The U.S. Geological Survey released new earthquake hazard maps for the nation on Thursday, boosting the risk of seismic damage up and down the Northern California coast — especially for tall buildings.
There’s a red zone forecast for much of the state, including the Sacramento, Bay and North Coast regions.
Long bridges and tall buildings are particularly susceptible to the longer wavelength of ground shaking seen in sedimentary basins, the kind of soils that underlie San Jose and Walnut Creek, according to the new assessment, announced at the annual meeting of the American Geophysical Union in San Francisco.
The maps are not meant to alarm current or future residents of those areas but to help them prepare and reinforce their homes before the next earthquake hits. The maps and supporting documentation also are used by municipal planners to create building codes.
“Before there were just estimates” of the long slow waves that flow through the region’s sedimentary basins, said Mark Petersen, a research geophysicist with the USGS in Golden, CO. “Now we have calculated a number.”
The geologic basins are caused when tectonic plates push and strain the earth, warping the rock, said Petersen.
“They’re warped like a bowl — with nice flat areas where people like to build,’ he said.
The sediments that settle into these geologic basins amplify an earthquake’s big slow waves. Since the previous map, scientists have taken images of the subsurface geology, using tools such as sonar.
For San Jose, estimates of ground motion have climbed 5 to 10 percent. Walnut Creek estimates increased 10 to 25 percent, according to Allison M. Shumway, USGS geophysicist.
Current building codes do not incorporate these new calculations, but future building codes will, said Nicolas Luco, research structural engineer with the USGS.
A committee of the FEMA-funded Building Seismic Safety Council has voted to use the new model in its 2020 recommendations. Then it must be adopted by the American Society of Civil Engineers, the 2024 International Building Code and local building codes.
The new maps are not ShakeMaps, which show ground shaking from a single earthquake or predicted ground motion from a simulated event, essential for planning exercises and response drills. Rather, they are probabilistic maps that show expectations for damaging shaking from all possible earthquakes over a 10,000 year period.
Seismic waves affect buildings in different ways, depending on height, according to USGS.
Small buildings experience more shaking from earthquake waves that are high frequency — short and frequent. They’re like small boats sailing in the ocean, which aren’t greatly affected by a large swell but can be capsized by several small waves in quick succession.
A tall building is more affected by earthquake waves that are long period, with slow shaking. It’s like an ocean liner, which isn’t disturbed by short waves in quick succession – but could sink with a large swell.
San Jose is home to many tall buildings, such as the 27-story San Jose Marriott, the 22-story The 88, Three Sixty Residences and Fairmont Hotel. San Jose City Halls stands at 18 stories.
Most buildings in Walnut Creek are 10 stories or shorter. Concord has one 15-story building, One Concord Center, and two 13-story buildings.
“If you have these big slow waves, modified by deep sediments that are several miles thick beneath San Jose and parts of the East Bay, it causes these buildings to swing back and forth,” said Petersen.
