Earthquakes in the Eastern U.S.
Just saw this story about the New Madrid fault, but have a bone to pick. The thrust of the story seems about right: The eastern United States has its own set of seismic hazards, not least of which is this rather mysterious fault that broke in 1811-1812 in a series of quakes that have been estimated as high as magnitude 8.1. New England and South Carolina have had major quakes in the past; in fact many of the major cities of America with no cultural memory of earthquakes (say, New York City) could be vulnerable to an earthquake. But -- a big but -- we tend when we write about these things to claim more knowledge than we actually possess. Take that 8.1 magnitude figure for the largest New Madrid quake in the 1811-1812 series. What is that based on? Did anyone have a seismometer then? I don't think the word "seismometer" existed. People didn't even know the world was old at that point. I believe the figure is based on the accounts of survivors. Basically they talked about chimneys falling down and landslides and so on, and scientists would then say: Let's call that an 8. You have to wonder if it's not roughly as scientific a process as the judging of figure skating. Recently the very smart seismologist/historian Susan Hough mentioned an interesting fact about New Madrid: Most people back then lived right along the rivers, on soft ground that is prone to liquefaction and increased shaking during a quake. Maybe their 8 was just a 6 back in the hills.
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