Chief stonemason Joe Alonso examines the damage at the Washington National Cathedral a day the earthquake. (Nikki Kahn/THE WASHINGTON POST)

When a 5.8-magnitude earthquake struck the Washington area last month, what seemed like one of the city’s strongest buildings turned out to have some of the worst damage: the National Cathedral.

Several slender carved pinnacles on top of the cathedral, which are 45 feet tall, were cracked or damaged. “It’s hard to see, but a lot of them just rotated,” said Joe Alonso, who manages the cathedral’s stonework.

One four-ton section of a pinnacle fell onto the roof of the cathedral’s 301-foot-tall central tower, as did several finials, which are pieces at the very top of a pinnacle. All the pinnacles on the main tower will have to be removed and fixed, Alonso said.

Why the cathedral?

Throughout the city, the damage caused by the earthquake was fairly mild. But the cathedral is different from your house in two important ways: It is made of stone, and it is very, very tall. Both of those factors exaggerated the impact of the shaking earth. The Washington Monument, another tall stone structure, was also damaged by the quake.

“The cathedral is a big, heavy building, and it’s stiff — it’s not made to be flexible,” said Bill Leith, a seismologist (earthquake scientist) with the U.S. Geological Survey. “Modern skyscrapers and steel buildings are made to be flexible . . . and not be damaged” by most quakes. Work on the cathedral began in 1907 and was completed in 1990.

When the earthquake hit, waves of energy from the shaking ground passed up through the cathedral, but the walls didn’t respond by bending or swaying, so pieces at the top broke off when the energy reached them.

The worst damage was at the cathedral’s tallest point. It’s a matter of physics: Motion increases the higher you go. You’ve seen this at work if you’ve ever built a Lego tower: When you move the base even slightly, the top sways like crazy.

This is also what happened at the Washington Monument, which also remains closed after several stones near the top were slightly dislodged or cracked. “At this point we really don’t know when it will reopen,” said Carol Johnson of the National Park Service. “But I can tell you it will reopen, and it is not leaning.”

Fixing it

The good news is that “the main structure of the cathedral is sound,” Alonso said. But the cathedral can’t reopen until all the damage is inspected and loose stones are secured. That could take weeks or months.

What Alonso does know is that it will take years, and millions of dollars, to completely restore the cathedral. One carved angel that broke off the tower, for example, would take about 50 hours of carving to replace.

“Every one of those pieces up there is hand-carved,” Alonso said. “And that’s just a lot of hand work.”

— Margaret Webb Pressler