By David Jacobson, Temblor
At 4:47 p.m. local time, a M=4.1 earthquake struck Delaware, near the capital city of Dover. The quake occurred at a depth of 8 km, and registered strong shaking near the epicenter. Additionally, based on felt reports, and the ShakeMap produced by the USGS, weak shaking would have been felt in Washington DC, and New York City. Less than 45 minutes after the earthquake, nearly 4500 people had reported feeling the quake on the USGS website, though over 55 million people would have been exposed to some degree of shaking. Due to the relatively moderate magnitude of this earthquake, damage should remain minimal.
Based on the focal mechanism from the USGS, this earthquake was both compressional and strike-slip in nature. Even though earthquakes on the east coast are much less frequent than on the West Coast, they can be felt over a much greater distance. For example, according to the USGS, a M=4 earthquake east of the Rockies, could be felt over an area ten times larger than a M=4 quake in California. This is because the eastern U.S. is predominantly made up of sediments, uninterrupted by crystalline rock. Furthermore, due to lower building codes east of the Rockies, smaller magnitude earthquakes can cause more damage than a comparable quake in California.
From data from the Delaware Geological Survey, this is the largest recorded earthquake in the state’s history. Previously, the largest recorded quake was a M=3.8 in 1973. Having said that, an earthquake in 1871 had an estimated magnitude of 4.1. This highlights how Delaware is not seismically active. Nonetheless, in 1997, the state was reclassified as a medium seismic risk state by the USGS and FEMA.
While earthquakes on the East Coast are infrequent, they can be damaging in nature. In 2011, a M=5.8 earthquake in Virginia resulted in $200-300 million in damage and over 140,000 reported feeling the earthquake on the USGS website. Therefore, it is always important to understand your seismic hazard and to check how you can mitigate your risk.
Delaware Geological Survey