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THE AUGUST 23, 2011 MAGNITUDE 5.8 VIRGINIA EARTHQUAKE IN THE EASTERN UNITED STATES ‐ AN ENGINEERING PERSPECTIVE
Jim E. Beavers Matthew R. Eatherton Ramon E. Gilsanz
James M. Ricles M Ricles
Ying‐Cheng Lin Principal, James E. Beavers Consultants, Knoxville, TN, USA Assistant Professor, Virginia Tech, Blacksburg, VA, USA Principal, Gilsanz Murray Steficek LLP, New York, NY, USA Professor Lehigh University Bethlehem PA USA
Professor, Lehigh University, Bethlehem, PA, USA Graduate Student, Lehigh University, Bethlehem, PA, USA SUMMARY
A magnitude 5.8 earthquake occurred in Virginia, U.S. on August 23,
2011 and was the largest magnitude earthquake in Virginia since 1897.
The earthquake was centered in Louisa County, Virginia, approximately
130 km South‐West (SW) of Washington D.C. Louisa County, a rural
area with a population of 33,153, has no hospitals and one high school.
The high school and one elementary school had to be closed
indefinitely due to structural and nonstructural damage.
The
earthquake also caused architectural and structural damage in
Washington D.C. and was felt by many in New York City and as far north
as Canada. A nuclear power plant, located 18 km from the epicenter,
was subjected to a peak ground acceleration more than twice its safe
shut down earthquake value, but only suffered minor damage to
noncritical facilities and was safely shut down during the earthquake.
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AUGUST 23, 2011 GROUND MOTIONS
DAMAGE TO SCHOOLS
Few ground motions with short distance:
• Charlottesville, Virginia (54 km from epicenter with PGA=0.121g)
• Fredricksburg, Virginia (58 km from epicenter with PGA=0.135g)
• Reston, Virginia (122 km from the epicenter with PGA=0.109g)
• North Anna Nuclear Power Plant (18 km from the epicenter with
PGA=0.27g)
Other key details
• Strong energy content at low periods (high frequencies).
• High frequency content is typical for East Coast U.S. ground motions
p
gground motions for Central Virginia
g
• USGS maximum expected
Seismic Zone (CVSZ) is 0.12g on rock. PGA was 0.27g at North Anna
• Two middle schools and one high school in Louisa County
• There was enough damage to one of the middle schools and the high school to require them to be shut down indefinitely
• Middle School structure consisted of steel roof joists bearing on masonry exterior walls
• Masonry is concrete masonry units (CMU) at bottom and clay brick at top as shown in figure (a) below
• Poor anchorage of masonry walls to roof structure for walls perpendicular to roof framing
• Fallen brick and CMU from top of walls observed
• Outward bowing of exterior bearing wall shown in (a) below
• One of the few reported injuries occurred due to falling contents such as shown in Figure (b) below
• High school structure was steel framing with masonry infill as shown in Figure (d) below
• Estimated repair costs are $43 million for the high school
Ground Motion Time Histories for the Four Closest Recording Stations
REGIONAL SEISMICITY
Tectonic stresses on the East Coast of the United States:
o Compressive stresses oriented perpendicular to the
Appalachian Fold Belt
o Responsible for the formation of the Appalachian Mountains
300 to 400 million years ago
o Responsible for several seismic areas capable of producing
moderate to severe earthquakes
o 1886 M7.3 Earthquake in Charleston, South Carolina
o 1811 / 1812, M7.0 to M8.0 in New Madrid Seismic Zone
(d)
(a)
Brick
Infill at
Top of
Wall
(b)
Hinge Point
Forming in
Exterior Wall
Compressive Stresses in U.S. East Coast [Adapted from Zobach and Zobach (1980) and Topographic Map from en.wikipedia.org] (c)
Damage to Schools in the Epicentral Region
NORTH ANNA NUCLEAR POWER PLANT
• 18 km from epicenter and in line with directionality of observed damage
Reactor containment designed to remain elastic for PGA = 0.12g
Measured PGA at the site was 0.27g
No safety related damage observed
Twenty Seven large steel storage casks (520 kN each) for spent fuel rods shifted as much as 114 mm relative to their concrete slab.
