There are a couple of ways to tell when there has been an earthquake in Southern California. One is to wait for signs of violent shaking. The other is to visit a Web site maintained by the U.S. Geological Survey.
The site plots recent earthquakes on a map of the Los Angeles area that is updated every half hour. The size of the plot point corresponds to the magnitude of the quake, and the color of the point indicates how recently the quake happened. The site can be found at https://www.quake.wr.usgs.gov/QUAKES/CURRENT/los-angeles.html.
The site also offers information on preparing for earthquakes.
Greg Miller covers high technology for The Times. He can be reached at (714) 966-7830 and at firstname.lastname@example.org.