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One of my students asked me if Southern California was separating from the rest of the continent. If this is happening: 1. At what rate is it happening? 2. What's causing the movement (San Andreas or a different fault?) 3. And when would Southern California be completely separate? (My student seems to think it would be in the next 30 years?). Thank you!!
Question Date: 2006-02-21
Answer 1:

One of my students asked me if Southern California was separating from the rest of the continent. If this is happening:
1. At what rate is it happening?
R1. About 2. 5 inches PER YEAR
2. What's causing the movement (San Andreas or a different fault?)
R2. Motion between the North American plate and the pacific plate is strike slip motion. The slip plane is the San Andras fault!
3. And when would Southern California be completely separate?(My student seems to think it would be in the next 30 years?).
R3. About 30 MILLION years from now the west side of the San Andras crystal block will slip UNDER the Aleutian island and be subducted BACK into the INTERIOR of the EARTH!

Answer 2:

Funny! The only way Southern California could separate from the rest of California in thirty years is if: (a) they establish themselves as an independent state, or (b) there was a GIGANTIC landslide and the entire land area fell into the ocean. If the latter did happen, Southern California would indeed separate from the rest of California, but would no longer be above the ocean.

Gigantic landslides have occurred (the Nu'uanu landslide, in which one-third of O'ahu fell into the ocean, e.g.) but I don't think this is a realistic assumption for California. The bulk of the US (the part that sits on the North American plate and includes northern and central California) is moving northwest and the little strip of landmass that is on the Pacific plate (from San Francisco south to the tip of Baja) is also moving northwest but at an even faster rate, so it's separating from the rest of California. (Geologists map faults in such a way as to indicate the relative direction of movement, so it looks like the North American plate is actually moving southeast, but it's not. This is if you look at long-term averages. The reality is that the plate boundary is really a system of faults, some of which are moving horizontally and some of which are moving vertically (as in the case of the North ridge earthquake), and none of which are really moving at the same rates or at the same time.

The most famous fault is the San Andrea's Fault. It is one of the best-studied faults because it's above ground and easy to study and because it generates high-energy earthquakes near densely-populated cities like LA and San Francisco, so there is a lot of work to try and predict the next "big one" so people can evacuate. There are many other faults that make up this system, including the Hayward, Calavaras, Concord, Greenville, Rodgers Creek, San Gregorio, and San Jacinto faults, to name a few.

Movement along faults isn't really uniform but is sort of "herky-jerky". The idea is that the faults are usually locked shut, causing pressure to build up in the rocks on either side of the fault from motion far away, along other regions of the plate boundary. This pressure causes the rocks to stretch and bend until they give way. At this point, the rocks along the fault rupture and move all at once. Then they lock up again, and the whole process starts over ("elastic rebound theory").

Based on past rates of movement, it took about 20 million years to move the Pacific Plate north about 200 miles. A good map/math exercise would be to have your students look at a map of plate boundaries, estimate the distance that the Pacific plate would have to move to separate LA from the rest of California by a narrow ocean (the map would have to have a scale on it), have them calculate the past rate of plate movement (200 miles/20 million years = 10 miles every million years, or use the more current estimate of 37 miles every million years) and then have them predict how long it would take, assuming the plates don't change direction or speed. (Hint: it's not 30 years.)

Geologists make these sorts of assumptions all the time. In technical terms, these assumptions are called "uniformitarianism" (the idea that "the past is the key to the future").

Answer 3:

Southern CA is not separating, but rather moving toward San Francisco at an AVERAGE rate of 50 mm per year. The motion happens in spurts we call earthquakes. Some information in websites give a good overview of the current best understanding about how convection currents in the molten core of the planet push on the more "crusty" stuff on the surface.

Interesting site with a video here. Play it to watch.

Answer 4:

In the 1970's there were fears that San Francisco would fall into the ocean, and that hasn't happened. I think there was a song about it. When a non-scientist makes a claim about something as unrealistic as Southern California breaking away from the continent in 30 yrs, you can reliably assure them that it isn't true and they don't need to worry. I've heard about the possibility of Southern California and Northern California becoming different states, and I suppose that's possible in the next 30 yrs; do you suppose that's what the student was hearing about?

Here's a website from the US Geological Survey about earthquakes hazards:

earthquakes it has Frequently Asked Questions and other info.

Answer 5:

I'm going to start from the beginning.The uppermost part of the Earth is composed of rigid plates called lithospheric plates, which move over time. This process of shifting plates is called plate tectonics. Where plates collide, mountain chains and volcanoes form. This is happening today along the northwestern coast of the U.S. (OR and WA), along the coast of Chile, and in the Himalayas, for example. Where plates separate or rift apart, large basins form. Major rifts occur, for example, at the bottom of the Atlantic Ocean, forming the ocean basin.

A third type of relative plate motion is where plates grind past one another but aren't colliding or separating. This is what is happening now in southern California. The San Andres Fault separates the rest of North American (which sits on the North American plate) from southern CA (which sits on the Pacific plate). Relative to North America, the Pacific Plate carrying southern CA is moving north at a rate of somewhere around 20-40 mm/yr. So, southern CA is not tearing away from the rest of the country only moving slowly north, a trip that will take millions of years.

Answer 6:

The San Andrea's Fault is dragging the western side of the fault, including Southern California, northward relative to the rest of North America. It's not so much separation as it is dragging, so it will never be completely separated. It moves a little bit each year (a few millimeters), or a bit more during a major earthquake (up to a few meters).

At the current rate, it will take western California about sixty million years to reach Alaska. >Baja California, on the other hand, is separating and already has, hence the Gulf of California's existence. It is now going to get dragged north with the rest of southwestern California northward to Alaska.

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