Quake Highlights East Coast Fault Lines

Thought big earthquakes in the United States only happened in California? Nope. They’re just more common there.

The temblor that shook Washington, D.C, New York and parts in between Tuesday afternoon was unusual, but it didn’t come as a total shock to geologists. While four-fifths of the world’s major earthquakes occur on the tectonic belt takes in California, Japan and Indonesia, smaller danger zones are scattered around the Eastern United States, as this earthquake hazard map shows. Recent research suggests some could be related to a mysterious, ancient fault that runs clear from Alabama to New York.

In fact, 39 of the 50 states – including New York and Tennessee – have moderate to high seismic hazard risk, ABC’s Good Morning America reported last year. The New Madrid fault, running from St. Louis to Memphis, is one of the country’s most dangerous. Between 1811 and 1812 it was the site of a series of quakes larger than any recorded in California, causing damage as far away as Washington, D.C. and Charleston, S.C.

Central Virginia is home to a smaller seismic zone, but minor earthquakes have been reported there periodically for centuries, according to the USGS. Until Tuesday, however, the largest quake in the region had been a modest 4.8, back in 1875.

The 5.3 event in Colorado earlier Tuesday was also out of the ordinary. The New York Times reports it was the state’s largest naturally occurring quake since 1882. A couple of more recent quakes—a 5.3 near Denver in 1967 and a 5.7 in northwestern Colorado in 1973—were human-caused.

New Yorkers aren’t safe either. The Ramapo Fault System, which runs from southeastern New York to eastern Pennsylvania, has been known to cause quakes as high as 5.2, according to Columbia University’s Earth Institute. Perhaps most concerning, from the standpoint of potential damage, is the much-studied 125^th St. fault line, which crosses northern Manhattan from the Hudson River to the East River. It hasn’t stirred much for the past 200 years, which has some seismologists worried it could be due. This Slate story on surviving an earthquake explains: “Geologists estimate a 20 percent to 40 percent chance of a significant earthquake in the next 50 years in New York, and they make a special point to say that a major quake is also a real possibility... What has experts especially concerned is the city's alarmingly high ratio of likelihood-to-preparedness.”

As Tuesday’s shaking showed, even a quake not centered on a major metropolitan area can menace big cities, especially in the East. According to an explainer from the USGS: “Earthquakes in the central and eastern U.S., although less frequent than in the western U.S., are typically felt over a much broader region. East of the Rockies, an earthquake can be felt over an area as much as ten times larger than a similar magnitude earthquake on the west coast.”

Easterners curious about their local earthquake risk shouldn’t get too hung up on the precise location of fault lines, yet another USGS explainer points out: “In California, a large earthquake can generally be associated with a particular fault because we have watched the fault break and offset the ground surface during the earthquake. In contrast, east of the Rockies things are less straightforward, because it is rare for earthquakes to break the ground surface. In particular, east of the Rockies, most known faults and fault lines do not appear to have anything to do with modern earthquakes. We don't know why.”