Hundreds of feet below the Earth’s surface, a groundbreaking event is brewing below the East Bay.
For Cal State East Bay and U.S. Geological Survey researchers and scientists, the question is not whether a powerful earthquake — like the 1989 magnitude 6.9 Loma Prieta temblor — will strike Hayward, San Leandro or surrounding unincorporated communities. The more pressing question, they say, is what it will do to local neighborhoods when it does strike.
“We can map the Hayward fault at the surface, so we can see it, but we don’t know what it does in the subsurface, and if you look at a map, you can see that there are a whole bunch of sub-parallel faults in the area, particularly one called the Chabot fault,” U.S. Geological Survey geophysicist Rufus Catchings said in an interview.
“These faults, if they were to move by themselves, could generate a very large earthquake, but we suspect that these faults may be connected in the subsurface, so if we have an earthquake on the Hayward (fault), it would generate a tremendous amount of energy traveling along those arterial faults. We need to know if they’re connected or not because, if they’re connected, then it’s not just the Hayward fault that we have to be concerned with — it’s all of these faults.”
To answer that question, the university and federal agency are conducting a joint experiment to determine what the subsurface looks like across the Hayward fault and measure how the ground in different Castro Valley and San Leandro neighborhoods respond to earthquake shaking.
“There are things that affect ground shaking — one of them is topography, whether you’re on sediment or hard rock, and the other is the fault structure itself,” Catchings said last Friday at Lake Chabot Regional Park, where experiment preparations were taking place.
The research team is installing more than 500 small, portable seismographs along a 9.3-mile line that crosses the Hayward fault through San Leandro and Castro Valley and digging 20- to 30-foot-deep holes inside which very small explosive devices will be set off by the end of this month. The small bangs will send seismic energy below ground and be recorded on the seismographs, creating a three-dimensional picture of the underground geology in the San Leandro and Castro Valley areas.
Cal State East Bay geology professor Luther Strayer, a project co-director, said talks to collaborate with the U.S. Geological Survey began in 2013, shortly after the federal agency teamed up with the university on another project: the demolition of 13-story Warren Hall on the Cal State campus. Scientists and researchers using the building implosion to analyze ground shaking effects discovered something unusual, Catchings said.
“If you have a house on top of a hill, even if it’s on hard rock, the shaking can be more intense on that hill than down in sediment areas,” Catchings said.
The Hayward, San Leandro and Castro Valley area is at risk for a major earthquake, based on recent estimates.
A 2014 working group charged with developing statewide earthquake rupture forecasts found there is a 72 percent probability that at least one 6.7-magnitude or greater earthquake will strike the Bay Area before 2043.
Of the five major Bay Area fault systems, there is a 33 percent probability that an earthquake of that power will happen along the Hayward fault. That’s the highest probability rate of any other major Bay Area fault line, compared to the Calaveras fault (26 percent), San Andreas fault (22 percent), Maacama fault (8 percent) and San Gregorio fault (6 percent).
“One thing that makes this area unique compared to other world-class, strike-slip systems in the world is the urban factor — we’ve got huge cities right on top of this fault,” said Adrian McEvilly, a Cal State graduate student participating in the study.
“When the catastrophic earthquake happens, and it will, we’ve got to try to mitigate the loss of life and damage to property, because the bottom line is that buildings, not earthquakes, kill people,” he said.
It’s an issue that has prompted Hayward and San Leandro leaders to take proactive approaches to building codes and safety.
In San Leandro, the city has two earthquake retrofit programs — one for older unreinforced masonry buildings and another for older wood-frame homes — that provide workshops, guides, financial help, and permit and building code information to residents, business owners and contractors.
Hayward offers an expedited permitting process to homeowners and free earthquake retrofits to qualified residents who own and live in their homes. The city also provides loans for qualified retrofit repairs to low-income, disabled or senior residents.
Hayward is now creating a program to retrofit multi-family, wood-frame buildings at risk of collapsing in an earthquake. The proposed program would apply to multi-story structures built before 1978 with at least five units and a large open space on the bottom floor, such as a parking area.
“In an earthquake, these buildings shake and the lateral motion causes them to collapse off that first floor, causing great damage to the floors above it,” Hayward management fellow Laurel James told the City Council in July.
There are about 900 such buildings in Hayward, including 8,500 apartments, according to Alameda County Assessor’s Office data.
“We do know that a big one is coming. We don’t know exactly where or when, but our odds are not good,” James said.
