The 7.1 magnitude earthquake that ravaged central Mexico earlier this week, leveling buildings and killing more than 270 people, is a warning for central California.
UC Berkeley seismologist Peggy Hellweg said the East Bay could experience a similar sized earthquake “tomorrow.” And some Bay Area communities, including Richmond, are not prepared.
Particularly vulnerable are “soft story” buildings: older, often wood-framed, multi-story structures with an unstable ground floor (think of an apartment building with a parking garage underneath it). San Francisco and other Bay Area cities have passed laws that mandate retrofitting these buildings, which experts say are at a high risk of collapse in a major quake. But Richmond officials have yet to implement similar safety measures.
Chris Castanchoa, an engineer and building official for Richmond, said city planners surveyed more than 280 potential soft-story buildings in December 2010. Building owners were notified the following summer, but retrofitting in Richmond is voluntary, and Castanchoa couldn’t provide numbers on how many, if any, of the buildings have been fixed.
“We don’t have a formal [seismic-retrofit] program at this point that is up and running,” Castanchoa said.
According to the Association of Bay Area Governments (ABAG), soft-story buildings made up more than half the housing lost during the 1989 Loma Prieta earthquake, and the group estimates they will account for approximately two-thirds of uninhabitable buildings after a major Hayward fault quake.
Berkeley, Fremont and San Francisco all require seismic retrofits of soft-story buildings, and those cities are nearly finished making them safer. Castanchoa said Richmond officials have not researched what other East Bay cities are doing, and drafting a similar ordinance would consume already limited time and labor.
“Quite honestly, we haven’t even started developing this,” Castanchoa said.
Oakland requires owners of soft-story buildings to have a licensed engineer or architect examine the structure’s ground floor. But, like in Richmond, that city’s officials have not passed an ordinance mandating seismic retrofits.
“From our perspective, most communities that haven’t done a screening have no idea if a building is vulnerable,” said Lin Chin, a coordinator with Oakland’s Department of Housing and Community Development, who helps run the city’s seismic-retrofit program. “For cities that have a mandatory ordinance, there are better standards for what a soft-story retrofit needs to be.”
The ABAG provides a model ordinance city officials can use to tailor a seismic-retrofit law to their specific needs.
But retrofits are time consuming and costly. And it’s hard for lawmakers to approve an ordinance landlords can’t comply with, or that cash-strapped tenants can’t handle.
But for seismologists such as Hellweg, the time spent debating whether or not to retrofit the buildings is time wasted — and could be deadly.
The spectre of a Hayward fault temblor is fresh in her mind. The Mexico quake was the second to hit the particularly vulnerable country in one month. An 8.2 magnitude earthquake killed at least 61 people there in early September.
Hellweg said the aftermath of a major Hayward earthquake could be worse. While the first earthquake started miles off the Mexican coast in the Pacific, the Hayward fault, which stretches from Mission San Jose to Point Pinole, runs through nine cities home to more than 2 million people.
“Lots and lots of communities are built right on the Hayward fault,” she said.
Earthquakes happen on the Hayward fault every 140 to 150 years, so, there’s a good chance it’s built up the stresses needed for another.
She explained that the Hayward fault is a strike-slip fault. If the tectonic plates that shape the East Bay’s terrain had their way, they would skate along at about a centimeter a year. The plates are creeping along at less than half that rate, though. And eventually, they’ll jump to catch up the lost distance.
Based on the amount of time that has elapsed since the fault’s last big earthquake, in 1868, and the creep that has occurred since, scientists can estimate how much the plates will jump. Hellweg said it could be 2 meters or more.
As a general rule in seismology, the longer the fault has to move, the bigger the earthquake. A 6.0 magnitude earthquake moves about 1 meter and lasts five to 10 seconds. If the Hayward fault moves the full 2 meters or more it has to catch up, the resulting quake would be magnitude 7.1 and last 30 seconds, Hellweg said.
During those 30 seconds, a century-and-a-half of mounting tension on the Hayward fault would yank the western plate (the flats) north of the eastern plate (the hills). Hundreds of miles of gas lines, water mains and road would split in two. The gas lines would ignite fires, and the damage to water mains and roads would prevent emergency responders from extinguishing them.
Near the bay, buildings would sink into gelatinous earth like quick sand. Those closest to the fault line would be destroyed. The ABAG estimates more than 10,000 homes in Contra Costa county will be uninhabitable and 100,000 people could be left homeless for at least a month.
“My opinion is that, as a city, we would be as prepared as one could be,” Castanchoa said of such a catastrophe. “There is always room for improvement, though.”