Early on Aug. 9, 2020 many across the Mid-Atlantic, and the Southeast awoke to their homes shaking around them as an earthquake centered in Sparta, NC rattled the east coast. Earthquakes are common, yes even here on the east coast usually they’re so weak very few if any people even notice them. Until the other day.

DYFI intensity map
A “Did you feel it map?” indicating where around the area Sunday’s (August 9th, 2020) quake was felt
Courtesy: USGS

This quake registered at a magnitude of 5.1 on the Richter Scale, which isn’t a particularly strong earthquake in the grand scheme of things. But when it comes to the east coast we don’t see very many earthquakes stronger than a 3.0 on the Richter scale.

What does XY.Z on the Richter scale even mean?

The Richter scale was developed in 1935 by Charles F. Richter at the California Institute of Technology, to provide a standard measurement tool to measure earthquakes. The scale increases by tenths, but for each whole number the scale increases, the energy released by an earthquake increases ten fold! To put it another way, a magnitude 10.0 earthquake is 1,000,000,000,000-percent (1 Trillion) stronger than a magnitude 2.0 earthquake. We’ve never recorded one that strong though, but we have gotten close. On May 22, 1960 a magnitude 9.5 earthquake was recorded off the southern coast of Chile. So far that’s the strongest earthquake on record.

Why and where do earthquakes normally occur?

Earthquakes are generally the result of a ‘slip’ on a fault line (location where two different tectonic plates meet). The tectonic plates are always moving, just very very slowly. They shift approximately 1-2″ a YEAR. But even this slight movement causes stress to build up along fault lines, especially since they don’t all move in the same direction. Take the North American plate and the Pacific Plate for example they meet along the west coast of the United States. One moves Northwesterly, while the other slowly drifts to the Southeast (approximately). Part of this fault line along these two plates runs through California, and yes this is the infamous ‘San Andreas Fault’.

Now there are many different ways earthquakes occur, some more common than others. The three most common forms of earthquakes occur along three common types of faults. Go figure.

  • Normal Fault
    • When the block above the fault has moved downward relative to the block below it.
  • Reverse Fault
    • When the upper block, above the fault plane, moves up and over the lower block.
  • Strike-Slip Fault
    • A fault on which the two blocks slide past one another. (San Andreas Fault)

(Information provided here courtesy of the USGS)

Animations of the three fault types

Now normally movement along these fault lines is slow, referred to as a ‘creep’. Sometimes though movement along the faults is restricted when one of the plates gets caught on the other. This causes stress (energy) to build up and eventually the stress (energy) becomes greater than the frictional force holding the plates and it gets released violently. Thus causing an earthquake!

What type of Earthquake occurred in Sparta?

The earthquake that occurred in Sparta was actually a combination of two of the different earthquake types discussed above. An ‘oblique slip fault’ happened which is a combination of a normal fault earthquake, and strike-slip fault earthquake. Specifically, because the quake didn’t occur along a fault line, it’s a ‘intraplate oblique slip fault earthquake’. Try saying that five times fast. Intraplate type earthquakes are relatively uncommon, but as was proved on Sunday (August 9) not impossible.

-StormTracker 59 Meteorologist Liam Healy