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The Mw 5.8 Virginia Earthquake of August 23, 2011 EERI, GEER, And

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The Mw 5.8 Virginia Earthquake of August 23, 2011 EERI, GEER, And EERI Special Earthquake Report — December 2011 Learning from Earthquakes The Mw 5.8 Virginia Earthquake of August 23, 2011 After the Mw 5.8 earthquake of social disruptions up and down the The epicenter of the August 23 August 23, 2011, EERI assembled East Coast that lasted from several quake was 8 km from Mineral, a team of East Coast EERI mem- hours to some months. The quake Virginia, 61 km from Richmond, bers to document various aspects caused widespread interruptions to and 135 km from Washington, of the event. Co-leaders were communications and transportation D.C. (Figure 1). A network of James Beavers, a consulting struc- systems, numerous school closings in seismographs installed following tural engineer from Knoxville, Ten- the epicentral area of Louisa County, the August 23rd main shock has nessee, and William Anderson, a other closures in the greater Wash- enabled accurate location of many sociologist and an EERI Board ington, D.C., area, and an automatic aftershocks. The aftershocks define member from Washington, D.C. shutdown of the North Anna Nuclear a 10-km-long, N28°E striking, Other team members included Power Plant. The earthquake is a 45°ESE dipping fault at between Matthew Eatherton, Virginia Tech; noteworthy reminder that communi- 2.5 and 8 km depth, in good agree- Ramon Gilsanz, Gilsanz Murray ties on the East Coast are not pre- ment with the focal mechanism of Steficek Inc; Frederick Krimgold, pared to cope with a major earth- the main shock. The locations of Virginia Tech; Ying-Cheng Lin, quake, and that steps could be taken aftershocks do not correspond to a Lehigh University; Claudia Marin, to improve their readiness and in- previously identified fault; previous, Howard University; Justin Marshall, crease loss reduction efforts. smaller, instrumentally recorded Auburn University; Jelena Pantelic, earthquakes in the zone have had the World Bank; and James Ricles, Seismology diverse focal mechanisms over Lehigh University. Marshall partici- an area with a length and width of The Mw 5.8 (USGS) earthquake re- pated remotely by mapping struc- about 120 km, rather than align- sulted from reverse faulting at shallow tural damage. Additional material ment in a pattern that might sug- depth (estimated at 6 km) on a north- was furnished by members of the gest a single causative fault. northeast-striking plane within a pre- Geotechnical Extreme Events Re- viously recognized seismic zone: the Figure 1 shows the regional dis- connaissance (GEER) Association Central Virginia Seismic Zone. This tribution of intensities associated team: Russell Green, Virginia Tech; zone has produced James Martin, Virginia Tech; Sissy small and moder- Nikolaou, Mueser Rutledge Con- ate earthquakes sulting Engineers; and Sam Lasley, since at least 1774, Virginia Tech. Other contributors to the previous most this report included Jeff Munsey, damaging shock Tennessee Valley Authority, and a having been in team of social scientists from the 1875. That quake Disaster Research Center at the predated the inven- University of Delaware: Yvonne tion of effective Rademacher, Alex Greer, Laura seismographs, but Keeley, James Kendra, Benigno its felt effects and Aguirre, and Lucia Velotti. area suggest that it Unless otherwise indicated, photos had a magnitude of in this report were taken by EERI about 4.8. It shook team members. bricks from chim- neys, broke plaster Introduction and windows, and overturned furni- The Mw 5.8 earthquake that struck central Virginia at 1:51 p.m. local ture at several lo- time on August 23, 2011, caused cations. An M4.5 no deaths and few injuries, but did earthquake on De- damage buildings and other struc- cember 9, 2003, tures within a 100-mile radius of also caused minor the epicenter. Perhaps its biggest damage. Figure 1. Epicenter and intensity map made from effects were the business and 148,281 respondents to USGS DYFI site. 1 EERI Special Earthquake Report — December 2011 Figure 2. Acceleration response spectrum with 5% damping recorded at the North Anna Nuclear Power Plant for periods up to 5 sec (left) and isolating low-period range (right) (adapted from Dominion, 2011). with the earthquake, as estimated Regional Geology at earthquake depths, but numer- from data contributed to the USGS ous smaller or deeply buried faults “Did You Feel It?” (DYFI) web site. The region is very old, with bedrock remain undetected. Accordingly, Reports of the shock came in from formed more than a billion years ago few, if any, earthquakes in the seis- most zip codes within 800 km of the and a complex surficial geology that mic zone can be linked to named epicenter, and additional observa- bears the imprints of continental colli- faults, and it is difficult to determine tions came in from zip codes at sions, long-dead mountains, and for- whether a known