The 1906 San Francisco earthquake and resulting fires marked one of the worst natural disasters in American history. Striking at 5:12 a.m. on April 18, the quake, estimated at magnitude 7.8, led to the deaths of some 3,000 people and destroyed more than 80 percent of San Francisco. This was just one of many earthquakes, quite a few disastrous, experienced in California.
The U.S. Geological Survey (USGS) estimates that in the Southern California area alone, about 10,000 earthquakes occur each year, although most are so small that they are not felt. Several hundred quakes measure greater than magnitude 3.0 and 15 to 20 measure greater than magnitude 4.0. “If there is a large earthquake,” the USGS states, “the aftershock sequence will produce many more earthquakes of all magnitudes for many months.” Why does California have so many earthquakes?
The earthquakes in California are caused by plate tectonics. The surface of the earth is made up of 15 major tectonic plates. Each plate consists of the outer crust of solid rock along with the much denser mantle underlying it. Together they make up a fairly rigid body of lithosphere, and these sections of lithosphere float atop the layer of hot, rocky, somewhat pliable material that makes up the next layer inside the earth, called the mantle.
The mantle material is not exactly fluid, but it does flow in convection currents that bring heat from the center of the earth to the surface. This flow is one of the forces that causes the tectonic plates to be pushed and pulled in different directions on the surface.
Another force is gravity, which pulls thickened, denser lithosphere down toward Earth’s center. There may be many other mechanisms causing motion of the tectonic plates that geologists are struggling to understand.
Geologists do know that tectonic plates move at a rate of about 1 centimeter per year, but the plates don’t move in concert. Rather, they are pulling from each other in some places and pressing against each other in other areas. In yet other locations they are forced to rub against each other as they move in opposite directions. Most earthquakes occur at plate boundaries.
The ground beneath you may seem firm, but in geological terms, Earth’s outer crust is brittle. In the areas near tectonic plate boundaries, the crust fractures along faults, or cracks, much like cracks in an eggshell. The plates are moving underneath, but at the surface, the top of the crust becomes layered with sediment. This causes the fractured sections of rock on each side of the fault to hold together. The sections become stressed until they suddenly and violently lurch, shifting along the fault and making the earth quake.
The San Andreas fault
The San Andreas fault that runs along the coast of California is near the tectonic plate boundary between the Pacific and the North American plates. The San Andreas is not a single, continuous fault. Rather, it is a fault zone consisting of a grouping of faults. The San Andreas fault system stretches more than 800 miles in length and occurs in segments.
The tectonic plate boundary along the California coast is of the type known as a transform boundary. Here, the plates are grinding past each other, with the Pacific plate moving in a northwesterly direction and the North American plate moving in a southeasterly direction. The fault zone in California is a continental transform fault.
The next “big one”
In 2008 geologists released a report estimating that the probability of a major earthquake (known as the big one) of a magnitude greater than 6.7 within the next 30 years was 21 percent for the northern segment of the San Andreas fault, which terminates in Mendocino, California, and 59 percent for the southern segment, which reaches the Salton Sea in the southeast corner of the state.