1989 Loma Prieta earthquake


The 1989 Loma Prieta earthquake occurred on California’s Central Coast on October 17 at local time. The shock was centered in The Forest of Nisene Marks State Park approximately northeast of Santa Cruz on a section of the San Andreas Fault System and was named for the nearby Loma Prieta Peak in the Santa Cruz Mountains. With an magnitude of 6.9 and a maximum Modified Mercalli intensity of IX, the shock was responsible for 63 deaths and 3,757 injuries. The Loma Prieta segment of the San Andreas Fault System had been relatively inactive since the 1906 San Francisco earthquake until two moderate foreshocks occurred in June 1988 and again in August 1989.
Damage was heavy in Santa Cruz County and less so to the south in Monterey County, but effects extended well to the north into the San Francisco Bay Area, both on the San Francisco Peninsula and across the bay in Oakland. No surface faulting occurred, though many other ground failures and landslides were present, especially in the Summit area of the Santa Cruz Mountains. Liquefaction was also a significant issue, especially in the heavily damaged Marina District of San Francisco, but its effects were also seen in the East Bay, and near the shore of Monterey Bay, where a non-destructive tsunami was also observed.
Because it happened during a national live broadcast of the 1989 World Series, taking place between Bay Area teams San Francisco Giants and the Oakland Athletics, it is sometimes referred to as the "World Series earthquake". Rush-hour traffic on the Bay Area freeways was lighter than normal because the game, being played at Candlestick Park in San Francisco, was about to begin, and this may have prevented a larger loss of life, as several of the Bay Area's major transportation structures suffered catastrophic failures. The collapse of a section of the double-deck Nimitz Freeway in Oakland was the site of the largest number of casualties for the event, but the collapse of man-made structures and other related accidents contributed to casualties occurring in San Francisco, Los Altos, and Santa Cruz.

Background

The history of earthquake investigations in California has been largely focused on the San Andreas Fault System, due to its strong influence in the state as the boundary between the Pacific Plate and the North American Plate; it is the most studied fault on Earth. Andrew Lawson, a geologist from the University of California, Berkeley, had named the fault after the San Andreas Lake and later led an investigation into that event. The San Andreas Fault ruptured for a length of during the 1906 shock, both to the north of San Francisco and to the south in the Santa Cruz Mountains region. Several long term forecasts for a large shock along the San Andreas Fault in that area had been made public prior to 1989 but the earthquake that transpired was not what had been anticipated. The 1989 Loma Prieta event originated on an undiscovered oblique-slip reverse fault that is located adjacent to the San Andreas Fault.

Forecasts

Since many forecasts had been presented for the region near Loma Prieta, seismologists were not taken by surprise by the October 1989 event. Between 1910 and 1989 there were 20 widely varying forecasts that were announced, with some that were highly specific, covering multiple aspects of an event, while others were less complete and vague. With a M6.5 event on the San Juan Bautista segment, or an M7 event on the San Francisco Peninsula segment, United States Geological Survey seismologist Allan Lindh's 1983 forecasted rupture length of for the San Juan Bautista segment nearly matched the actual rupture length of the 1989 event. An updated forecast was presented in 1988, at which time Lindh took the opportunity to assign a new name to the San Juan Bautista segment – the Loma Prieta segment.
In early 1988, the Working Group for California Earthquake Probabilities made several statements regarding their forecasts for the northern San Andreas Fault segment, the San Francisco Peninsula segment, and a portion of that segment which was referred to as the southern Santa Cruz Mountains segment. The thirty year probability for one or more M7 earthquakes in the study area was given as 50%, but because of a lack of information and low confidence, a 30% probability was assigned to the Southern Santa Cruz Mountains segment. Two moderate shocks, referred to as the Lake Elsman earthquakes by the USGS, occurred in the Santa Cruz Mountains region in June 1988 and again in August 1989. Following each event, the State office of Emergency Services issued short term advisories for a possible large earthquake, which meant there was "a slightly increased likelihood of an M6.5 event on the Santa Cruz Mountains segment of the San Andreas fault". The advisories following the two Lake Elsman events were issued in part because of the statements made by WGCEP and because they were two of the three largest shocks to occur along the 1906 earthquake's rupture zone since 1914.

