|CaliforniaEarthquake Map 01/20/2012|
Lurking offshore of Northern California and running north toward Vancouver, Canada, lies the Cascadia Subduction Zone. This fault has the potential to produce an earthquake and tsunami as devastating as the March 11, 2011 quake that struck off Honshu, Japan.
While the San Andres fault system has produced a long and obvious scar on the landscape of California and the frequency of small to moderate, non-damaging earthquakes has drawn the attention of the media and scientists, Cascadia remains locked quietly offshore for centuries, building stress until it suddenly releases in massive quakes.
Geological evidence shows seven great quakes have occurred along Cascadia in the last 3500 years. Intervals range from 200 to 900 years with most occurring around 300 years apart.
|Cascadia Subduction Zone|
Between these major quakes, Cascadia sits virtually silent. In fact, scientists were aware of its presence in the Pacific northwest, but it was not considered a threat as it appeared to never experience quakes.
Brian Atwater's 2005 study of drowned coastal forests lead to the discovery of Cascadia's hidden threat. It appears to remain virtually locked and silent for centuries, pressure and stress building until it is released in a giant quake.
According to Iris, Incorporated Research Institution for Seismology:
"...Just a quarter century ago, hardly anyone suspected that a giant underwater earthquake – or the fast-arriving tsunami that such an earthquake would spawn – could ever occur in the Pacific Northwest. The region’s written history, which begins a few decades before the arrival of Lewis and Clark, is devoid of such catastrophes. Yet today the Northwest has a widely acknowledged history, thousands of years long, of earthquakes as large as magnitude 9 and of associated tsunamis. "
|Oregon Earthquakes1841 - 2002|
Now look at this USGS map showing the Cascadia fault registering a 4.3 magnitude earthquake on January 17, 2011. This is a highly unusual place for movement to occur. The map also shows several small quakes in the days before the larger one.
Foreshocks, Aftershocks or Anomaly?
Large Earthquakes are often preceded by 'foreshocks' which can occur from minutes to even years before the main shock. They occur in about forty percent of moderate to large quakes and perhaps seventy percent of quakes greater than magnitude 7.0.
Studies show foreshocks may increase in intensity over time leading to the main quake:
"The observation of foreshocks associated with many earthquakes suggests that they are part of a preparation process prior to nucleation. In one model of earthquake rupture, the process forms as a cascade, starting with a very small event that triggers a larger one, continuing until the main shock rupture is triggered. "
|4.3 Cascadia quake 01/17/2012|
Aftershocks are smaller quakes which occur as plates readjust following a larger event. They decrease in frequency as time passes.
Perhaps the small quakes in the preceding days were foreshocks to a main 4.3 quake on the 17th? This is certainly possible, however if the 4.3 was the culmination of this series, it should have been followed by decreasing aftershocks as the huge plates settled back into a locked position. There have been no such aftershocks.
Earthquake prediction is notoriously unreliable. Tantalizing hints of patterns give hope that someday we may be able to recognize the precursors to great quakes and evacuate populations before they strike.
The question arises: Is Cascadia stirring? The recent anomalous quake on the fault is noteworthy if only for its rarity.
There are great concerns that this area is not prepared for a quake of historical size. Cities from Eureka, California to Vancouver, Washington are potential targets.
|Tsunami Evacuation Route, Oregon|
"... ...Pacific Northwest cities are full of buildings with slender structural frames and fewer and smaller shear walls. In a mega-quake, many of the region's iconic tall buildings would probably collapse. "The Cascadia Region Earthquake Workgroup has prepared a hypothetical scenario of the effects of a rupture of Cascadia. Predictions include:
Without a doubt, the Cascadia Subduction Zone will someday unleash a great quake in the Pacific Northwest. The odds may be as high as 45% that it will strike during the lifetime of todays residents. Whether the quake that occurred on January 17 was a foreshock, aftershock or simply an anomaly, Cascadia will rupture someday. Perhaps it will happen tomorrow or a decade from now. Please prepare now. The clock is ticking."...• Depending on location, strong shaking might be felt for several minutes.
• Injuries and fatalities could number in the thousands, and
hundreds of buildings could be destroyed.
• A destructive tsunami will quickly hit the Cascadia coast
and travel across the Pacific Ocean.
• Aftershocks up to M7 are common, creating the potential
for additional damage."
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images: Oregon quake history: http://www.oregongeology.com/sub/earthquakes/eqepicentermap.htm
Cascadia Subduction Zone: http://en.wikipedia.org/wiki/Cascadia_subduction_zone
tsunami evac: http://www.flickr.com/photos/cornelii/2066817391/sizes/m/in/photostream/