Something rather strange happened in Delaware yesterday: the ground shook.
At around 4.47pm local time (9.47pm GMT), an earthquake registering as a 4.1M hit within the Bombay Hook National Wildlife Refuge, at a relatively shallow depth of 8.1 kilometers (5 miles). The tremors were felt in New Jersey, Pennsylvania, Maryland, New York, Virginia, and Washington DC.
Earthquakes do happen in this part of the world, but ones this powerful are exceedingly rare. The last time Delaware experienced anything comparable was way back in 1871 when another 4.1M quake shook the state.
As plenty of you know, earthquakes are most commonly associated with fault lines, particularly those along tectonic plate boundaries. Plates sliding across each other (e.g. the San Andreas Fault) and plates subducting beneath another one (e.g. the Cascadia Subduction Zone) are where most major earthquakes occur, as their movements permit the most extreme build ups of stress.
The section of North America east of the Rocky Mountains, however, doesn’t have any tectonic boundaries – so what caused the Delaware earthquake?
Technically, it’s known as an intraplate earthquake, one that occurs in the middle of a tectonic plate. A region famous for such quakes is the New Madrid Seismic Zone (NMSZ), one that’s centered on southeast Missouri. Again, there are no tectonic boundaries here, but there are ancient faults that some have referred to as “mantle scars”.
The NMSZ’s origins can be traced back to the attempted split of the continental landmass we now see as the contiguous United States. Although this severance never succeeded, old faults remained, and every now and then, they slip. Between 1811 and 1812, for example, several earthquakes struck the region, with one possibly as powerful as a 7.7M event.
Ancient fault networks streak through the area east of the Rocky Mountains too, and although they aren’t as hazardous as those in the NMSZ, they can slip from time to time. The North American Plate experiences external forces from surrounding plates all the time, and this can sometimes get transferred to these quiet faults, causing them to become temporarily active.
The United States Geological Survey (USGS), in a statement, explained that “most earthquakes in North America east of the Rockies occur as faulting within bedrock, usually miles deep.”