When a fault beneath Delaware slipped and triggered an earthquake felt for hundreds of miles Thursday afternoon, a computer decided it was magnitude 5.1.
But that wasn't correct — the U.S. Geological Survey later determined it was magnitude 4.1, a level that is actually one-tenth as strong as originally thought.
Here's how geologists figured that out.
Thursday’s quake occurred at 4:47 p.m., about 5 miles beneath the Bombay Hook National Wildlife Refuge, according to the USGS. It was felt across much of Maryland.
When the ground started shimmying from Virginia to New York, a computer produced the initial magnitude estimate using the traditional Richter scale, created by seismologist Charles Richter in the 1930s. That scale assumes earthquakes emanate from a single point underground, said Paul Caruso, a geophysicist at the National Earthquake Information Center in Golden, Colo.
It works well for smaller earthquakes, with magnitudes up to 2 or 3 on the scale, Caruso said.
But larger earthquakes occur along larger planes. Seismologists use complex formulas to analyze the energy an earthquake produces across that entire area, he said.
“It’s much more complicated and it takes a while for us to get that magnitude calculated,” Caruso said.
That’s why the magnitude of Thursday’s earthquake was, at times, erroneously reported as 5.1 or 4.6 or 4.4. Because the magnitude scale is logarithmic, a 5 magnitude earthquake is actually 10 times stronger than a quake of magnitude 4.
As for what exactly caused the earthquake, seismologists did not immediately know. Caruso said that will take some extra work on the part of regional geologists, or possibly the USGS, given that earthquakes in this part of the country are so rare.
All earthquakes occur on faults, but Caruso said he did not know the characteristics of the fault that caused Thursday’s quake or if it even had a name.
The bedrock beneath this part of the country is relatively stable, settling over hundreds of millions of years — compared to the relatively young age, 28 million years, of the infamous San Andreas fault in Southern California.