Foreshocks

The 5.3 June 1988 and the 5.4 August 1989 events also occurred on previously unknown oblique reverse faults and were within of the M6.9 Loma Prieta mainshock epicenter, near the intersection of the San Andreas and Sargent faults. Total displacement for these shocks was relatively small and although they occurred on separate faults and well before the mainshock, a group of seismologists considered these to be foreshocks due to their location in space and time relative to the main event. Each event's aftershock sequence and effect on stress drop was closely examined, and their study indicated that the shocks affected the mainshock's rupture process. Following the August 8, 1989, shock, in anticipation of an upcoming large earthquake, staff at the University of California, Santa Cruz deployed four accelerometers in the area, which were positioned at the UCSC campus, two residences in Santa Cruz, and a home in Los Gatos. Unlike other nearby seismographs that were overwhelmed by the large magnitude mainshock, the four accelerometers captured a useful record of the main event and more than half an hour of the early aftershock activity.
The June 27, 1988, shock occurred with a maximum intensity of VI. Its effects included broken windows in Los Gatos, and other light damage in Holy City, where increased flow was observed at a water well. Farther away from the Santa Cruz Mountains, pieces of concrete fell from a parking structure at the Sunnyvale Town Center, a two-level shopping mall in Santa Clara County. More moderate damage resulted from the August 8, 1989, shock when chimneys were toppled in Cupertino, Los Gatos, and Redwood Estates. Other damage included cracked walls and foundations and broken underground pipes. At the office of the Los Gatos City Manager, a window that was cracked had also been broken in the earlier shock. Also in Los Gatos, one man died when he fell or jumped through a window and fell five stories.

Earthquake

The Loma Prieta earthquake was named for Loma Prieta Peak in the Santa Cruz Mountains, which lies just to the east of the mainshock epicenter. The duration of the heaviest shaking in the Santa Cruz Mountains was about 15 seconds, but strong ground motion recordings revealed that the duration of shaking was not uniform throughout the affected area. At sites with rocky terrain, the duration was shorter and the shaking was much less intense, and at locations with unconsolidated soil the intensity of the shaking was more severe and lasted longer. The strong motion records also allowed for the causative fault to be determined – the rupture was related to the San Andreas Fault System.
While a Mercalli Intensity of VIII covered a large swath of territory relatively close to the epicenter farther to the north, portions of San Francisco were assessed at intensity IX. At more than distant, the San Francisco Bay Area recorded peak horizontal accelerations that were as high as 0.26g, and close to the epicenter they peaked at more than 0.6g. In a general way, the location of aftershocks of the event delineated the extent of the faulting, which extended about in length. Because the rupture took place bilaterally, the duration of strong shaking was about half of what it would have been had it ruptured in one direction only. The duration of a typical M6.9 shock with a comparable rupture length would have been about twice as long.

Characteristics

, a seismologist with Stanford University, made several distinctions regarding the 1906 and 1989 events. Near Loma Prieta, the 1906 rupture was more shallow, had more strike-slip, and occurred on a fault that was near vertical. The 1989 event's oblique-slip rupture was at 10 km and below on a fault plane that dipped 70° to the southwest. Because much of the slip in 1989 occurred at depth and the rupture propagated up dip, Beroza proposed that the overlying San Andreas Fault actually inhibited further rupture and also maintains that the occurrence of an event at the location that was forecast by the WGCEP in 1988 was coincidental.
The contrasting characteristics of the 1906 and 1989 events were examined by seismologists Hiroo Kanamori and Kenji Satake. The significant amount of vertical displacement in 1989 was a key aspect to consider because a long-term sequence of 1989-type events normally result in regions with high topographic relief, which is not seen in the Santa Cruz Mountains. Three scenarios were presented that might explain this disparity. The first is that the geometry of the San Andreas Fault goes through a transition every several thousand years. Secondly, slip type could vary from event to event. And lastly, the 1989 event did not occur on the San Andreas Fault.

