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Author: JPLenny Big gold star, 5000 posts Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: of 33  
Subject: California Earthquake Answer Date: 1/12/2005 3:54 PM
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This will be worse when it hits CA, won't it? I feel awful for the people whose homes have been lost to the mud slides in CA.

Arleen,

Grab a chair and sit back. Let me tell you what I know.

The San Andreas Fault is actually a transform plate boundary. The Pacific plate is sliding northward past the North America plate. The Sumatra quake was caused by plates that were coliding against each other.

As most people know this fault has produced some large earthquakes in the past in California, but none at the Richter scale level of what has occurred in Sumatra, Alaska, or Chile. California's quakes don't get into the top 10 list of largest earthquakes in the world and only three make the top 20 recorded in the US in the past 200 years. Most of them are in Alaska. Still, this fault produces some major earthquakes.

Using the Richter scale, each whole number step represents a tenfold increase in the size of seismic waves measured on a seismograph. The more important fact is each step on the Richter scale corresponds to a over thirty fold increase in the amount of energy released in an earthquake. An 8.0 (San Francisco 1906) earthquake release the equivalent energy of 1 billion tons of TNT while a 9.0 earthquake like the Sumatra one released the equivalent energy of 32 billion tons of TNT.

With all of that said an earthquake approaching 8.0 is still a devastating event. With the large population of San Franciso and Los Angeles near the San Andreas fault, when the next major quake hits one of these areas, there will be the possibility of many casualties and widespread damage.

The amount of damage and casualties depends on where an earthquake occurs and the composition of the ground in an area and the population density. Soft and water-saturated soils tend to amplify the effects of an earthquake.

California's population density makes it a big focus for potential disaster. While Alaska has three earthquakes in the top 10 in the world in the past 100 years, two of them occurred in the Aleutian chain and the other near Anchorage. While Anchorage is a large city it is nowhere near what Los Angeles or San Francisco is today and certainly not back in 1964.

Scientists actually use another scale when discussing earthquakes. To measure the effects of an earthquake on the surface, the Mercalli Intensity Scale is used. This scale, composed of 12 increasing levels of intensity that range from unnoticeable to total destruction. The scale is designated by Roman numerals. The scale is not a mathematical one, but a scale based on observed effects.

http://www.pgc.nrcan.gc.ca/seismo/eqinfo/intensty.htm

In the US, U.S. Geological Survey will often mail out questionnaires after a larger earthquake. They also have a form on their website to gather this information.

This scale is valuable at assessing the impact of the quake. The October 17, 1989, Loma Prieta near San Francisco was a magnitude 6.9. There were ratings up to IX on the Mercalli Intensity scale near the epicenter and in the portions of Oakland and parts of San Francisco near the water, due to the soil composition.

Compare that to the 7.9, Nov 2002 Denali Fault, Alaska earthquake that was about the same size in magnitude as the Great San Francisco earthquake of 1906 and released 30 time more energy. There were only a few responses that reached IIX on the Mercalli Intensity scale.

I'll bet there are very few outside of Alaska even know or remember that an earthquake the size of the great San Francisco of 1906 hit Alaska in 2002. Low population density and composition of the ground made for less damage and no reported loss of life.

Sorry for staying off the board topic and being so long winded....

JP


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