pipeline that wasn't there in 1964."
There is seismic monitoring in the state, particularly the urban areas, "but there's large parts of Alaska that are not that well-studied — magnitude 4 earthquakes go undetected all the time in some of these places," Abers said. "There's just a lot of real estate to cover."
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