Ground effects

While the effects of a four-year drought limited the potential of landslides, the steep terrain near the epicenter was prone to movement, and up to 4,000 landslides may have occurred during the event. The majority of landslides occurred to the southwest of the epicenter, especially along road cuts in the Santa Cruz Mountains and in the Summit Road area, but also along the bluffs of the Pacific Coast, and as far north as the Marin Peninsula. Highway 17 was blocked for several weeks due to a large slide and one person was killed by a rockfall along the coast. Other areas with certain soil conditions were susceptible to site amplification due to the effects of liquefaction, especially near the shore of San Francisco Bay and to the west of the epicenter near rivers and other bodies of water. Minor lateral spreading was also seen along the shores of San Francisco Bay and to the south near Monterey Bay. Other ground effects included downslope movement, slumps, and ground cracks.

Injuries and fatalities

Fifty-seven of the deaths were directly caused by the earthquake; six further fatalities were ruled to have been caused indirectly. In addition, there were 3,757 injuries as a result of the earthquake, 400 of which were serious. The highest number of deaths, 42, occurred in Oakland because of the collapse of the Cypress Street Viaduct on the Nimitz Freeway, where the upper level of a double-deck portion of the freeway collapsed, crushing the cars on the lower level, and causing crashes on the upper level. One section of the San Francisco–Oakland Bay Bridge also collapsed, leading to a single fatality, Anamafi Moala, a 23-year-old woman. Three people were killed in the collapse of buildings along the Pacific Garden Mall in Santa Cruz, and five people were killed in the collapse of a brick wall on Bluxome Street in San Francisco.
When the earthquake hit, the third game of the 1989 World Series baseball championship was about to begin. Because of the unusual circumstance that both of the World Series teams were based in the affected area, many people had left work early or were staying late to participate in after work group viewings and parties. As a consequence the normally crowded freeways contained unusually light traffic. If traffic had been normal for a Tuesday rush hour, injuries and deaths would certainly have been higher. The initial media reports failed to take into account the game's effect on traffic and initially estimated the death toll at 300, a number that was corrected to 63 in the days after the earthquake.

Magnetic disturbances

After the earthquake occurred, a group led by Antony C. Fraser-Smith of Stanford University reported that the event was preceded by disturbances in background magnetic field noise as measured by a sensor placed in Corralitos, about from the epicenter. From October 5, they reported that a substantial increase in noise was measured in the frequency range 0.01–10 Hz. The measurement instrument was a single-axis search-coil magnetometer that was being used for low frequency research. Precursor increases of noise apparently started a few days before the earthquake, with noise in the range 0.01–0.5 Hz rising to exceptionally high levels about three hours before the earthquake. The Fraser-Smith et al. report remains one of the most frequently cited claims of a specific earthquake precursor, more recent studies have cast doubt on the connection, attributing the Corralitos signals to either unrelated magnetic disturbance or, even more simply, to sensor-system malfunction.

