Could it happen here? That was the big question in the U.S. in the hours and days after a magnitude 9.0 earthquake and resulting tsunami crippled the Fukushima Daiichi nuclear power plant and destroyed the surrounding region. Most American worries focused on the integrity of our nation's rapidly aging nuclear power plants, many of which are still churning through uranium long past the reactors' original expected lifetimes. Fears were piqued again August 23 when a magnitude 5.8 temblor struck central Virginia, threatening power plants up and down the east coast. But a quieter danger lingers near many of our cities and towns. Tens of thousands of dams, many built before seismic engineering came of age, have the potential to release tsunami-like flash floods in the event of a seismic breach.

In 2009 the American Society of Civil Engineers released a survey of the state of infrastructure in the U.S. The group found that dams are, on average, in terrible disrepair. Of the more than 85,000 dams, more than 4,000 are unsafe or deficient, and nearly 1,800 of those are located where a breach would cause severe damage to life or property. With so many dams, it is hard to know where the gravest danger lies. The average budget for dam inspectors is distressingly low. For instance, Texas employs just seven inspectors to keep an eye on 7,400 dams, and in many states inspectors lack the authority to inspect private dams, including those built to hold back the chemical by-products of mining operations. A report by Switzerland's Paul Scherrer Institute estimates that dams are the most potentially hazardous source of energy. A catastrophe at an average dam has the potential to kill 11,000 people. The second-most-hazardous energy source? Nuclear.

Decades ago, engineers frequently built dams using a slurry of soil and water that would eventually settle into place. Unfortunately, an earthquake might liquefy the core of the dam. Although these dams were most often built in locations that were not thought to suffer from earthquakes (the Lower San Fernando Dam that nearly failed in a 1971 earthquake in the Los Angeles area is a notable exception), our knowledge of just where and when an earthquake may strike is no longer limited to the west coast. "New York City wasn't supposed to be seismic 20 or 30 years ago," says Tarek Abdoun, a civil engineer at Rensselaer Polytechnic Institute, "but the standard for what should require seismic loading has changed." In the past few decades, seismologists have discovered potentially serious fault lines everywhere from the Carolinas to Missouri.

Engineers have begun the process of retrofitting dams that suddenly are found to be located in earthquake-prone areas. But progress is slow and expensive. The Association of State Dam Safety Officials estimates that $50 billion would be needed to repair all the nation's faulty dams. Until that money comes through, fragile nuclear reactors should not be our only worry.