What made the August 2011 Virginia quake so unique?
The magnitude 5.8 (M5.8) earthquake which struck the state of Virginia in August 2011 was unusual in that it was the largest to occur in the eastern United States in over a century (it was the second largest ever recorded in the state).
Now research into the extent of damage has revealed that it also caused damage over an area many times larger than predicted or seen before.
The Virginia Earthquake of August 2011
The M5.8 Virginia earthquake occurred in an area known as the Central Virginia Seismic Zone which, according to the United States Geological Survey, “has produced small and moderate earthquakes since at least the 18th century.”
The seismic zone is one of local intra-plate faulting: the area is remote from any major tectonic plate boundaries which are associated with major earth tremors in the western United States.
Earthquakes on the CVSZ are not uncommon but tend to be relatively small in magnitude compared with the western states: the largest on record in the eastern US was an M5.9 which occurred over a century earlier in 1897, also in Virginia. The tremors experienced in the area are typically shallow and appear not to be linked to a single fault.
Although the August 2011 quake did not cause significant damage, it was felt across a wide area: the USGS reports of the area over which the quake could be felt reported the extent of shaking as reaching as far away as Texas, Wisconsin and Florida, a maximum distance of 1,673 km (USGS, quoted in Jibson and Harp).
Landslides Caused by the Virginia Earthquake
Following the earthquake, scientists from the USGS used field techniques to map, in painstaking detail and down to a small scale, the extent of the landslides attributable to the earthquake. The paper describes earth movements triggered by the earthquake at distances of up to 245km from the epicentre of the earthquake. The total area within which disturbance occurred was around 33,400 square kilometres.
These findings are highly significant in that, when compared with landscape studies of previous intra-plate earthquakes, they were up to 20 times larger. “The fact that the landslide distances and areas so far exceeded historical observations from other earthquakes is the key finding,” lead author Dr Randall Jibson told Decoded Science. “The implication of this observation is that earthquake shaking can travel much farther in the eastern U.S. (an intraplate area) than the western U.S. (a plate-boundary region)…. Our findings indicated that shaking extends even farther than was predicted by the seismological predictions.”
Learning From the Virginia Earthquake
It’s long been known that shaking from an earthquake east of the Rockies is felt over a wider area than from a tremor of comparable size to the west. There are sound geological reasons for this: away from plate boundaries the earth’s crust is much older and is therefore relatively colder and denser. Under these conditions, seismic waves travel further before losing their energy (attenuating) and thus are felt at greater distances.
The close study of the 2011 earthquake, more detailed than for any previous tremor of its size and setting, has established that the previously understood and fixed limits for earthquake shaking have been well short of what has now been shown to be the case.
“The study confirms that for the same magnitude earthquake, the area affected in stable continental interiors at a given intensity is likely to be ~20 times larger than it would be in plate-boundary regions,” says Dr Jibson. These findings, then, have implications for future assessment of risk and hazard. “The way we plan for such earthquakes in these two types of regions is thus very different,” he concludes.
Jibson, R.W. and Harp, E.L. Extraordinary distance limits of landslides triggered by the 2011 Mineral, Virginia, Earthquake. (2012). Bulletin of the Seismological Society of America. Accessed November 6, 2012.
USGS. Magnitude 5.8 Virginia. (2012). Accessed November 6, 2012.