The Virginia Earthquake – An Intraplate Event
The Virginia Earthquake (M5.8) occurred at roughly 2 pm on August 23, 2011 in the Virginia Seismic Zone (central Virginia). In Mineralogy lab on the fifth floor at Virginia Tech, my microscope nearly toppled over. After the shaking ceased, students rushed down the stairs to check out the live stream from nearby seismographs. Needless to say, this earthquake struck close to home.
A surprise to many, the Virginia Earthquake was an unexpected, relatively large East Coast intraplate event. Unlike most of the events that occur in the North America (Californa, Alaska, etc.) this event happened far from any active tectonic boundary. Events like this remind us that the stresses and strains we use to describe such boundaries, are actually present all over.
Figure: Seismic events (Mw > 4.0) since 1980 for eastern North America. Location of the Virginia Earthquake is shown in blue. (USGS)
Intraplate earthquakes are technically unrelated to plate boundaries and have an interval on the order of a thousand years or much, much more. Intraplate earthquakes have different source properties than interplate earthquakes. For example, while both types of events have a moment relationship to length of
the equivalent relationship between slip and length is
and includes different values for alpha. For interplate earthquakes, values of alpha are usually around 1×10-5 whereas for intraplate events values for alpha fall closer to 6×10-5. So while both sets of earthquakes “obey the same scaling laws, intraplate earthquakes have, on average, 6 times more slip on the fault.” This further indicates that stress drop is much larger for intraplate earthquakes.
These differences are likely a direct result of differences in frictional properties between the two types of earthquakes. Those occurring away from plate boundaries have more slip on shorter lengths with slower slip velocities.
Another interesting aspect of East Coast intraplate earthquakes is the distance from which they are able to be felt. For similar magnitude events in California, shaking is usually felt within the region, like most middle magnitude earthquakes in the East Coast, the Virginia Earthquakes was felt at much larger distances. This is a result of crustal difference between the eastern United States and the West Coast. The older, denser crust in the East allows seismic energy to propagate much further.
For more information, see Scholz et al. JGR 1986