U.S. emergency agencies constantly rehearse for a disaster like the one unfolding in Japan, and American first responders will learn from the experience of their Japanese counterparts, Homeland Security Secretary Janet Napolitano said Tuesday.

Napolitano, speaking at a conference in Denver, said the United States was already planning a drill based on a hypothetical major earthquake along the New Madrid fault in the central U.S. when the earthquake, tsunami and nuclear-reactor crisis struck Japan.

"We are constantly practicing, using scenarios that are worst-case scenarios, to make sure we are as prepared and as up-to-date and as ready to go as we can be in any kind of a crisis," she said.

Napolitano said her department works with the Nuclear Regulatory Commission to practice responding to a crisis at a nuclear reactor.

"We think about how we would manage a crisis where you lose all your communications capability, all your critical infrastructure, there's no electricity, you can't even pump water for people to drink," she said.

Napolitano said it's too early to say whether U.S. practices or preparations will change because of the disasters in Japan. But she said the U.S. will learn from Japan's experience, as it did from Hurricane Katrina in 2005 and the oil spill in the Gulf of Mexico last year.

The U.S. is focusing now on providing whatever assistance the Japanese government asks for, Napolitano said. She said the NRC and the Energy Department have told her Japan is responding to the nuclear crisis the way U.S. agencies would.

She also cited NRC assurances that any fallout from the Japanese reactors would not put the U.S. at risk.

Napolitano spoke at a conference on the U.S. network of "fusion centers," which gather and share federal, state and local intelligence on terrorism and other threats.

She said terrorist plots by U.S. residents or citizens are increasing, and the centers are a key part of the U.S. counter-terrorism strategy. There are about 70 fusion centers nationwide.

Napolitano said the Colorado fusion center, called the Colorado Intelligence and Analysis Center, played a "significant role" in the arrest of Najibullah Zazi, a former Denver airport shuttle driver who pleaded guilty to plotting to detonate explosives in New York City around the anniversary of the Sept. 11, 2001, attacks.