This morning's Illinois earthquake registered a 3.8 on the Richter Scale. How does that stack up against past US tremors?
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.