February 8, 2012
by John Vidale
As the seismic activity in Oregon and Washington is mild this week, today I'll review a sequence of activity from about a decade ago.
Earthquakes can come either as a big pop and then some aftershocks, or else as a fairly steady dribble of earthquakes for hours, days, or months. The former is known as the mainshock-aftershock pattern. The latter, persistent activity, is known as an earthquake swarm.
Spokane in 2001 had the most noticed swarm in the northwest in recent decades. Dozens of earthquakes sputtered on for nearly a year. The largest of the quakes was only M4, so nerves were frayed but little damage done. As the fault moving in the earthquake was very shallow, in the top km or two, even earthquakes of magnitude 2 and less were felt as sharp pops. We installed several new seismic stations to watch carefully.
In June and in November, there were days with numerous felt events. The November booms were especially disconcerting in the aftermath of 9/11. We've found no report of a previous earthquake in the 120 years Spokane had been inhabited. Perhaps the earthquakes formed a line along a dimly perceivable fault trace, or perhaps the locations are not so accurate - not many instruments were in the area nor close when the action started.
The events eventually stopped. To this day, we do not know exactly what process triggered the action. Scientists speculate about fluid migration through the ground, perhaps natural, perhaps induced by man, or else faults that are episodic in their activity for other reasons.
In retrospect, from the knowledge we have from watching swarms around the world, the danger of a disastrous quake never rose very high, but one never knows for sure what might have been coming. Or might come next year. In L'Aquila Italy in 2009, a similar swarm led to a devastating M6.3 earthquake, and the ongoing trial of the national seismologists for manslaughter for failing to predict it!
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- Seismic recordings of a gas explosion in North Bend, WA
- Seismic signals generated by the March 22nd Oso Landslide
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- Ice avalanches on Cascade volcanoes
- A New View On What's Shaking on the Cascade Volcanoes
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- The Football Game Experiment Continues
- July (1)
- May (1)
- April (1)
- March (2)
- February (3)
- January (4)
- October (1)
- September (1)
- August (1)
- June (1)
- April (3)
- March (4)
- February (1)
- January (2)
- December (1)
- November (2)
- October (3)
- September (1)
- August (3)
- July (2)
- June (4)
- May (4)
- April (2)
- The wech-o-meter takes over all of Cascadia
- Keystone Cops: Italy prosecutes seismologists for failure to predict deadly quake
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- 15 years of mostly silent magma inflation near Three Sisters, Oregon
- Mount Hood earthquake swarm of Feb 23, 2012
- Web glitches: duplicate (and even triplicate!) earthquakes
- How earthquake magnitude scales work
- Mine blast masquerades as volcanic tremor
- The Spokane Swarm about 10 years ago
- Another hum around Mount St. Helens
- Slow slip: A new kind of earthquake under our feet
- PNSN and social media
- 3am M3.4 earthquake in St. Helens Seismic Zone
- The wrong kind of volcano noise
- Fast chatter on Rainier an hour ago
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- Rainier Repeating Earthquakes Update and Comparison with Weather Patterns
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- Mount Rainier popping away
- Repeating Earthquakes on Mount Rainier - are glaciers the culprit?
- Debunking another SEC football myth by the PAC-12
- One year ago, Seattle Seahawks 12th Man Earthquake
- The odds this year of a megaquake on the Pacific Northwest coast
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- Good vs evil in central US earthquake hazard analysis
- Why does a volcano scream?
- Predicting big quakes from patterns of little ones
- 1-hour warning for Japanese M9 earthquake?
- Sound Transit train under Interlaken keeps a rollin'
- Invisible changes under the hood at the PNSN
- Sound Transit Tunneling Noise
- "Visionary" toads
- Earthquake early warning in the PNW
- November (1)
- December (13)