Several devastating earthquakes have rumbled beneath the Pacific in the past 15 months. In Feb­ruary 2010 a magnitude 8.8 temblor slammed central Chile; last September a 7.0 quake walloped Christchurch, New Zealand, leading to a 6.3 aftershock in February. The magnitude 9.0 mega quake that flattened Japan in March is tied for fourth largest in the past 110 years.

These events have led many people to wonder if they are somehow linked. Most likely, scientists say, their near coincidence is merely a statistical fluke. That doesn’t mean, however, that it is necessarily safe to come out from under the bed. The best gauge of quake risk is the geologic record. And new data on that record tell a disturbing story, especially in the northeastern Pacific.

Although most people may consider southern California to be the most earthquake-prone region in the nation, the Cascadia subduction zone is arguably the biggest seismic hazard in the U.S. It parallels the coast and poses a seismic threat to cities such as Vancouver, B.C., Seattle, and Portland, Ore. At that subduction zone, the tiny Juan de Fuca plate slides eastward underneath North America between 30 and 40 millimeters a year—but this interface has apparently been locked for centuries. “This subduction zone stands out as the big elephant in the corner,” says Chris Goldfinger, a marine geologist at Oregon State University. “It sits quiet for hundreds of years and then goes off all at once.”

New data suggest that the northern portion of the subduction zone, from the middle of Vancouver Island to the Washington-Oregon border, has a 10 to 15 percent chance of suffering a magnitude 8.0 or greater quake in the next 50 years. The southern portion, stretching from the Washington-Oregon border to California’s Cape Mendocino, has a 37 percent chance of the same-size quake over that same interval. Goldfinger and his colleagues expect to publish the data in an upcoming USGS report. The next big one, he says, “is going to happen. It’s just a matter of narrowing down the timeline.”

How do scientists measure the speed of tectonic plates?
The best way to measure how quickly two tectonic plates converge is to use the Global Positioning System. By repeatedly checking the distances between specific points on two different plates, researchers can assess long-term rates of convergence and measure sudden movements, such as Japan’s 2.4-meter (eight-foot) leap eastward during the March 11 quake. Before the advent of GPS, scientists relied on rocks in the ocean floor, which, when they cool, record the direction of the earth’s magnetic field. Knowing when and how often the field has flipped in the past enables researchers to calculate the rate at which new ocean crust forms at mid-ocean ridges. Another technique is to sample and map rock formations on both sides of a tectonic interface—especially formations that have a distinctive composition or an unusual assemblage of fossils. 

2 Comments

1. 1. OBagle 08:55 AM 5/3/11

   What? No comments? I guess it's true what they say - no one really cares until it happens to them. Then, if we know that something has already happened to somebody somewhere, so many times already, then something will eventually happen to us. Unfortunately, the list is so long that the threat of a massive earthquake doesn't raise anyone's pulse anymore. If you lose your house and livelihood to unemployment and foreclosure, then it's no different than if your house was flattened in an earthquake - you're homeless all the same. Same with losing your health insurance and then coughing up blood the next day. So, excuse us, SA, if we fail to run for the exits after reading this article.

   Reply | Report Abuse | Link to this

2. 2. Max Rockbin 12:33 PM 6/7/11

   I was trying to figure out if we're really overdue for a Big One or not. The usual statement is that we get a big Cascadia fault quake every 250 years or so and it's been 300+ years so Look Out.
   
   Which is true... IF you include the 9.x and 8.x magnitude quakes lumped together. That makes about 41 quakes in 10,000 years. According to the graph.
   
   But only 19 have been the huge 9.x quakes. (By the way, 1 point increase = 32x increase in energy released. Not 10x like some of the newspapers and TV say. It's logarithmic, but not base 10).
   
   So the HUGE quakes are more like 500 years apart (10,000/19)
   
   Which makes it seem like we're not at all overdue for another one of those 9.x's. We should have some breathing room since it has only been 300 years.
   
   Also, in the past 10,000 years there have never been more than two 9.x earthquakes within a thousand years of each other.
   
   Except that's not exactly completely true.
   The last 1000 years breaks that trend. There have been three 9.x quakes since 1300. Plus another 8.x just for good measure.
   So the last 1000 years have been much much more active.
   
   That's really good! (or bad).
   
   If you believe that earthquakes relieve stress built up and that the stress is building up at more or less the same rate (10,000 years isn't much time in plate tectonics earth changing scales), then whoopee! We've relieved a huge amount of energy buildup with those 3 quakes (the 8.x hardly matters. it's just 1/32nd as much energy as each of the other 3 quakes since 1300).
   
   On the other hand, maybe quakes happen in swarms and we're in the biggest swarm in 10,000 years. Maybe the plates are moving faster and more stress is building up this millennium? I haven't found info about the speed of these plates over the past 10,000 years.
   
   Historically though, it looks as though we've relieved way more stress than our epoch's share. We've relieved 97 "units" (if a unit is one 8.x quake) of stress in 700 years (3x 32x quakes and one 1x quake). A normal average stress relief would be less than 44 units in 700 years.
   
   So we've got 200 years before a 9.x hits (on average) and
   we've had enough stress relief over the last 700 years to be owed another 700 years of tranquility!
   
   Maybe some geologist will say I'm full of crap. I'd love to know if I am...

   Reply | Report Abuse | Link to this