On August 23, 2011, a magnitude 5.8 Mw earthquake hit central Virginia, the largest in the area in more than a century. The quake occurred at an approximate depth of 3.7 miles and shaking was felt as far away as Atlanta, Detroit, and Toronto. Buildings were evacuated in D.C. and business was disrupted in New York. Seventy homes were heavily damaged and four collapsed. Three decorative pinnacles at Washington National Cathedral fell. The Washington Monument was closed due to cracks in the top section. The quake caused an estimated $70 million in damage.
The quake shook the North Anna nuclear power plant, just miles from the epicenter, with more force than it was designed to withstand. North Anna was shut down after the quake, becoming the first nuclear reactor to shut down after an earthquake in the 53-year history of commercial nuclear power in the United States. In response to the March earthquake and tsunami that triggered large releases of radiation at Japan’s Fukushima Daiichi nuclear power plant, the NRC ordered a review of seismic risks to 27 U.S. nuclear power plants, including North Anna.
The Southeast is seismically unique
Although most of us associate earthquakes with the West Coast, seismological studies show that although earthquakes happen less frequently in the southeastern U.S., the region is vulnerable to large, damaging earthquakes. In 1897, an earthquake with an estimated magnitude of 6.0 rocked southwestern Virginia and was felt from Pennsylvania to Georgia.
There are important differences in the seismic zones of the Southeast and West Coast. According to James R. Martin, Ph.D., professor of civil and environmental engineering and Director of the Disaster Risk Management Institute at Virginia Tech, “East Coast geology is a much bigger actor than we once thought.” He explains that the bedrock is much harder and closer to the surface in the Southeast, causing the soil to amplify shock waves moving from the epicenter of a quake more than in California. As a result of the geologic differences between the East and West Coast regions, a quake in the Southeast creates waves that travel farther and cause more damage at a greater distance from the source than when a shock of similar magnitude hits California.
The Southeast tends to have large earthquakes separated by long periods of quiet. Often, strong quakes are not even felt in the Southeast because the tectonic strain rates are different than those in California. Faults are buried deeper below the surface and are consequently harder to study, making interval prediction more difficult. The densely populated East Coast has a preponderance of older buildings not designed to withstand significant seismic shaking. Unreinforced masonry structures in particular are brittle, highly vulnerable to catastrophic damage, and the number one nemesis of human safety during seismic activity.
Analyzing the risk
Before the catastrophic earthquake and fire in San Francisco in 1906, the most damaging seismic event in the U.S. occurred in 1886 near Charleston, South Carolina. It caused structural damage as far away as Richmond and Atlanta, and reached an estimated magnitude of 7.3 on the Richter scale. Of course, the density of structures in the area was much less than today. A quake of the same magnitude killed more than 17,000 people in Turkey in August 1999.
James Martin has a particular research interest and expertise in the areas of adaptive risk management of large-scale infrastructure systems and earthquake and foundation engineering. He visited Turkey to study the effects of the 1999 earthquake. Martin points to a frightening commonality between Turkey and the southeastern U.S. Both have dangerously inadequate structural protection of buildings. “Earthquakes don’t kill people, buildings kill people,” Martin notes. He warns that if another 7.3 magnitude earthquake hit Charleston today, the city would suffer much the same damage as cities in Turkey. He adds that if the epicenter of the recent quake in Virginia was just 30 miles closer to D.C., the scope of damage and fatalities would have reached startling proportions.
Even though experts agree on the potential for major earthquakes in the Southeast, there are few engineering studies or emergency response plans in place. In response to that concern, James Martin and Martin Chapman, director of the Virginia Tech Seismological Observatory co-founded the Earthquake Engineering Center for the Southeastern United States in 2000. Presently, Martin is working on seismic analyses for the Smithsonian Institution and Washington Monument and is scheduled to report his findings to Congress later this month.
Creating better codes
Martin helped develop the newest seismic codes and teaches continuing education courses on code implementation. He points out, however, that 86% of new structures in the Eastern U.S. are built with simplified code procedures. “There is a potential for more damage than we currently anticipate. Current provisions don’t work in the Southeast. All the factors that went into the development of code – design curves, soil conditions, recorded quakes – are based on data from California.”
After four years of numerical analyses, the August earthquake showed exactly what Martin’s team predicted. Martin emphasizes the importance of developing region-specific provisions for the Southeast. “The biggest difference in material earthquake waves will encounter is the last 100 feet between the source and a structure, which changes the character of the motion that affects the building. Three- or four-story buildings, like low-rise office or apartment buildings are the most affected. This geologic uniqueness of this region demands a different approach. We now recognize the need to use more site-specific analyses, tailored to the individual situation, until we have developed provisions specifically for this region. I expect it to take about six years for this new information to get through the code committee and implemented in the engineering community.”
As always, SEAA strives to keep members informed of developments that affect standards for the commercial construction business and closely follows refinements to existing standards. The Association maintains a keen interest in news that impacts our members and the industry, and promises to stay involved in these activities as events unfold.
