A seismogram. :The record from station HFP (Fremont Peak; 9 miles SW of Hollister, 18 miles SSE of Gilroy; 36.754, -121.492) is shown below. The closeness in time and location of these earthquakes to each other is no coincidence. Earthquakes "interact", meaning that the stress changes produced by one earthquake can encourage others to occur nearby, thereby producing clusters of events. The most common form of cluster is the main shock and its aftershocks, the main shock being first and having the largest magnitude. When an earthquake (like the M=1.7 event here) triggers a larger earthquake (the M=3.3 event here), the first is called a "foreshock".

Taken from http://quake.wr.usgs.gov/recent/helicorders/Examples/Fore_main_after.html

See below.