Damage

The earthquake caused severe damage in some very specific locations in the Bay Area, most notably on unstable soil in San Francisco and Oakland. Oakland City Hall was evacuated after the earthquake until a US$80 million seismic retrofit and hazard abatement work was completed in 1995. Many other communities sustained severe damage throughout the region located in Alameda, San Mateo, Santa Clara, San Benito, Santa Cruz and Monterey counties. Major property damage in San Francisco's Marina District from the epicenter resulted from liquefaction of soil used to create waterfront land. Other effects included sand volcanoes, landslides and ground ruptures. Some 12,000 homes and 2,600 businesses were damaged.
In Santa Cruz, close to the epicenter, 40 buildings collapsed, killing six people. At the Santa Cruz Beach Boardwalk, the Plunge Building was significantly damaged. Liquefaction also caused damage in the Watsonville area. For example, sand volcanoes formed in a field near Pajaro as well as in a strawberry field. The Ford's department store in Watsonville experienced significant damage, including a crack down the front of the building. Many homes were dislodged if they were not bolted to their foundations. There were structural failures of twin bridges across Struve Slough near Watsonville. In Moss Landing, the liquefaction destroyed the causeway that carried the Moss Beach access road across a tidewater basin, damaged the approach and abutment of the bridge linking Moss Landing spit to the mainland and cracked the paved road on Paul's Island. In the Old Town historical district of the city of Salinas, unreinforced masonry buildings were partially destroyed.
An estimated 1.4 million people lost power following the quake, mainly due to damaged electrical substations. Many San Francisco radio and television stations were temporarily knocked off the air. KRON-TV was off the air for about half an hour, while KGO-AM was off the air for about 40 minutes. About an hour and 40 minutes after the quake, Fox affiliate KTVU resumed broadcasting, with their news anchors, Dennis Richmond and Elaine Corral reporting from the station's parking lot. KCBS-AM switched immediately to backup power and managed to stay on air despite a subsequent generator failure. KCBS later won a Peabody Award for their news coverage, as did KGO-TV.
Power was restored to most of San Francisco by midnight, and all but 12,000 customers had their power restored within two days.
The quake caused an estimated $6 billion in property damage, becoming one of the most expensive natural disasters in U.S. history at the time. Private donations poured into aid relief efforts and on October 26, President George H. W. Bush signed a $1.1 billion earthquake relief package for California.

Marina District

Four people died in San Francisco's Marina District, four buildings were destroyed by fire, and seven buildings collapsed. Another 63 damaged structures were judged too dangerous to live in. Among the four deaths, one family lost their infant son who choked on dust while trapped for an hour inside their collapsed apartment.
The Marina district was built on a landfill made of a mixture of sand, dirt, rubble, waste and other materials containing a high percentage of groundwater. Some of the fill was rubble dumped into San Francisco Bay after the 1906 San Francisco earthquake, but most was sand and debris laid down in preparation for the 1915 Panama-Pacific International Exposition, a celebration of San Francisco's ability to rebound after its catastrophe in 1906. After the Exposition, apartment buildings were erected on the landfill. In the 1989 earthquake, the water-saturated unconsolidated mud, sand and rubble suffered liquefaction, and the earthquake's vertical shock waves rippled the ground more severely.
At the intersection of Beach and Divisadero Streets in San Francisco, a natural gas main rupture caused a major structure fire. The San Francisco Fire Department selected civilians to help run fire hoses from a distance because the nearby hydrant system failed. Since the bay was only two blocks from the burning buildings, water from the bay was pumped by the fireboat Phoenix, to engines on the shore, and from there sprayed on the fire. The apartment structures that collapsed were older buildings that included ground floor garages, which engineers refer to as a soft story building.

