This bridge spans across two tectonic plate boundaries. The North American Plate to the east side of the bridge and the Pacific Plate on the west side of the bridge. The San Andreas Fault goes right down the middle. So what causes this to happen?
Before I go into any more details lets first discuss the basic fundamentals of how fault lines are created and now they work. The answer can be found in your kitchen, literally. When boiling pasta on the stove, the hotter pasta floats to the surface then when it cools it sinks. This effect has a name. Convection currents in your pot at home and in the earth are nearly identical. There are a few difference though. One is with water, rather than boiling rocks and the other is that one is less a few inches while one is miles deep. Convection currents are continues cycles in the earth's mantel that make hotter rock rise, and cooler rock fall and the cycle begins over and over again. When this happens, it causes fractures in the earth's crust. When fractures occur, a fault line is created.
Now at this point I'm sure you are asking, "What is a fault line?" Well, it is place where two tectonic plates meet and create friction. When this friction is released, an earthquake is formed. This friction is formed by these convection currents. When they move in a circular pattern in one direction, they can push rock with them. When tension is released you get an earthquake in an that moves in waves on the X, Y, and Z axises. X is up or down, Y is right or left, and Z is pivoting side to side.
Depending on the pressure, it can create a magnitude 1 earthquake on the Richter Scale or a 10. A 10 has never happened in recorded history, but is believed to have only occur when large meteors hit the earth. The largest earthquake ever recorded happened in Chile on May 22, 1960. It happened in open waters, just off the coast in the Pacific where it created a Tsunami. You can see my earthcache about that Tsunami
here.
A Richter Scale works like this. Say there was a 5.0 earthquake, which are common, and a 7.0, a little less. A 7.0 on the Richter scale is 100 times greater of that of a 5.0 on the same scale. Each decimal point in-between is twice as as bad as the preceding one.
So how do we predict such event? We can use a number of devices that monitor the fault line or zone, as it is more scientifically correct. The fault zone can be monitored by using lasers or GPS. Both have downsides, but both are extremely precise in measuring the activity. By using lasers, we can accurately measure how far the fault moves in a year, which is the same rate as an average human's finger nails grow. GPS systems are a little bit better in tracking the movement and direction of the fault zone. By using survey disks and benchmarks, we can accurately measure the direction and move rate of the fault zone and predict what it will look like in the future.
Now that you know how a fault works, you can now understand what is happening on this site. Here, the bridge is on two tectonic plates. The fault here is creeping (see the map above) which means that it's moving at a constant rate of about 2 inches per year. This is better for a fault since it doesn't build up stress for any earthquake. It is being constantly released and that's why earthquakes here are almost non existent. That being said, the Fort Tejon earthquake happened just a few miles south of here in 1857. That was the last major earthquake in this region.
Because the fault is creeping it moves constantly. If you build something over it, wait a few years and you will obviously see an offset. A great example of a world famous offset is in Hollister just north of here. You can see that earthcache that I created here. to learn more information on it. That offset is on the Calaveras Fault, a offshoot fault of the San Andreas Fault system. At this site, parts of the bridge are offset by a few inches if you look closely enough. Given a few hundred years, this bridge will collapse due to the fault literally pulling it apart. But for now, the bridge acts as a major link into the small town of Parkfield, the earthquake capital of the world.
Works Cited
http://pubs.usgs.gov/gip/dynamic/tectonics.html
http://earthquake.usgs.gov/earthquakes/states/events/1857_01_09.php