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Monday, 01 May 2006

The San Andreas Fault

Written by Dr. Ronald Francis
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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.

 


 

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.

 

image
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.

 


 

faultline
US Geological Survey Image
I walked along the path designated to learn about the fault zone and was wondering if there would be a clear section where the earth had been shifted and so you could see the earth just broken up along this line. I walked with my friend along the path and eventually we came to a little sign that explained that there had been a major earthquake in 1906 in San Francisco and that the earth had slid by as much as 5 or 6 meters! This was devastating for the city even if it was interesting for geologists.

 

The sign explained that we were standing next to two pieces of a fence that had run perpendicularly across the fault line. One section of the fence was shifted by about 4 meters or so from another section. The fence was basically broken in two. I stayed there for a while trying to see if there were patterns in the land, like pieces of a puzzle that had been shifted, giving more evidence of the earthquake. Unfortunately, I didn’t find any so had to make do with the fence.

 

There are other places along the fault line where one can see clear movements of the earth even over a period of ten to twenty years: streets that have lines down the middle of the road that have been shifted a few inches to even a foot over a decade or so of slow movement. These shifts are best seen in cities farther south of San Francisco.

 

If you have a question for our resident physicist about any natural wonder please send your question along with a couple of pictures (.jpg) of the place to submissions [at] intravelmag [dot] com.


© Dr. Ronald Francis

 

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Last modified on Sunday, 16 December 2012

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