The Quake That Wasn't

At 4:55 this afternoon, all the cellphones in our office started whooping wildly at us. It's been a while since we had a major alert from the earthquake early warning system, but the sound is unmistakeable to anyone who was here for the months following 3/11. 

The Japan Meteorological Agency pushes out alerts when its sensors detect fast-traveling P-waves (the primary waves that vibrate in the same direction as they travel). Ideally, these alerts reach people before the slower, more destructive S-waves, secondary waves that shake perpendicularly to the direction of travel, get to where they are. 

Today's warning was for a quake in Nara Prefecture, at 60 kilometers depth, with a magnitude of 7.8. This would be an immensely powerful shake for Nara, Osaka, and other surrounding prefectures; the top intensity predicted was 7, which is what some Tōhoku communities felt in the 9.0 megaquake two years ago. The word went out like lightning: tweets like this one from @eew_jp got more than 10,000 retweets in short order as people flocked to Twitter to see what their phones were warning them about. Some reports said that Yahoo Japan's servers were knocked offline briefly by the deluge of clicks looking for information. JR halted service on its high-speed Shinkansen lines as soon as the alert arrived; as of 5:15 all trains were running again, though.

In the end, the only quake that struck the area was an insignificant 2.3 shaker in Wakayama Prefecture at 4:56—not even strong enough to mark a 1 on the intensity scale. False alarms are better than real destruction, but the JMA will be working hard to figure out how such a miscalculated alert could go out. (PD)

UPDATE: NHK is now reporting (in Japanese) that electrical noise may have affected a quake sensor, causing it to trigger the alert. 

earthquake science warning