fault is still active greater distances (Figure 1). Ac- gotten seas (Mittelbach & Crewdson, and could slip and cause an earth- cording to the USGS, the large felt 1998). The regional geology is char- quake. area is not unusual for an eastern acterized by a distinct “fall line” that U.S. shock; east of the Rockies, an separates inboard bedrock (the earthquake may be felt over an area Piedmont province) from the out- ten times larger than that for a simi- board Atlantic Coastal Plain. lar magnitude earthquake on the The fall line runs southwesterly West Coast. from the District of Columbia- Montgomery County boundary A maximum intensity of VII is esti- near Silver Spring, Maryland, mated from DYFI data, when obser- across the Potomac River north vations are averaged over zip code of Roosevelt Island (Figure 3). regions. However, field observa- tions indicate that an area in rural Most bedrock beneath central Louisa Country close to the causa- Virginia was assembled as tive faulting had shaking correspond- continents collided to form a ing to Modified Mercalli Intensity supercontinent about 500-300 VIII; the average DYFI intensity is million years ago, raising the VII for the entire 500-km² zip code Appalachian Mountains. The region in which higher-intensity rest of the bedrock formed when effects were observed. the supercontinent rifted apart about 200 million years ago to A time history recording of the earth- form what are now the north- quake from the North Anna Nuclear eastern U.S., the Atlantic Ocean, Power Plant approximately 17 km and Europe. The Central Vir- from the epicenter shows a peak ginia seismic zone is far from horizontal ground acceleration of the nearest plate boundaries, 0.27g (Figure 2). The response which are in the center of the spectrum also shows high energy Atlantic Ocean and in the Carib- Figure 3. The fall line (dashed) and terrace content at short periods as com- bean Sea. The seismic zone deposits in the vicinity of the Washington pared to typical west coast earth- is laced with known faults, Monument (source: Collective Works of J. quake spectra. though they are poorly located P. Gould). 2 EERI Special Earthquake Report — December 2011 lie directly over the rupture plane. There may have been other lique- faction sites, but the undergrowth in the region is thick and sand depos- its sparse. Several heavy rains associated with Hurricane Irene in the days after the main shock likely washed away any features in the river and creek beds. Slumps and Subsidence. One small slump was found on the bank of the South Anna River very close to the liquefaction features, again likely directly over the rupture plane. What is believed to be earthquake- induced subsidence was found in an abandoned gold mine. Rockfalls. Four rock falls were Figure 4. Aerial image of the epicentral region showing locations of observed found in the epicentral region, two liquefaction sand boils. Also shown are the epicenters of the main shock and along the banks of the South Anna of >M3.0 aftershocks within two days of the main shock (source: GEER). River and two in road cuts. The team determined that the area of Coastal Plain deposits include deposition of relatively fine grained rockfalls extended from just north Cretaceous sediments, Pleistocene alluvium in river channels. of Harper’s Ferry on the north, to Terrace deposits, and river depos- the Virginia/West Virginia border The epicentral area is drained by ited alluvium of relatively recent on the west, and to about 10 miles South Anna River and its major, sec- origin. The Cretaceous sediments, north of the North Carolina border, ondary, and tertiary tributaries. Allu- known as the Potomac formation, along the Blue Ridge Parkway, on vium (unconsolidated clay, silt, sand, were deposited in relatively shallow the southwest. They were not able and gravel of Tertiary to Quaternary seas on the sloping bedrock sur- to determine the eastern limit. The age) appears along portions of these face by streams flowing eastward determined limits are shown in streams, typically at leeward-side out of the continental interior. These Figure 5. point bars and horseshoe bend cut- sediments are primarily hard clays offs, ranging from a few meters to Details of the geotechnical effects and compact sands, and a succes- tens of meters thick. Topographic of this earthquake will be published sion of river terrace deposits of sideslopes and hilltops are typically in a forthcoming GEER report Pleistocene times overlies them. underlain by saprolite (decomposed (GEER, 2012). While glacial ice did not reach south bedrock chemically weathered in- to the current Washington, D.C., place) beneath soil horizons; saprolite Damage to Structures area, the terrace deposits were can range from a few meters to tens formed by debris carried in streams Most historic engineered structures of meters thick. charged by glacial melt water flow- located in the densely populated corridor along the Atlantic coast ing from the northwest.
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