Originally measured as a 4.3 quake, the shock quickly seized national headlines. But later estimates lowered the count to a 3.8 on the Richter Scale.
How big of a difference is that? And how common are 3.8-magnitude earthquakes?
The Richter Scale considers anything between a 3.0 and 3.9 to be a "minor" tremor. Last month's Haiti quake, by constrast, was a 7.0 or "major" shake. But just because today's measurement is about half of Haiti's doesn't mean it was half the power. In fact, the January earthquake was about 1,600 times the magnitude that hit Illinois. That's because the Richter Scale is logarithmic – the jump from 3.0 to 4.0 represents 10 times higher magnitude, 3 to 5 means 100 times, and so on.
"Minor" earthquakes are actually quite common. Approximately 130,000 earthquakes between 3.0 and 3.9 strike around the world every year – that's 356 a day. In fact, three other such shakes hit the US today: two in Alaska, one in Puerto Rico. And in all, 773 earthquakes have shaken the US in the past 7 days. About 40 were above a 3.0.
Big tremors are far less common. On average, 7.0 to 7.9 magnitude shocks hit 17 times a year around the world, but they rarely occur as close to cities as Haiti's recent quake, which happened just 16 miles west of Port-au-Prince. Many earthquakes occur underwater or in areas with few people.
So, which US state deals with the most seismic activity? People think of California as the earthquake capital of the US, but the USGS says that Alaska should share the title. Alaska was home to the biggest recorded earthquake in US history, a 9.2 that hit in 1964. Worldwide, it's second only to a 9.5 that struck Chile in 1960.
Outside of California, 18 major American cities lay vulnerable to "significant seismic activity," according to a Congressional report from last month. They include East Coast cities such New York, Boston, and Charleston, S.C.
Even if you feel the rattle, the vast majority of earthquakes don't cause any real harm. In most cases, it takes at least a 4.0 for "light damage" to occur. Every state experiences earthquakes from time to time, but the USGS says Florida and North Dakota feel the fewest.
If even today's "minor" earthquake rattled your nerves, the USGS suggests you move south – way south. "Antarctica has the least earthquakes of any continent," it says, "but small earthquakes can occur anywhere in the World."
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