THE GREAT QUAKE OF ’89
Despite scientific advances, predicting quakes is still in the future.
It would be an understatement to say that earthquake science has evolved since the 1989 Loma Prieta earthquake, when the best known early warning system was to keep tabs on your dog or cat for signs of stress.
The advances, many inspired by the devastating 6.9magnitude quake that ripped through Northern California 30 years ago, have been remarkable — including everything from laser mapping and the use of the Global Positioning System to a burgeoning alert system that will stop BART trains and shut off gas lines.
But none of the detection equipment or hightech computergenerated models helped geologists predict the 4.5magnitude earthquake that rolled through Pleasant Hill on Monday night. The fault that shook more than 70,000 people was unknown, unmapped and 9 miles under the Earth’s surface.
“We now realize that faults are very complicated,” said Stephen Hickman, director for the U.S. Geological Survey’s earthquake science center in Menlo Park. “We know there are a lot more faults in the Bay Area than we knew about, and we know much more about them. Nevertheless, blind faults are still a critical player in California earthquake hazards.”
The real question, though, is whether science has made Bay Area residents any safer since Loma Prieta leveled houses in San Francisco’s Marina District, destroyed a freeway viaduct in Oakland, collapsed a portion of the Bay Bridge and killed 63 people, all while a nationwide television audience was watching the World Series.
“I think that what the man or woman on the street should do hasn’t changed much since 1989,” said Peggy Hellweg, operations manager at the UC Berkeley Seismological Laboratory. “People dropping, covering and holding on is always going to be an important thing for safety in an earthquake.”
It’s a sobering reality that, despite the best efforts of scientists, jumping under a desk is still the best advice the experts can give to the public. But it doesn’t mean society isn’t safer than it was 30 years ago, Hickman said.
“Back then we didn’t have the internet, cell phones, GPS. We relied on TV and radio for communications. That was a different world,” Hickman said. “The impact of an earthquake identical to Loma Prieta would be different today than it was then, thanks in large part
to all the retrofitting that has occurred since then. There would almost certainly be fewer casualties.”
Andy Michael, a USGS geophysicist, said Parkfield (Monterey County) has the most seismic instruments because it has the largest concentration of magnitude6 earthquakes in California. It is on the San Andreas Fault halfway between San Francisco and Los Angeles and averages a 6.0 earthquake about every 20 years. The last 6.0magnitude earthquake there was in 2004.
The biggest lifesaver could be ShakeAlert, a sophisticated earthquake warning system being developed by the USGS, UC Berkeley, the California Institute of Technology and the universities of Washington and Oregon.
The plan, once all 1,675 seismic sensors have been installed in the early 2020s, is to be able to send warnings to every smartphone, business, public agency, utility and home where shaking is predicted in California, Oregon and Washington. The alerts, sent a few seconds to a minute before the shaking starts, would automatically brake trains, open firehouse doors, close valves in fuel pipelines and halt elevators at the nearest floor.
Almost 60% of the ShakeAlert sensors have been installed in the three states, but the greater Bay Area, stretching from Santa Rosa to Monterey, almost has its full complement. The region has 157 of the 188 planned ShakeAlert stations working. The rest should be operational within the next year, officials said.
ShakeAlert is the culmination of a series of technological advances that have pushed realtime earthquake monitoring, information dissemination and science into a new realm, Hickman said.
It started with a statewide program to map every urban fault, under the California Seismic Hazards Mapping Act of 1990. Since then, computer image processing techniques, laser scanning of surface topography, 3D modeling and Google Earth have vastly improved the speed, quality and accuracy of documenting and recording fault lines and rupture zones, which used to be done with topographic maps and compasses.
Aerial reconnaissance using drones, digital photography and videography have helped geologists find and map previously unknown faults. Laser scanners known as Lidar can be used from airplanes to map fault line contours in overgrown, inaccessible areas without showing trees and other obstacles. The information, like an Xray image of the land, is recorded on supercomputers, allowing maps and graphics to be displayed in high definition.
