On shaky ground.
New early warning networks could provide alerts a few seconds before earthquakes hit, as Alexandra Witze described in "Buying time" (SN: 4/19/14, p. 16). These systems work by detecting the primary seismic waves, or P waves, that arrive before the more damaging secondary waves. Reader David Reynolds e-mailed the story of his frightening experience with a magnitude 9.2 earthquake, the second largest in recorded history, which led him to believe P waves could help provide early warnings. "I was at the corner of 15th Avenue and Juneau Street in Anchorage when the Good Friday quake struck in 1964. A week after the quake, I was working in an office with about 20 other people at Ft. Richardson when, without saying anything, everyone in the office suddenly jumped to their feet and started running out of the building. No noticeable shaking had started. About 15 or 20 seconds later, we had a 7.5 aftershock. Although none of us knew what we were experiencing at the time, I'm sure now that we must have been sensing the P waves."
People can absolutely feel P waves, says Witze. "Trained seismologists in particular can detect them and they will start counting as soon as they feel them, to determine the seconds elapsed until the secondary waves arrive. This allows the researchers to calculate a rough distance to the epicenter."