Earthquakes have shifted the ground beneath Libby Rose's historic Craftsman house several times in the last half-century, but still her home is standing. The chimneys, however, haven't fared as well.
When the 1989 Loma Prieta quake struck, the Roses' front chimney came tumbling down. Two weeks ago, the magnitude 6.0 Napa earthquake sent bricks from a second chimney raining into her backyard.
"They just fell all over the back. They broke the back gate," said Rose, 87. "This one was by far the worst."
Although brick chimneys have received little attention from seismic safety experts — overshadowed by more life-threatening issues such as unreinforced masonry and older concrete-frame buildings — they account for the most common form of damage during larger California quakes.
When chimneys collapse, bricks can become deadly projectiles. At least 15,000 brick chimneys were damaged in Los Angeles during the 1994 Northridge earthquake. In Napa, about half of the residential buildings damaged were due to brick chimneys.
"Chimneys are the first thing to go, we've seen it time and time again," structural engineer David Cocke said.
Older brick chimneys — generally those built before 1980 — are too stiff and brittle to withstand major shaking. The mortar holding them together can be ground down.
"Once the cracks form [in the mortar], then it's very vulnerable," said Laura Whitehurst, a San Francisco-based structural engineer. "If there's nothing to keep it together, once it starts bending, then you get catastrophic failures when the bricks start falling out."
Many California cities prohibit putting unreinforced brick chimneys in new homes. Others, including Los Angeles, have imposed stringent requirements that discourage homeowners from using brick.
If an older brick chimney sustains damage in an earthquake, it is not allowed to merely be fixed, Los Angeles Building and Safety spokesman Luke Zamperini said. The chimney must be replaced or at least retrofitted to meet stronger codes.
Across California, older brick chimneys are generally not required to be reinforced, and few have been strengthened or replaced.
Minimal retrofitting options for a single-family home usually cost less than $5,000.
To keep bricks from falling through the roof during an earthquake, one option is to add layers of plywood on the roof around the chimney. Another is to remove the upper part of the chimney, which can twist and turn during shaking, and replace it with metal. A third option is to add a diagonal steel brace to the chimney.
Even with those improvements, the chimney still could collapse. A more extensive retrofit, which could cost tens of thousands of dollars, involves removing the chimney completely or rebuilding it.
If the homeowner doesn't need the chimney to be functional, the retrofitting could be as simple as placing a steel tube down the structure and filling it with concrete.
Janiele Maffei, a structural engineer and chief mitigation officer for the California Earthquake Authority, said the threat to family members and neighbors makes it worth the cost of retrofitting.
Maffei notes the last known death in California caused by a chimney collapse. In the 7.3 earthquake that struck Landers, Calif., a 3-year-old boy in a sleeping bag next to the fireplace died after the upper half of the chimney came crashing down and struck him in the head. In another earthquake — the 5.2 temblor that hit Napa Valley in 2000 — a 5-year-old boy was left fighting for his life after he was buried by the fireplace during a birthday party sleepover. He survived, but underwent more than two dozen operations.
After the most recent Napa quake, Maffei said, she saw two chimneys that had crashed onto a street that would have been busy if the shaking had occurred at rush hour. She said homeowners should ask themselves: "If this does fall, could this fall on my neighbor's house? Could this fall where I always park my car? Could this fall where my children play?"
The family of a 13-year-old Napa boy who was struck by bricks in the Aug. 24 earthquake said they had no idea their chimney could be a hazard.
Nicholas Dillon was having a sleepover at the home, which he shares with his grandparents, and was lying on a mattress with his head closest to the fireplace. When the quake hit, he jumped up and woke his friend, who was on the couch. Nicholas began crawling to get to the door, but couldn't get far enough before the lower half of the chimney fell.
"I just remember falling on the floor after being hit by the brick," he said. "My back hurt tremendously."
The lights went out, and Nicholas screamed for help — he couldn't move his legs. With 911 phone lines jammed, his mother rushed to a fire station to summon rescuers.
Nicholas' pelvis was fractured in six places; it could be several months before he walks again.
Now his family wants the remains of the brick chimney ripped out of their home.
"My mother always liked having a fireplace for the nostalgia," said Carmen Rosales, Nicholas' aunt. "After this, she said, 'It needs to go. I want it gone. I never want to see it again.' "
Twitter: @ronlin @RosannaXia