There have been many small earthquakes affecting the state of Oregon, but the smaller ones usually cause little if any damage. The serious concern is the long overdue “big one.”
The above graphic is a look at the earthquakes in the Pacific Northwest over the last 10 days as of 8.18.20. The lower magnitude earthquake are occurring every day, but we must be aware of the overdue “big one.”The “big one” is the subduction earthquake that has been studied and talked about for years. We have seen some pretty serious earthquakes fairly recently. March 11th 2011 will be remembered for the 9.0 earthquake which was the strongest ever recorded in Japan. There was an Magnitude 8.2 earthquake off the coast of Chile late Tuesday night April 1st 2014 that produced a relatively small tsunami. A 5.8 earthquake off the coast of Panama the following day April 2nd caused light and moderate shaking with no serious damage or injuries reported. A 5.1 earthquake struck Los Angeles, California on March 28th again no serious damage or injuries reported even with the many aftershocks including a 4.1 jolt on Saturday March 29th. The experts say there is no connection among these quakes that would indicate any major activity around the famous Pacific “Ring of Fire.” The Oregon earthquake Sunday April 6th, 2014 at about 8:30 PM was a Magnitude 3.5 quake centered near Portland, Oregon. Oregon hasn’t seen as many serious earthquakes as our neighbor California, but we are holding our collective breath waiting for the “Big One” which is the subduction earthquake that could devastate the Pacific Northwest. I’m sure you have heard of plate tectonics. The earth’s surface is not uniformly solid around the planet. There are various sections called “plates” that float on the molten layer below.
Here in the Pacific Northwest our landmass is on the North America Plate which goes across the United States and Canada to the middle of the Atlantic Ocean. On the other hand, off the Pacific Northwest coast are the smaller Juan de Fuca Plate, the Gorda Plate, and the Explorer Plate. Together they are referred to as the Cascadia Subduction Zone.Those plates have been digging under the North American Plate for a long time and when enough pressure builds up they push farther under the North American plate causing an earthquake. That action of subduction (going under) can cause an explosion of energy which would be a very strong subduction earthquake and, depending on where its epicenter was, could be devastating to the whole Pacific Northwest. The worst damage would be closest to the epicenter.
How are earthquakes measured? A seismometer is the instrument that scientists use to measure the motion of the ground and that includes the sonic waves caused by earthquakes and volcanics. A seismograph the device that shows a tracing by means of a graph depicting the magnitude and intensity of the waves. There are various scales that measure the magnitude of earthquakes.
The best known scale with which to measure the magnitude of earthquakes was established in 1935 by Charles Richter who was a Caltech seismologist. The scale is based on how much a seismograph needle is deflected by the seismic wave caused by an earthquake. When the magnitude of an earthquake is reported it is designated as M1, M2, etc. The scale’s numbers can be deceptive though. A Magnitude 5.0 quake is 31 times stronger than a 4.0 quake. It is a logarithmic progression not an arithmetic one, so the single whole number difference is much greater than it would seem. The intensity of an earthquake is measured by how much actual damage is caused by the temblor. That scale is give in Roman Numerals from I to XII. An Intensity III earthquake would mean no damage and that it was not felt by everyone, while Intensity IX or X would cause considerable damage. Intensity XII would cause total destruction. A study done in 1980 by Jim Savage of the USGS (the U.S. Geological Survey) and his colleagues came to the conclusion that the reason there were no earthquakes on the Cascadia Subduction Zone because it is “completely locked.” Assuming they are correct that would mean the pressure will continue to build up in the fault zone, that it would continue to grow the 1.6 inches per year as it has been, and eventually the result will be a rupture causing a massive earthquake. That’s the “big one” we are waiting for.
The experts say it’s not a matter of if, but a matter of when the big subduction earthquake will occur. Where the epicenter is will be paramount in determining who gets the most damage. If it happens in Washington or far northern Oregon the South Willamette Valley would probably see a lesser catastrophe. The same would be true if the epicenter were well to the South. According to Robert S. Yeats, of Oregon State University, an earthquake with an epicenter off the central Oregon coast would be the worst case scenario for the South Willamette Valley. According to one of his colleagues Chris Goldfinger the 50-year probability of a M 9 earthquake for the entire subduction zone is about 10% and there is a 30% chance of it in just the south part of the Cascadia Subduction Zone (that’s us, in Lane County). For more information on how to prepare your home for an earthquake go to the FEMA Earthquake Safety website. The information contained in this column was gleaned from many sources including classes I took in college, the web, and from Living With Earthquakes In The Pacific Northwest, A Survivor’s Guide by Robert S. Yeates, Professor Emeritus in Geology at Oregon State University. If you like to keep track of earthquakes or just want to know when and where they happen you can go to The Live Earthquakes Map website whenever you like.
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