Geek 101: Why Canadians Felt an Earthquake in Virginia

On Tuesday, an earthquake measuring 5.8 on the Moment magnitude scale struck near Mineral, Virginia--around 38 miles away from the state’s capital city of Richmond.

The most interesting aspect of this earthquake was not only its location--earthquakes on the East Coast of the United States are far less common than they are along the West Coast--but how people hundreds of miles from the epicenter could readily feel the quake. New York City residents could clearly feel the tremors, 316 miles away from the epicenter. The quake could even be felt in parts of Canada.

So, why is it that a fairly moderate earthquake that's incapable of causing significant amounts of serious damage was felt from such a distance away? While some websites have pinpointed perhaps one reason why the Virginia quakes could travel so far, there are actually a few factors at play that could explain why earthquakes of varying magnitudes can go the distance. First, let’s take a look at some earthquake basics.

Basics

As you probably already know, earthquakes are caused by a sudden release of energy from tectonic plates that make up the Earth’s Crust. These plates are constantly shifting as part of continental drift, and the boundaries between them consist of a series of fault lines.

Not all faults (or portions of faults) can cause earthquakes--some exhibit aseismic creep, where the land masses on both sides of the fault slowly slip by each other without causing a quake. But if the two plates can't slip by each other, stress builds up along a fault, and it can result in an earthquake when that stress is relieved.

To match the energy of a 5.8 earthquake, you would need between 6000 and 7000 tons o fTNT--imagine the amount of destruction that amount of TNT could do in a confined space.

Although quakes are very common overall, they are often too small to notice. The shaking from an earthquake is caused from vibrations called seismic waves; these waves are capable of travelling either across the surface and through the Earth's mantle and core--this is why sometimes earthquakes that happen on one side of the Earth can be picked up by seismographs on the other side of the world.

We usually think of faults as being around the edge of tectonic plates--after all, the vast majority of earthquakes occur at these plate boundaries--,but as Tuesday's quake shows, earthquakes can occur in the middle of a tectonic plate. A paper published to UC Berkeley's website gives some more detail on why these interplate quakes occur, but the basic idea is that they may be caused by ancient earthquake faults that spring back to life.

Virginia is part a fault zone

A "shake map" of the Virginia quake. [Image: USGS]

While Virginia is not near a plate boundary like California, there is still a fault area (bear in mind faults are different to fault lines) through the state: the Virginia Seismic Zone. Generally, although the zone is active, its quakes are largely unnoticeable and deep underground, so an earthquake of 5.8 is quite large for Virginia--in fact, it’s the biggest quake to hit the area since 1897.

This zone sits in the middle of the North American plate, and covers about 3000 square miles around the Piedmont Province. A glance at the state may also be a hint that it sits on a seismically active area, as the Appalachian Mountains pass right through it, and the Virginia Seismic Zone could link to various other nearby faults. Holly Ferrie, a Geology student at the Open University, explains how the waves in Virginia probably moved:

“The fault [the quake] happened along led up to the location of Mineral, Virginia, which was [where] the first and strongest... seismic waves were felt.

“Although [seismic] waves can travel through the earth internally, they most commonly fan out to nearby locations, and energy decreases the further away it gets, unless a fault connects certain locations.”

And as the Washington Post points out, "the released energy traveled 'along the grain' of the Appalachian Mountains to the northeast and southwest." In other words, the quake's waves could have been amplified through a seismic chain reaction of sorts.

Scratching the Surface

Depth could have played a key factor in the Virginia quake’s coverage as well. The Central Virginia Seismic Zone is hard to find on seismic activity maps because the quakes within the zone usually happen very deep underground—so by the time the seismic waves reach the surface, they’re hardly noticeable unless you have a seismograph. However, the Virginia quake was only 3.7 miles below the surface--quite shallow for an earthquake.

So, due to the shallow depth of the quake’s epicenter, which happened in the Virginia Seismic Zone, which is connected to other faults, the quake’s seismic energy was able to travel at a stronger force over a wider distance than a similar-sized quake in, say, California.

The Ground Matters

As strange as it seems, the ground can play a big part in how well an earthquake travels.

Due to the positioning of the North American Plate boundary and nearby fault lines, the West Coast sees a lot of activity. Because of all this activity, the crust along the West Coast is generally a lot hotter and also a lot weaker in comparison to the East Coast, where the plate boundary is further out to sea. Seismic waves find it much easier to travel through colder, stronger, less abused areas of the Crust than more offset surfaces such as in California. As Holly explains:

“The West Coast is worse due to the collision of the Pacific and North American plate, creating the San Andreas fault line. The East Coast's closest plate boundary is Mid-Atlantic ridge, and that's pretty far away from it! [The East Coast's crust is] cooler and stronger, but [it] does not necessarily have a thicker crust though. But the colder surface definitely makes waves travel faster!”

The actual bedrock can have an impact on how earthquakes travel. In Virginia, the Piedmont contributes to the Appalachian Range, created millions of years ago, out of very old rock. The mountain ranges were created out of faulted marine sedimentary rocks, volcanic rocks and were part of the original Pangaea supercontinent. The old and faulting rock of the Appalachian range, combined with the coolness of the east coast, and potential activity in the Seismic Zone encourages further-moving waves due to reverberation. The breaks in the bedrock of California usually stop this kind of reflex.

“Virginia is part of the Appalachian Range, so a similar formation to Scotland and Norway [once all of these countries were attached and the range ran throughout]--very old rock squashed in Pangaea supercontinent collision 440 Ma [million years ago]," Holy tells us. "This created loads of faulting, hence the wide spread of seismic propagation.”

When you bring all these factors together, it makes a clearer picture as to why the Virginia quake carried so magnificently: cool, barely-ruptured ancient ground combined with an activity zone will let a sudden powerful surge energy equivalent to 7000 tons of TNT less than 4 miles from the surface travel at a fair pace across lots of connected faults and fault lines to reach other parts of the continent.

Of course, why such a sudden moderate-heavy quake happened at such a shallow depth is yet to be really uncovered--perhaps it's a mystery fault that went previously undetected--but if there's one thing that geologists, seismologists, and scientists can agree on, it's that the Earth will always continue to surprise.

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