Santa Cruz and Monterey counties

In Santa Cruz, the Pacific Garden Mall was severely damaged, with falling debris killing three people, half of the six earthquake deaths in Santa Cruz and Monterey Counties. Some 31 buildings were damaged enough to warrant demolition, seven of which had been listed in the Santa Cruz Historic Building Survey. The four oldest were built in 1894, the five oldest withstood the 1906 San Francisco earthquake.
Immediately, a number of civilians began to free victims from the rubble of Ford's Department Store and the Santa Cruz Coffee Roasting Company – both buildings had collapsed inward on customers and employees alike. Two police officers crawled through voids in the debris, found one victim alive and another dead inside the coffee house. Santa Cruz beach lifeguards assisted in moving the victims. Police dogs were brought in to help locate other victims. A woman was found dead inside Ford's. The civilians who were helpful initially, were soon viewed by police and fire officials as a hindrance to operations, with frantic coworkers and friends of a coffee house employee thought to be trapped under the rubble continuing their efforts in the dark. Police arrested those who refused to stop searching. This became a political issue in the following days. The body of a young woman coffee worker was found under a collapsed wall late the next day.
During the first few days following the quake, electric power to most Santa Cruz County subscribers was out, and some areas had no water. Limited phone service remained online, providing a crucial link to rescue workers. Widespread search operations were organized to find possible victims inside the remains of fallen structures. As many as six teams of dogs and their handlers were at work identifying the large number of damaged buildings that held no victims.
The quake claimed one life in Watsonville: a driver who collided with panicked horses after they escaped their collapsed corral. In other Santa Cruz and Monterey county locations such as Boulder Creek and Moss Landing, a number of structures were damaged, with some knocked off of their foundations. Many residents slept outside their homes out of concern for further damage from aftershocks, of which there were 51 with magnitudes greater than 3.0 in the following 24 hours, and 16 more the second day. The earthquake damaged several historic buildings in the Old Town district of Salinas, and some were later demolished.
Damage to the Salinas River rail bridge and subsequent repairs led to reduced traffic on the Monterey Branch Line, which contributed to the discontinuance of freight rail services in western Monterey County.

San Francisco–Oakland Bay Bridge

The San Francisco–Oakland Bay Bridge suffered severe damage, as a section of the upper deck on the eastern cantilever side fell onto the deck below. The quake caused the Oakland side of the bridge to shift to the east, and caused the bolts of one section to shear off, sending the section of roadbed crashing down like a trapdoor. Traffic on both decks came to a halt, blocked by the section of roadbed. Police began unsnarling the traffic jam, telling drivers to turn their cars around and drive back the way they had come. Eastbound drivers stuck on the lower deck between the collapse and Yerba Buena Island were routed up to the upper deck and westward back to San Francisco. A miscommunication made by emergency workers at Yerba Buena Island routed some drivers the wrong way; they were directed to the upper deck where they drove eastward toward the collapse site. One of these drivers did not see the open gap in time; the car plunged over the edge and smashed onto the collapsed roadbed. The driver, Anamafi Moala, died, and the passenger, her brother, was seriously injured. Caltrans removed and replaced the collapsed section, and re-opened the bridge on November 18.

Oakland and Interstate 880/Cypress Viaduct

The worst disaster of the earthquake was the collapse of the double-deck Cypress Street Viaduct of Interstate 880 in West Oakland. The failure of a section of the viaduct, also known as the "Cypress Structure" and the "Cypress Freeway", killed 42 and injured many more.
Built in the late 1950s and opened to traffic in 1957, the Cypress Street Viaduct, a stretch of Interstate 880, was a double-deck freeway section made of nonductile reinforced concrete that was constructed above and astride Cypress Street in Oakland. Roughly half of the land the Cypress Viaduct was built on was filled marshland, and half was somewhat more stable alluvium. Because of new highway structure design guidelines – the requirement of ductile construction elements – instituted following the 1971 San Fernando earthquake, a limited degree of earthquake reinforcement was retrofitted to the Cypress Viaduct in 1977. The added elements were longitudinal restraints at transverse expansion joints in the box girder spans, but no studies were made of possible failure modes specific to the Cypress Viaduct. Caltrans has since received widespread backlash for not thoroughly studying the structure. When the earthquake hit, the shaking was amplified on the former marshland, and soil liquefaction occurred.
When the earthquake struck, the freeway buckled and twisted before the support columns failed and the upper deck fell on the lower deck. 42 people were crushed to death in their cars. Cars on the upper deck were tossed around violently, some of them flipped sideways and some of them were left dangling at the edge of the freeway. Nearby residents and factory workers came to the rescue, climbing onto the wreckage with ladders and forklifts and pulling trapped people out of their cars from under a four-foot gap in some sections. 60 members of Oakland's Public Works Agency left the nearby city yard and joined rescue efforts. Employees from Pacific Pipe drove heavy lift equipment to the scene and started using it to raise sections of fallen freeway enough to allow further rescue. Local workers continued their volunteer operation nonstop until October 21, 1989, when they were forced to pause as U.S. President George H. W. Bush and California Governor George Deukmejian viewed the damage. On October 21, survivor Buck Helm was freed from the wreckage, having spent 90 hours trapped in his car. Dubbed "Lucky Buck" by the local radio, Helm lived for another 29 days on life support, but then died of respiratory failure at the age of 57.
Although the freeway reopened in stages between 1997 and 1999, it was not fully rebuilt until 2001 due to safety and reinforcement concerns. In the meantime, traffic was detoured through nearby Interstate 980, causing increased congestion. Instead of rebuilding Interstate 880 over the same ground, Caltrans rerouted the freeway farther west around the outskirts of West Oakland to provide better access to the Port of Oakland and the San Francisco–Oakland Bay Bridge, and to meet community desires to keep the freeway from cutting through residential areas. Street-level Mandela Parkway now occupies the previous roadbed of the Cypress structure.

