September 9, 2013 – 200 Year Old Earthquake After Shocks: In 2009, Science Daily cited a study published in the academic journal Nature by U.S. researchers who found that most of the earthquakes felt in the Midwest were in fact aftershocks of 7- or larger magnitude quakes that struck the region in 1811 and 1812.
“Aftershocks happen after a big earthquake because the movement on the fault changed the forces in the earth that act on the fault itself and nearby,” the story quoted one of the researchers, University of Missouri-Columbia geologist Mian Liu, as saying. “Aftershocks go on until the fault recovers, which takes much longer in the middle of a continent.”
The main variable, the researchers found, was the speed at which the sides of a fault line move past each other. California’s San Andreas fault, for example, moves relatively quickly, meaning aftershocks peter out after 10 years.
Tangshan, it should be noted, is located on the Bohai Sea, so it’s not clear whether the comparison with the American Midwest is really apt. Tangshan suffered a devastating earthquake on July 27, 1976; which was a 7.9 magnitude monster and a follow-up aftershock of 7.4 the next day. In all, 240,000 people were killed. Picture above is the city of Tangshan, China. (Credits: Photo Getty Images, Narrative – The Wall Street Journal).
The Master of Disaster