A network of GPS receivers has also been set up, allowing seismologists to measure the movement and create threedimensional maps of faults, including the dangerous Hayward in the East Bay and the San Andreas, which runs up the Peninsula and off the coast of San Francisco before returning to shore at Point Reyes.
The receivers can be used — along with recordings of ground shaking from past earthquakes — to calculate how much pressure has built up, how fast energy is accumulating and how much ground shaking would happen in a future tremor.
That’s how scientists determined the Hayward is now slipping at a faster rate deep underground than it is on the surface.
“The Hayward will have to have an earthquake to balance that out,” said Belle Philibosian, a USGS research geologist who studies soil disruptions in trenches dug across fault lines and dates historic earthquakes. “We try not to use the word overdue ... but I would say it would not be surprising if the next big earthquake happened tomorrow.”
Philibosian said radio carbon dating using highprecision mass spectrometry, which didn’t exist 30 years ago, has documented 12 large quakes on the Hayward Fault over the past 1,900 years, the last one in 1868. Based largely on that information, the U.S. Geological Survey now says there is a 72% chance of an earthquake of 6.7magnitude or greater hitting the Bay Area by 2043.
Hickman said every earthquake has something to teach scientists. The 1989 quake, which was centered near Loma Prieta peak in the Santa Cruz Mountains, showed how a fault can cause damage on a large scale, farther away, than was previously known. It also highlighted the danger of liquefaction — when the ground liquefies — for structures built on wetlands and landfill, like San Francisco’s Marina District, which suffered major damage in 1989.
The 7.3magnitude quake that hit the Mojave Desert town of Landers in 1992 taught scientists how earthquakes are often made up of a multitude of interconnected fault segments, rather than a single fault, creating a kind of ripple effect.
The 1983 Coalinga earthquake, a 6.5magnitude shaker that occurred far from faults that had been seen at the surface, and the 6.7magnitude Northridge earthquake, which hit the San Fernando Valley in 1994, showed how some faults can go undetected until they burst.
“These faults — being hidden below the surface — can produce earthquakes on faults that were previously not recognized,” Hickman said. Scientists are working on ways to detect blind faults using spacebased geodetic techniques to find regions of the crust that are rapidly deforming, he said.
The Ridgecrest quakes, which hit on July 4 and 5, culminating in a 7.1magnitude monster, are teaching geologists important lessons about the eastern California shear zone, a swath of the PacificNorth America plate boundary zone, where the Landers quake also occurred.
Hickman said the 6.0magnitude Napa quake in 2014 demonstrated how the earth can continue to slip long after the shaking stops. That fault slipped about 50 centimeters in the north and only 10 centimeters in the south when the earthquake struck. The southern portion continued to slip after the shaking and “had pretty much caught up” a year later, Hickman said.
“The reason this matters is you don’t want to repair a water main or a gas line across a fault when it is still slipping,” he said. “ShakeMaps show where shaking was strongest during an earthquake, to focus emergency response, damage inspections and rebuilding.”
It’s especially important, he said, because building and structure collapses are what kill most people in earthquakes. It is why Bay Area businesses, individuals and cities have spent up to $80 billion retrofitting and replacing structures since 1989. Building codes throughout the region have also been updated using computergenerated maps forecasting likely levels of shaking from future earthquakes.
With all the progress made in making people aware of an earthquake as soon as the ground starts shaking, forecasting the Big One is still something no one has figured out how to do. So people cling to the old ways, like in China, where they look for hibernating snakes streaming out of their hollows, or here in the West, where people keep a sharp eye out for panicky pets.
“We’re learning more, but there are still lots of questions we can’t answer, like when is the next one,” UC Berkeley’s Hellweg said. “We keep looking for signs, but none of them are good enough. Unfortunately, earthquake prediction is really, really, really challenging, and I don’t expect it to happen within my lifetime.”