RICHMOND, Va. (WTVR) – Two years ago, perhaps during the time of day after lunch when we let our minds wander, we were startled by the sudden and undeniable shaking of the ground in central Virginia. It was felt up and down the East Coast states at 1:51 p.m. ET. Compare that area of impact with California’s 2004 earthquake of similar magnitude:
Damage repairs continue today from the Epicenter near Mineral, VA to our Nation’s Capitol.
(CLICK HERE to see this graphic in higher resolution)
The Virginia earthquake of 2011 August 23 occurred as reverse faulting on a north or northeast-striking plane within a previously recognized seismic zone, the “Central Virginia Seismic Zone.” The Central Virginia Seismic Zone has produced small and moderate earthquakes since at least the 18th century. The previous largest historical shock from the Central Virginia Seismic Zone occurred in 1875. The 1875 shock occurred before the invention of effective seismographs, but the felt area of the shock suggests that it had a magnitude of about 4.8. The 1875 earthquake shook bricks from chimneys, broke plaster and windows, and overturned furniture at several locations. A magnitude 4.5 earthquake on 2003, December 9, also produced minor damage.
Previous seismicity in the Central Virginia Seismic Zone has not been causally associated with mapped geologic faults. Previous, smaller, instrumentally recorded earthquakes from the Central Virginia Seismic Zone have had shallow focal depths (average depth about 8 km). They have had diverse focal mechanisms and have occurred over an area with length and width of about 120 km, rather than being aligned in a pattern that might suggest that they occurred on a single causative fault. Individual earthquakes within the Central Virginia Seismic Zone occur as the result of slip on faults that are much smaller than the overall dimensions of the zone. The dimensions of the individual fault that produced the 2011 August 23 earthquake will not be known until longer-term studies are done, but other earthquakes of similar magnitude typically involve slippage along fault segments that are 5 – 15 km long.
Earthquakes in the central and eastern U.S., although less frequent than in the western U.S., are typically felt over a much broader region. East of the Rockies, an earthquake can be felt over an area as much as ten times larger than a similar magnitude earthquake on the west coast. A magnitude 4.0 eastern U.S. earthquake typically can be felt at many places as far as 100 km (60 mi) from where it occurred, and it infrequently causes damage near its source. A magnitude 5.5 eastern U.S. earthquake usually can be felt as far as 500 km (300 mi) from where it occurred, and sometimes causes damage as far away as 40 km (25 mi).
The initial 5.8 earthquake, and the subsequent “settling” aftershocks, released the energy that had built up along those complex, folded and buried faults. The vast majority of the pressure was relieved on August 23, 2011. With that relieved pressure, tension in the Central Virginia Seismic Zone could take several lifetimes to rebuild.
The USGS focused on Virginia in the months immediately after the earthquake, tracking 369 aftershocks greater than about magnitude 1.0 through December 31, 2011. That includes micro-earthquakes. The USGS says, “These earthquakes were located by the USGS National Earthquake Information Center (NEIC) using data provided by portable seismographs deployed by Virginia Polytechnic Institute (Virginia Tech), Columbia University Lamont- Doherty Earth Observatory, University of Memphis Center for Earthquake Research and Information, Lehigh University, IRIS (Incorporated Research Institutions for Seismology (IRIS), Cornell University, and USGS. A couple hundred of the aftershocks greater than about M1.7 were felt locally. Many more aftershocks smaller than M1.0 (and unlikely to have been felt) have likely occurred in the epicentral area during this time, but the data have not been thoroughly analyzed for events this small.”
CLICK HERE to find out how long we can expect aftershocks, and if another big earthquake is likely in our lifetime.
Meteorologist Carrie Rose compiled the official list of the USGS recorded events since August 23, 2011 (that list currently has 65 aftershocks, not including micro-earthquakes), plus other “micro-earthquakes” not reported by the USGS, but measured by other groups studying the Central Virginia Seismic Zone in the wake of the quake. This list below is not an official record, but it gives us a good idea of the number of well-measured, documented tremors. There are 90 aftershocks and the initial earthquake listed here, in order by date. You’ll notice the vast majority occurred within Louisa County.
DATE MAGNITUDE LOCATION