Virginia: An Old State with Deep Faults

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Data from the USGS online database show that seismic activity in Virginia is low level and limited. Image by USGS

Virginia is among the oldest of the states, laden with history. It’s an old state geologically, too, with its exposed rocks “record the advance of ancient shorelines, the collision of continents, the tracks of long-extinct dinosaurs, and over one billion years of geologic history,” according to the College of William and Mary’s ‘Geology of Virginia.

Old age doesn’t necessarily equate with turbulence and Decoded Science’s seismological stopover in the Old Dominion State always looked likely to be short and quiet. But it would be a dull place indeed where an Earth scientist found nothing of interest.

Virginia, for all its quiescence, at least had the decency to produce one of its largest earthquakes recently enough for it to raise eyebrows.

Geology and Earthquake History of Virginia

Before we get on to the events of August 2011, it’s worth taking a quick look at why Virginia is so invulnerable, relatively speaking, to earthquakes. A geological map of the state is a beautiful thing, showing the relationship between geology and topography; but the map of historic seismicity (all recorded earthquakes since 1915, drawn from the USGS database) doesn’t reflect that. There’s a sprinkling of minor tremors in the mountainous south-west and a cluster in the flatter but older east-central part of the state. And..er.. that’s it.

But it’s never wise to place all your trust in recorded data. Seismology is a young science, and areas with long written and oral histories tend to offer more than just a map. Virginia’s biggest earthquakes probably predate the recording of seismic events; the USGS earthquake history of the state identifies significant shocks in 1774, 1883, 1852 and 1861, among others.

Earthquakes in cold continental crusts are felt more widely than those of a similar magnitude elsewhere. Image by USGS.

There’s no information as to the exact location or the magnitude, and it isn’t beyond question that some may have occurred in neighbouring states. We shouldn’t forget that large earthquakes are felt at great distances and the major shocks of the New Madrid earthquake sequence, in Missouri in 1811-12, will certainly have been felt in Virginia.

Historically, the earthquakes are more accurately measured and their locations better constrained. The USGS tells us that a series of tremors in the Roanoake area in 1897 affected an area of 725,000 square km and was “The largest earthquake to originate in Virginia in historic times” and mentions other small tremors but that’s about as exciting as it gets, with nothing in the state larger than around M4…until 2011.

The M5.8 Earthquake of 2011

In actual fact, the state’s biggest earthquake, which rolled in at M5.8, wasn’t that large in the worldwide scheme of things; but only two larger tremors were recorded in the US in the year up to the time of writing (both of them in California) and in tectonically-stable eastern North America, this quake it was very much a big deal.

Fox News led with the headline: “Magnitude 5.8 Earthquake Hits Virginia, Sends Shockwaves Throughout East Coast” while CBS News reported that a third of the US population may have felt the ‘quake, a fact confirmed by the USGS, which reported that “it may have been felt by more people than any other earthquake in U.S. history.”

There’s a reason for this: In old, cold continental crust, earthquakes are generally felt over greater distances than they are in areas where the crust is newer.

The previously unknown fault zone which caused the M5.8 earthquake in Virginia in 2011 was identified from the pattern of aftershocks. Image by USGS.

So where did the big quake come from? Well, here at Decoded we regularly attribute earthquakes in otherwise stable areas to ‘movement on a previously unknown fault.’ This sounds like a copout (and sometimes feels like one, too) but the fact is that much of the bedrock in the US, as elsewhere, is ancient, even when it’s overlain by more recent sedimentary deposits; and there are very many unmapped faults.

In the case of Mineral, Virginia, the old refrain holds true. According to the USGS: “Scientists who study eastern and central North America earthquakes often work from the hypothesis that modern earthquakes occur as the result of slip on pre-existing faults that were formed in earlier geologic eras and that have been reactivated under the current stress conditions.”

In February 2015, research published by the USGS and others identified just such a fault zone by using the location of the many minor aftershocks. It’s worth noting, however, that there’s still debate about the age of the fault zone, although it’s certainly true to say that it was previously unknown.

Seismic Lessons For Virginians — and Others

Virginia, like so many of the other states, may seem pretty quiet — but the M5.8 earthquake shows that we don’t know everything about what lies deep beneath our feet. More than that; it’s a reminder, in the words of the USGS, that “damaging earthquakes, though infrequent, are a part of living in the eastern United States.

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