October 18, 2011

Loma Prieta earthquake anniversary

Posted by Austin Elliott

Twenty two years ago this very day the San Francisco Giants were preparing to play their cross-bay rivals, the Oakland Athletics, in the 1989 World Series–the so-called Battle of the Bay. It was just after 5pm on a beautiful October afternoon, and the nation’s eyes were glued on television coverage from Candlestick Park in southeastern San Francisco. Hordes of Bay Area residents had swarmed bars and gathered at home after early departures from their offices in order to watch a game so crucial to everyone’s pride in the rivalry.

Evening news had begun, and classes were still in session at UC Santa Cruz, 70 miles down the peninsula. At 5:04 and 15 seconds, residents of the forested mountains around Santa Cruz were thrown from their places by a giant lurch of the ground. A sizable fault plane adjacent to the San Andreas beneath the Santa Cruz Mountains had slipped, although the rupture didn’t make it to the ground surface. As the sides of the fault ground past each other, seismic waves emanated westward toward the coast, and rocked the city of Santa Cruz.

USGS animation of shaking intensity produced by the Loma Prieta earthquake

Downtown Santa Cruz was devastated, and people fled buildings on the UC campus, all before cities up the peninsula had even an inkling of an idea that this earthquake was underway. As the seismic waves raced northward through San Francisco, BART operators were ordered to halt their trains, and Candlestick Park started bouncing. With scores of miles separating them from the epicenter, the baseball fans had the roaring rumble of P-waves to herald the quake’s violent shaking, but with nowhere to go and not nearly enough time, the stadium erupted into screams as S-waves rippled through and yanked the decks back and forth. As TV transmission cut out from the stadium, the rest of the country stared shocked and agape at the World Series logo silently standing in on their screens.

Meanwhile the seismic waves continued racing outward to the east, high frequencies getting stifled with every additional kilometer, until they began rocking Sacramento and the Central Valley, many minutes after they’d wreaked their havoc in SF.

Rocking from this earthquake was felt as far away as Nevada and Los Angeles, but it was a relatively modest event when compared with the earthquakes northern California’s major faults are capable of.

"Did You Feel It?" survey results for the 1989 quake

This earthquake–named Loma Prieta after the hills nearest its epicenter–was a truly remarkable event made famous by a still rather unbelievable coincidence of conditions: Hitting the urban population of northern California alone would have made it remarkable, but it did so during live nationwide TV coverage of a major sporting event;  that would have been remarkable in itself, but the baseball game in question was between both of the Major League teams from the region, and deeply involved all of their local fans.

Failure of the Bay Bridge during a 6.9 earthquake 70 miles away... Image courtesy USGS

Gradually (surely aided by the wealth of attention already focused on the Bay Area for the World Series game) information started to spread about the extent of this quake’s effects. The waves had bounced around, slowed down, sloshed, and churned in the poorly consolidated sediments and made-land beneath San Francisco’s Marina District and the industrial coastal stretches of Oakland and the east bay, collapsing houses and double-decker roadways, and igniting fires that for a while brought back visions of 1906.

The eastern span of the Bay Bridge had wriggled and stretched, and during a particularly tensile yank, a segment of the upper deck was pried away and flopped down onto the eastbound deck below.

These details may describe familiar sights to many people. Certainly some of the footage from the Loma Prieta earthquake is the quintessential earthquake footage for many Americans far from significant seismic hazard and exposed to earthquakes only through their interest in baseball.

As you re-live that moment through some of this footage, the most important thing to note is that this was not San Francisco’s earthquake. It was largely Santa Cruz’s. The quake was far south of the peninsula, and had a relatively modest magnitude of 6.9, about an order of magnitude smaller than the real Big One in 1906. The havoc wreaked in 1989 doesn’t come close to illustrating the real effects of a large rupture on one of the faults bounding the Bay Area. Fortunately we can use modern instruments and computing power to compare the Loma Prieta earthquake with simulations of potential earthquakes on both the Hayward and San Andreas faults. We have maps showing the shaking hazard as well as other associated hazards, including landsliding and liquefaction. The USGS has a neat zoomable map of liquefaction susceptibility in the Bay Area–check out where your house stands:

http://geomaps.wr.usgs.gov/sfgeo/liquefaction/susceptibility.html

The 1989 failure of the Bay Bridge spurred the construction of an entirely new bridge span that is soon to be completed, and implements some excellent (and super cool) earthquake-savvy devices. (Watch the video at the link below)

http://baybridgeinfo.org/projects/corridor-overview/seismic-innovations#.TpyuNZyppWo

As is the nature of earthquake science, earthquakes that occur are our primary way about learning what to expect and what to do in the future. San Francisco is likely to have bigger and closer earthquakes than this one, so it’s best to be prepared and know what you’ll be dealing with. Try ShakeOut!