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Monday, 01 May 2006

The San Andreas Fault - Page 2

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.


The drive to Point Reyes went along a “sag pond”. These ponds are formed when the earth in a fault zone drops down during earthquakes (big and small) and goes below the water table. It’s a bit like sticking your foot into the dry sand at the beach and then discovering that there is water down there. Except it is the whole earth falling down a little bit as two pieces of the earth separate and leave a hole if you like.


Picture by Joseph Dellinger
As I went along the sag pond I was struck about the lengthwise nature of the pond, only to learn later on at the visitor center that the long lengthwise nature of the fault often leads to lengthwise sag ponds. One researcher even claims that part of the reason why San Francisco Bay is there is because it is sort of like a sag pond but connected to the ocean.


I was wondering if the land in the fault zone was going to be somehow different because it represented shifting earth. Was it going to have a lot of hills and valleys because the earth was shifting around so much, bumping up and down? I looked, but didn’t notice anything unusual. Come to find out, however, that the types of vegetation on one side of the fault line can be quite different than on another side because the land has shifted a lot over the years. Thousands of years that is. So the type of soil that is typically further south might show up on the northward slipping side of the fault.


This is in fact that case: the soil on the northward slipping section comes from way down south by the Sierra Nevada Mountains. Had I driven to the other side (southward slipping side) of the sag pond then I would have seen a different type of vegetation.


I got to the visitor center for the peninsula and was glad to see that some attention had been paid to the fault. Mostly the peninsula is a wildlife sanctuary and people come to watch birds, animals and flora. But there is a little section near the visitor center where one can learn about the fault. There is even a path that one can walk on to see a remnant of an earthquake.


The visitor center is worth a quick visit if only to glance at the books that discuss the special nature of the area. There is also a seismograph that is recording the hundreds of earthquakes that occur each week, mostly of very low magnitude.


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Last modified on Sunday, 16 December 2012

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