Loma Prieta 25 years later: Lessons learned on earthquakes and engineering

On Oct. 17, 1989, the Loma Prieta earthquake rocked Northern California, causing extensive damage, thousands of injuries and more than 60 deaths. Though the strongest part of the quake only lasted about 15 seconds, its aftershocks can still be felt a quarter-century later.

This is one of three stories marking the anniversary of the major 1989 Bay Area quake.
 

That is because Loma Prieta didn't just shake up the Bay Area. According to various San Francisco State University experts, it shook up understanding of earthquakes and engineering, along with how we can be smarter about the next seismic event.

The epicenter of the 6.9-magnitude earthquake was in the Santa Cruz Mountains, not far from a peak called Loma Prieta. The nearby city of Santa Cruz was particularly hard hit, but there was major property damage throughout the Bay Area -- worth an estimated $6 billion. Some buildings and highway overpasses collapsed, a section of the Bay Bridge failed, gas lines were ruptured and large fires broke out.

Despite the tragedies, most Bay Area edifices held up during the Loma Prieta earthquake. "Structures designed to or retrofitted to the standards at the time performed at or above expectation," according to Zhaoshuo Jiang, an SF State assistant professor of civil engineering and an expert on seismic structural design.

Photo of Assistant Professor of Civil Engineering Cheng Chen

Assistant Professor of Civil Engineering Cheng Chen

A lot was learned from the Loma Prieta quake, and that knowledge has been used to strengthen designs and building codes. "The earthquake-engineering cycle is always like that," said Assistant Professor of Civil Engineering Cheng Chen. "There's a big earthquake and we have big findings, and then we study and improve things. And then we have to wait for the next quake."

At SF State, researchers are preparing for future natural disasters today by diving into the science of earthquakes and developing innovative ways to minimize losses from them.

Down to earth 

Although Loma Prieta's destruction was spread widely, it was not spread evenly. For Associate Professor of Geology John Caskey, it quickly became apparent that certain areas were affected more than others, and not because they were closer to the epicenter. "The parts of San Francisco that suffered the most damage were landfill areas," he said.

The city's Marina District, which originally was marshland that was filled with dumped rubble from the massive 1906 earthquake, felt the effects of the '89 quake more intensely than other areas. The soppy ground and loose rock on which the neighborhood was built absorbed and amplified the seismic waves of the quake. "When the waves get amplified, they cause more damage," added Caskey, who teaches neotectonics and other geology courses and is part of a team reviewing seismic hazards near the Diablo Canyon nuclear-power plant for PG&E.

Photo of Professor of Geosciences Karen Grove

Professor of Geosciences Karen Grove

Earth & Climate Sciences Professor Karen Grove said the same effect that wreaked havoc in the Marina might have played a role in the quake's deadliest damage. In Oakland, Calif., an elevated section of Interstate 880 called the Cypress Viaduct collapsed, killing 42 people. "It was built along the edge of the bay in soft sediment, and those soft sediments have really high amplitude of shaking compared with solid bedrock," Grove noted. "For the same magnitude earthquake, the response is going to be very different depending on what you're sitting on. Loma Prieta really emphasized the importance of that."

Scientists are revisiting this notion in the wake of the recent magnitude 6.0 earthquake near Napa, Calif. Its epicenter, geologists suspect, was in a place where soft sediments -- marshes and mud basically -- magnified the quake's force. SF State scientists have been studying the aftereffects of that temblor by monitoring fault creep (surface displacement along fault lines) in the surrounding area. A large amount of creep could signal that the Napa earthquake changed how the faults in the Bay Area rub against each other -- perhaps increasing the odds of another, larger quake. Read more about fault-creep research at SF State.

But even though today's researchers have a better understanding of why and when earthquakes happen, Caskey does not foresee a time when seismologists are forecasting them like TV weather reporters. "Earthquakes are funny things. They don't always behave the same way leading up to a quake," he said. "We might be able to predict some of them one day, but we're never going to predict them all."

Engineering room

The last 25 years demonstrate the dramatic role that building design and construction have in earthquake country. Even as the Napa region was jolted in August this year, leaving dozens injured and one person dead, damage was not extensive. "If that kind of earthquake had been in another country, it would have been much more devastating," according to Wenshen Pong, director of the SF State School of Engineering.

Photo of Wenshen Pong, director of the School of Engineering

School of Engineering Director Wenshen Pong

Until recently, designs could only be tested via computer simulations or by constructing models and subjecting them to simulated earthquakes on a device called a "shake table." Chen, Pong's colleague, has been exploring a way to meld these techniques with what is called "real-time hybrid simulation." Backed by more than $400,000 in grants from the National Science Foundation, this method allows engineers to test a structure's response to a temblor.

A single element of a proposed design is built and tested on a special shake table in SF State's Structural and Earthquake Engineering Laboratory, and the results are then combined with sophisticated computer models that mimic an earthquake's effect on the building as a whole. "By integrating everything together, we can calculate the true structural response during an earthquake," Chen explained.

SF State's Jiang, for his part, is working on improving California's construction standards by exploring what he hopes will be the future of building design: "a bio-inspired symbiotic structural-health monitoring and control system." In other words, Jiang and engineers like him are trying to figure out how to create a building that can detect damage to itself and take appropriate action to address it, just like a living organism. "Using a human body as an analogy, if a person sprains their ankle, upon noticing the pain they will shift their weight to the other foot spontaneously," he said. "Imagine if a structure could do the same, all by itself."

So the key regarding earthquakes is an ongoing commitment to research, evident at SF State. "The structural-engineering community here [in the San Francisco Bay Area] is very welcoming to innovative ideas," Pong said. "I would say it's the most aggressive and progressive when it comes to the safety of the public."

For more on Loma Prieta 25 years later, see our additional coverage: SF State project monitors Bay Area faults and Be ready for the rumble.

-- Steve Hockensmith

Last update: 
2016-01-27 12:01
By: 
AL