A Little More Earthquake Talk

Last night during our discussion on the Family Preparedness Guide blog talk radio show James asked me about the Richter scale and I’m afraid I miss led you. With the bad connection, wind blowing so hard the blinds in the house were rattling like they would if the windows were open, and lightning and thunder, I just blew it. I got myself confused so I thought I should clarify today.

The Richter magnitudes are based on a logarithmic scale (base 10). What this means is that for each whole number you go up on the Richter scale, the amplitude of the ground motion recorded by a seismograph goes up ten times. Using this scale, a magnitude 5 earthquake would result in ten times the level of ground shaking as a magnitude 4 earthquake (and 32 times as much energy would be released). To give you an idea how these numbers can add up, think of it in terms of the energy released by explosives: a magnitude 1 seismic wave releases as much energy as blowing up 6 ounces of TNT. A magnitude 8 earthquake releases as much energy as detonating 6 million tons of TNT. Pretty impressive, huh? Fortunately, most of the earthquakes that occur each year are magnitude 2.5 or less, too small to be felt by most people.

While the statistics above are accurate the following are just an example so you can visualize in terms you can relate to. In terms of destruction think of it as sticks of dynamite. A category 1 earthquake on the Richter scale being 1 stick 20 miles down. No damage, we wouldn’t even feel it. A 2 would be like 10 sticks. Still nothing. A 3 would be 100 sticks, maybe a little swaying like a big truck going by. A 4 would be like 1000 stick, a 5 like 10,000 sticks, a 6 like 100,000 sticks, a 7 like 1 million sticks, an 8 like 10 million sticks, a 9 like 100 million sticks and a 10 like 1 billion sticks. You can see how quickly the severity rises.

Most of the area as I listed when we talked about where quakes are over due, Seattle, The Wasatch front, San Fransisco, LA and many others are expecting between a 7 and 8 magnitude earthquake. The largest earthquake recorded in the United States was a 9.2 in 1964 centered in Prince Island Sound in Alaska. It is the second most powerful ever recorded. The most powerful being the 1960 quake in Chile which was a 9.5.

The north east coast of the United States is overdue for a category 5-6 earthquake. That may seem pretty mild when you consider our example above, however, buildings have not been constructed according to earthquake codes as have building on the west coast. This will increase the destruction of a more mild quake.

Just a little story about the San Fransisco earthquake. Did you know that there is no record of that quake from San Fransisco itself. On the day of the earthquake the man in charge of the office where the actual recording device was housed borrowed the pencil because he could not find another and he forgot to replace it. The quake occurred later that day and…no record.

I hope this helps to clarify just what is meant when we refer to the Richter scale.

http://www.geo.mtu.edu/UPSeis/intensity.html

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