The Delaware Earthquake Was a 5.1 Before It Was a 4.4

On Thursday, just as late-afternoon fatigue set in for millions of office workers along America’s east coast, a tremor rumbled through the earth. The shaking wasn’t especially strong, so most people weren’t sure what they had just felt. Looking to the United States Geological Survey’s Twitter account immediately after the quake showed that even the country’s experts were a bit confused as well.

In a tweet, the USGS confirmed that the shaking had indeed been caused by a quake whose epicenter fell 6.6 miles east of Dover, Delaware and 55 miles south of Philadelphia. What the USGS wasn’t entirely sure about was how strong the earthquake was in the first place.

In its first tweet, posted shortly after the quake hit at 4:48 EST, the USGS said the magnitude was a 5.1 on the Richter scale. But then, 18 minutes later, it downgraded its original measurement to a measly 4.4.

The USGS quickly backtracked, leaving us to wonder what happened in the 18 minutes between tweets.

When Inverse asked the USGS, the agency responded with a tweet effectively suggesting that, like the rest of us, even experts can’t be too sure about their work when they’re in a rush.

The agency shifts the blame to its seismic monitors, which have varying speeds of data transfer. Some 150 seismographic stations are scattered throughout the world as part of the Global Seismographic Network, constantly scanning for seismographic waves rippling through the earth. Some of these are closer to the epicenter of an earthquake than others, so they take longer to pick up on waves, and not all of them relay their information to the USGS in real time.

“Some of the seismic waves used in magnitude analysis can take more than an hour to propagate around the earth and reach stations farther from the epicenter,” writes the USGS. “There is no physical way to include these measurements in the initial magnitude release because the energy used in the analysis has not yet arrived at all seismic stations.”

Furthermore, some of these seismographs, according to the GSN, are still laughably low-tech:

A small number of stations do not have telemetry, and data from those sites are transmitted to the USGS via media such as tapes or CDs. As part of GSN and backbone operations, waveform data are reviewed during quality control. GSN and backbone data are available from the IRIS Data Management Center.

What most likely happened is that on Thursday, the first bit of data that reached the USGS National Earthquake Information Center showed a magnitude of 5.1, then that number was revised as experts checked the quality of the data and as new data from other seismographic stations came in. According to the USGS, the official magnitude measurement for Thursday’s earthquake may even be revised again over the next couple of days.