A magnitude 4 earthquake occurred Thursday afternoon near Dover, Del., and was felt across Maryland.
The U.S. Geological Survey confirmed that the quake occurred at 4:47 p.m., about 5 miles beneath the the Bombay Hook National Wildlife Refuge. Richard Ortt, director of the Maryland Geological Survey, said geologists believe its precise magnitude was about 4.4, though the latest estimate USGS posted on its website Thursday night was magnitude 4.1.
Ortt, who said he didn’t feel the quake because he was busy trying to fix the transmitter on Maryland’s only seismograph, said geologists were still trying to confirm the details. At times, estimates of the quake’s magnitude ranged as high as 5.1 in the hour after it occurred.
“There’s a lot of data being brought in, and we’ll be analyzing,” Ortt said.
Maryland Emergency Management Agency officials said they are monitoring for any reports of damage, and encouraged residents to report anything they felt or saw to the USGS.
Geologists were receiving reports of people noticing ripples in their drinks or wondering whether a large truck was driving by, typical observations during minor earthquakes. Such rumbling is about equivalent to an earthquake of magnitude 1 or 1.5, Ortt said.
“It was very mild, but I am a native of Los Angeles, CA, so it was unmistakably the feeling of an earthquake,” one Northeast Baltimore resident wrote in a report collected by the European-Mediterranean Seismological Centre. “My cats were startled, but only one ran from where he was (he is very anxious).”
The quake was less intense than the 5.8-magnitude earthquake in August 2011 that was centered near Mineral, Va. That quake was felt up and down the East Coast.
The magnitude scale is logarithmic, so the 2011 quake was 25 times bigger than Thursday’s, and it released 126 times more energy.
People reported feeling Thursday’s quake across Delaware, into the Philadelphia area, Northern Virginia, and as far away as Long Island, N.Y., and Connecticut. Ortt said he had not yet received reports from Maryland’s Eastern Shore, but that officials he spoke with in Delaware said they did not feel significant shaking.
Earthquakes are relatively rare in this part of the world. A handful of quakes large enough to be detected are reported every year in Maryland — such as one of magnitude 1.5 felt around Howard County in October.
When they do occur in this region, they can often be felt across wide areas because seismic waves travel easily through the spongy soil along the Atlantic coastal plain.
At a depth of about 5 miles, Thursday’s quake is considered shallow, Ortt said. But it was deep enough to have likely occurred in the bedrock beneath the sandy coastal plain, he said.
Geologists are refining their analysis with data from nearly 40 quake monitors across Pennsylvania, where geologists are watching closely for any impact from the natural gas-harvesting technique known as fracking, and half a dozen monitors across Delaware.
Maryland has not contributed any data yet because its equipment, maintained in the Reisterstown area, has been having frequent problems with its radio transmitter. But Ortt said the seismograph was nonetheless collecting data when the quake occurred.