Q: I keep hearing about aftershocks years...

Q: I keep hearing about aftershocks years after an earthquake has occurred. How do they know it is an aftershock and not a new quake?

A: Earthquakes virtually always come in clusters centered in a small area. The largest is called the mainshock, any that occur before the mainshock are called foreshocks, and those that occur after it are called aftershocks.

According to Lucile Jones of the U.S. Geological Survey, the length of the cluster is determined by the amount of time it takes for the rate of earthquakes at that site to return to the background rate that existed before the cluster began. That time can vary widely. The cluster surrounding the magnitude 5.9 temblor in North Palm Springs in 1986, for example, lasted a year, while the cluster around the 1987 Whittier Narrows quake of the same size lasted only six months. In contrast, the magnitude 7.5 Landers quake in 1992 is producing many more aftershocks and the cluster is expected to last 10 years, she said.