North Anna Nuclear Power Plant
•
•
•
•
5% Damping Response Spectra for the Four Ground Motions
DAMAGE IN EPICENTRAL REGION
DAMAGE IN EPICENTRAL REGION
New
Madrid
Seismic
Zone
Aug. 23, 2011
Epicenter
Central Virginia
Seismic Zone
Charleston
Seismicity of Central and Eastern United States
[Adapted from USGS] 8/23/11 Epicenter
Regional Earthquakes from 1977‐1999
Showing Three Distinct Seismic Zones Along Appalachian Mountains
[Adapted from VTSO Figure] General Information
• Epicenter in rural area of Virginia in Louisa County
• Louisa County Population is 33,153
• Damage is estimated to be $200 million to $300 million. Less than
$100 million was insured for earthquakes
(a)
114 mm sliding
(b)
Epicentral Region Information
• Building stock is mostly one to two story residences and low rise
commercial structures
• North Anna Nuclear Power Plant 18km away
• Damage was generally oriented along a SW / NE line adjacent to the
epicenter. Strong directionality observed
• Unreinforced masonry buildings performed poorly
• Brick veneer failures were common
• Damage to foundations, carports, collapsed chimneys, and more
DAMAGE – WASHINGTON D.C. REGION
Unreinforced Masonry Buildings
Cracking
in Stone
Masonry
Eastern Tennessee
Seismic Zone
August 23, 2011 at 1:51pm local time
Moment Magnitude 5.8
Maximum Modified Mercalli Intensity of VII
Largest earthquake in Virginia since 1897 Giles County Earthquake
Intraplate faults such as these on the East Coast U.S. transmit ground
motions much further than interplate faults on West Coast U.S.
• Damage is therefore more widespread on East Coast U.S.
• Washington D.C. experienced Intensity VI and widespread damage
• Hurricane Irene hit East Coast four days later
Modified Mercalli
Intensity for East Coast U.S. [from USGS] Regional Modified Mercalli Intensity [from USGS] Cracked Chimneys at the
Cracked Chimneys at the Smithsonian Institute Castle Built in 1850s
Typical Damage to Smithsonian Institute Pods (storage for items not currently on display)
Central Virginia
Seismic Zone
VIRGINIA EARTHQUAKE 2011
Ground Motions at North Anna Nuclear Power Plant Compared to Design Response Spectra • Much damage to the Smithsonian Institute
• Cracking of Washington Monument, $15 million in damage and closed
• National Cathedral experienced $20 million in damage
Giles County
Seismic Zone
•
•
•
•
•
(c)
Sliding of 540 kN Steel Casks Holding Spent Fuel Rods
Historic Cuckoo House
Gilboa Christian Church Built in 1857 Built in 1819
Unreinforced Masonry Building Unreinforced Masonry Building
Suffered $500,000 in damage.
Residential Damage
Shifts in
Exterior
Foundation
Wall
15 cm or more
movement
Yielding
locations,
paint
flaked off
Sheared
Anchors Bolts
CONCLUSIONS
Failed Brick Veneer and Collapsed Chimneys Were Common in the Epicentral Region Shifting and Cracking of Foundation Walls in the Epicentral Region
Multiple Collapsed Carports in the Epicentral
Region Due to Failure of Anchorage to the House
The August 23, 2011 Virginia Earthquake should be a wake‐up call for
the Central and Eastern U.S. If the next large East Coast U.S. earthquake
occurs in a more populous area, the result could be much more severe
damages, injuries, and fatalities. Some of the lessons learned from this
earthquake that may help prepare for future Central and East Coast U.S.
earthquakes include:
• The mapped seismic hazard associated with the Central Virginia
Seismic Zone is underestimated. The ground motions recorded from
this earthquake exceeded expected maximum ground motions by a
factor of two. Furthermore, the low period energy content in East
Coast U.S. ground motions may not be captured adequately in
current seismic hazard characterization.
• Unreinforced masonry buildings are dangerous in earthquakes.
Although this is not a novel observation, it was found that there is a
surprising amount of URM in the building stock including store
fronts, chimneys, and old URM structures.
• Schools in the epicentral area were found to be more susceptible to
damage than typical buildings.
• Some of our nation’s most iconic structures were found to be the
more susceptible to damage since they are largely unreinforced
masonry. Seismic retrofit is recommended.