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Monday, 01 May 2006

The San Andreas Fault

Written by  Dr. Ronald Francis
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The Fault is a dividing line that separates one part of California that is sliding northward from the other part that is sliding southward. If you have one foot on one side of the fault and the other foot on the other side then your body would begin to twist a little bit as the two tectonic plates of the earth slide by each other.

I was outside San Francisco and saw the San Andreas Fault; it seemed fascinating. Can you tell me more about what’s happening in the earth there? - Rachel S.

 

The Fault is a dividing line that separates one part of California that is sliding northward from the other part that is sliding southward. If you have one foot on one side of the fault and the other foot on the other side then your body would begin to twist a little bit as the two tectonic plates of the earth slide by each other. The twist would only happen if you were to stand there for about 5 years because then your feet will have slid only about 25 centimeters, about 10 inches. The plates of the earth move very slowly, at a rate that even a snail would call a snail’s pace! It’s only about five centimeters per year, or about a couple of inches, each year.

 

Our current understanding of geology is that different plates on the surface of the planet move independently to some extent. The surface, in this case does not necessarily mean continental lands; the bottoms of the ocean also represent solid surfaces of the earth. That means that there are faults lines that are under the oceans as well. We are lucky to have a transform (sliding) fault that is on land, since most are under water.

map
US Geological Survey

On my first trip to San Francisco, I was awestruck at the prospect of standing on the fault line. It’s actually not a line because the width of the fault is several miles while its length is over 1000 miles extending from Northern California into Mexico. So I was standing in the Fault “zone,” if you will. So what did I see? Well, off to Point Reyes I went.

 

Point Reyes is the far point on San Marin County north of San Francisco. The county of San Marin sticks out as a peninsula jutting into the Pacific Ocean and at its tip, at Point Reyes, there is a fascinating lighthouse. Part of San Marin County is on one side of the fault and part is on the other side. I half expected to hear some rumbling of the earth as I was driving toward Point Reyes. Part of me was scared, I guess, since at any time Mother Nature can exert her will and shift the earth on our planet, causing a major earthquake. But, I figured that as long as I was on one side of the fault I would just be sliding very slowly and I didn’t think it would be a big deal. If only it were that simple.

 

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Last modified on Sunday, 16 December 2012

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