Shaken, not Stirred?

February 5, 2007 11:01 AM

Seismologists: how likely is a major Pacific Northwest earthquake? The Canadian Geological Survey seems to think a big quake is likely. The Pacific Geoscience Center thinks the probability went up, but is still very low.

Is there cause for concern, or is this just more of the typical Pacific Northwest "Big One" worry?

posted by b1tr0t to science & nature (5 comments total)

Emergency services are on 24-hour alert in BC this week. They don't do that for run-of-the-mill quake warnings.
posted by solid-one-love at 11:14 AM on February 5, 2007

The undersea seismic activity stopped suddenly on the weekend, which had the scientists scratching their heads a bit, but it looks like their concern may have faded.

Anyway, this is how one of the stories explained the increase risk (paraphrasing): If driving in normal traffic brings a risk of an accident, and driving in rush hour carries a slightly higher risk of an accident... that's how much greater the risk of a quake is right now.
posted by evilcolonel at 11:15 AM on February 5, 2007

Oddly, I don't see any Canadian quakes on the USGS site.
posted by b1tr0t at 12:16 PM on February 5, 2007

Geological Survey of Canada - Pacific - Seismology. The site also has some very clear, well illustrated discussion of the southern BC earthquake risk and the recent heightened risk alerted by the recent swarms of very, very small tremors.
posted by Rumple at 12:21 PM on February 5, 2007

b1tr0t, the reason it doesn't show up on the USGS site is because the current seismic activity (or lack of) is because it's part of a "slow tremor" event. A big earthquake releases a whole lot of energy within a matter of seconds or minutes, whereas a slow tremor event occurs when two plates sort of roll past one another for weeks or months. In the case of the Cascadia subduction zone, it happens about every fourteen months pretty reliably. The tremor doesn't indicate that a big earthquake is imminent, although there's some concern when that slow energy release stops unexpectedly.

The USGS site relies on an automated system where seismometers are triggered by a certain magnitude event, and then that information gets relayed to the USGS. A tremor event is different because a lot of energy is released very slowly. If you were to look at a seismometer during a tremor event, it would just look very noisy, but it wouldn't look like the signature of an earthquake (and therefore wouldn't show up on the USGS site most of the time). In fact, these kinds of slow tremors were only discovered rather recently, when people looked at energy released over long periods of time and noticed that there was in fact a signal on the seismometers, just not one you notice on a day-to-day basis.

Anyway, the tremor increases stress along part of the fault, which means that it is more likely that a big earthquake would happen during these tremors. However, we know that at least in the last few years, this has happened every fourteen months, and it's been three hundred years since the last Big One. So while you should have an emergency kit together anyway (because you live on a big subduction zone in the Pacific Northwest), you don't need to rush to the store and buy your bottled water today or anything. Although a lot of people are studying the tremor in the PNW and elsewhere in the world (particularly Japan), it's so new that we don't really understand what is and isn't typical behavior because we don't have data that goes far enough back in time.

(IAAS.)
posted by limicoline at 5:23 PM on February 5, 2007