5.1 NC quake felt in Stephens County is largest in region since 1916

USGS warns of 56% chance of aftershock of magnitude 3 or higher within one week

A “moderate” earthquake was felt throughout Stephens County this morning. The epicenter was 2.6 miles southeast of Sparta, North Carolina and struck at 8:07 a.m. The quake registered as a 5.1 magnitude on the Moment Magnitude scale, which is similar to the Richter Scale and is the modern standard for classifying earthquakes.

If you felt this morning's earthquake, visit USGS to add your report of the experience to the official database.

No structural damage or injuries have been reported from this morning’s quake.

The largest quake reported within 100 miles of today’s earthquake epicenter took place in 1916 and is recorded as a 5.2 magnitude event.

According the U.S. Geological Society Agency (USGS), “moderately damaging earthquakes strike the inland Carolinas every few decades, and smaller earthquakes are felt about once each year or two.”

Source: USGS

On an open-source reporting database for volcanos and earthquakes, more than 10,000 reports of the quake were reported within an hour of the event, with reports from North Carolina, South Carolina, Georgia, Virginia, Eastern Kentucky, Tennessee, southern West Virginia and even reports of a ‘light shake” Maryland and Washington D.C., “mainly from the upper floors of tall buildings.”

According to the USGS, “The August 9th, 2020 M 5.1 earthquake near Sparta, North Carolina, occurred as a result of oblique-reverse faulting in the upper crust of the North American plate. Focal mechanism solutions for the event indicate rupture occurred on a moderately dipping fault either striking to the northwest or south. This earthquake occurred in the interior of the North American plate. Such mid-plate earthquakes are known as intraplate earthquakes and are generally less common than interplate earthquakes that happen near tectonic plate boundaries. This earthquake was preceded by at least four small foreshocks ranging from M 2.1-2.6, beginning about 25 hours prior to the mainshock.”

Based on geological records, quakes of magnitude 5 or higher are rare, but occur every 5-10 years on average in the eastern US region.

One of the strongest and most damaging earthquakes in recent decades was the 5.8 quake that hit the Piedmont region of the Commonwealth of Virginia on August 23, 2011 at 1:51:04 p.m. EDT. It was felt across from Florida to Canada to Florida and caused widespread, but mostly minor damage.

According to the USGS, there is a 56% chance of aftershocks within a week.

The chance of an earthquake of magnitude 3 or higher is 56%, and it is most likely that as few as 0 or as many as 54 such earthquakes may occur in the case that the sequence is re-invigorated by a larger aftershock.

The chance of an earthquake of magnitude 5 or higher is 5%, and it is most likely that as few as 0 or as many as 2 such earthquakes may occur.

The chance of an earthquake of magnitude 6 or higher is 1 in 200, such an earthquake is possible but with a low probability

The chance of an earthquake of magnitude 7 or higher is 1 in 2,000, such an earthquake is possible but with a low probability.

Quake Data:

Date & time: Sunday, 9 August 2020 12:07 UTC Local time at epicenter: 2020-08-09 08:07:37 am Magnitude: 5.1 Depth: 3.7 km Epicenter latitude / longitude: 36.47583°N / 81.09333°W (United States) Nearest volcano: Carrizozo (2272 km) Estimated released energy: 2.8 x 1012 joules (783 megawatt hours, equivalent to 674 tons of TNT)

Graphical description of the Moment Magnitude scale fo

Source: Michigan Tech

The Moment Magnitude scale was introduced in 1979 by Tom Hanks (no, not that Tom Hanks) and Hiroo Kanamori as a successor to the Richter scale and is used by seismologists to compare the energy released by earthquakes.

The Moment Magnitude Scale in layman terms:

The moment magnitude scale is based on the total moment release of the earthquake. Moment is a product of the distance a fault moved and the force required to move it. It is derived from modeling recordings of the earthquake at multiple stations. Moment magnitude estimates are about the same as Richter magnitudes for small to large earthquakes. But only the moment magnitude scale is capable of measuring M8 (read ‘magnitude 8’) and greater events accurately.

Magnitudes are based on a logarithmic scale (base 10). What this means is that for each whole number you go up on the magnitude scale, the amplitude of the ground motion recorded by a seismograph goes up ten times. Using this scale, a magnitude 5 earthquake would result in ten times the level of ground shaking as a magnitude 4 earthquake (and 32 times as much energy would be released). To give you an idea how these numbers can add up, think of it in terms of the energy released by explosives: a magnitude 1 seismic wave releases as much energy as blowing up 6 ounces of TNT. A magnitude 8 earthquake releases as much energy as detonating 6 million tons of TNT. Pretty impressive, huh? Fortunately, most of the earthquakes that occur each year are magnitude 2.5 or less, too small to be felt by most people. (Source: MIchigan Tech)

The Moment Magnitude Scale in more scientific terms

Earthquakes occur when the ground ruptures. Stresses build up over time (usually caused by the slow movements of tectonic plates) and eventually a piece of the Earth's brittle crust deep under ground breaks (the technical term is ruptures). This rupture then grows until eventually a large area has shifted (the rupture propagates at a velocity of 2–3km/sec). The magnitude of the earthquake is related to the size of the rupture.

Seismic moment (Mo) = μ* rupture area * slip length

where μ is the shear modulus of the crust (approx 3x1010 N/m)

Moment magnitude (Mw) = 2/3log(Mo) - 6.06

Nowadays the moment magnitude scale is the one used by seismologists to measure large earthquakes. The historic Richter magnitude is calculated by measuring the deflection on a seismometer corrected for distance from the event. Richter magnitudes underestimate the size of large events and are no longer used. However, the constants used in the definition of Moment magnitude (Mw) were chosen so that the magnitude numbers for Richter and Moment magnitudes match for smaller events. For the largest events (the Mw 9.3 event on 26 December 2004) the rupture area can be 1200 km long by 100 km deep with a slip length of up to 15 m (it had a seismic moment of 1.1x1023 Nm)


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