"What causes the rupture to come to a stop?"

The answer to that question is currently being researched. While it may seem a simple question, the factors that control the size of a fault rupture, and thus determine where it stops, are complex and do not easily lend themselves to being reproduced in laboratory experiments. Obtaining the answer would give us great insight into the processes at work in fault ruptures, and might provide more understanding of what leads to the onset of rupture and the earthquakes which result.

Since laboratory experiments fall short of duplicating the natural proces of fault rupture, researchers must use data from actual earthquakes in their studies. Southern California is an excellent place to obtain such data. Not only are earthquakes common here, but the area is under constant surveillance by a large network of seismic monitoring equipment. These instruments are shown on the map at right; each green box represents the location of an instrument collecting data, and the red lines represent the surface traces of major faults. We will study this network more closely in later sections. Surely, though, you can at least appreciate the size and density of it from this map alone.

In addition to the obvious benefit of excellent instrumental coverage, southern California has an advantage for researchers that some parts of the world do not -- every kind of slip can be found here, on at least one of the dozens of significant, active fault zones. The sheer number of active faults in southern California presents researchers with an abundance of opportunities.