Effects on transportation

Immediately after the earthquake, Bay Area airports were closed so officials could conduct visual inspection and damage assessment procedures. San Jose International Airport, Oakland International Airport and San Francisco International Airport all opened the next morning. Large cracks in Oakland's runway and taxiway reduced the usable length to two-thirds normal, and damage to the dike required quick remediation to avoid flooding the runway with water from the bay. Oakland Airport repair costs were assessed at $30 million.
San Francisco Municipal Railway lost all power to electric transit systems when the quake hit, but otherwise suffered little damage and no injuries to operators or riders. Cable cars and electric trains and buses were stalled in place – half of Muni's transport capability was lost for 12 hours. Muni relied on diesel buses to continue abbreviated service until electric power was restored later that night, and electric units could be inspected and readied for service on the morning of October 18. After 78 hours, 96 percent of Muni services were back in operation, including the cable cars. Amtrak intercity rail service into Oakland from the California Zephyr continued, but the Coast Starlight was temporarily suspended north of Salinas because of damage to the Southern Pacific's Coast Line.
The earthquake changed the Bay Area's automobile transportation landscape. Not only did the quake force seismic retrofitting of all Bay Area bridges, it caused enough damage that some parts of the region's freeway system had to be demolished. Damage to the region's transportation system was estimated at $1.8 billion.
The 1989 World Series featured the Oakland Athletics and the San Francisco Giants in the first cross-town World Series since 1956. Game 3 of the series was scheduled to begin at San Francisco's Candlestick Park on October 17 at 5:35 PDT, and American TV network ABC began its pre-game show at 5:00 PDT. When the quake struck at approximately 5:04 PDT, sportscaster Tim McCarver was narrating taped highlights of Game 2, which had been played two days prior across the Bay Bridge in Oakland. Television viewers saw the video signal begin to break up, heard McCarver repeat a sentence as the shaking distracted him, and heard McCarver's colleague Al Michaels exclaim, "I'll tell you what, we're having an earth –." At that moment, the signal from Candlestick Park was lost.
The network put up a green ABC Sports “World Series” technical difficulties telop graphic while it scrambled to repair the video feed, but audio from the stadium was restored after a few seconds via a telephone link. Michaels cracked, "Well, folks, that's the greatest open in the history of television, bar none!" accompanied by the combined screams of excitement and panic from fans who had no idea of the devastation elsewhere. ABC then switched to episodes of Roseanne and The Wonder Years, which was on standby for a rain delay situation, while attempting to restore electricity to its remote equipment. Local San Francisco ABC affiliate KGO-TV lost power for almost 15 minutes upon the start of the earthquake, before beginning its coverage with anchor Cheryl Jennings.
With anchorman Ted Koppel in position in Washington, D.C., ABC News began continuous coverage of the quake at about 5:40 PDT, at the same time as CBS News; Al Michaels, in the process, became a de facto on-site reporter for ABC. NBC News also began continuous coverage, with Tom Brokaw, about an hour later. The Goodyear blimp had already been overhead to cover the baseball game, and ABC used it to capture images of damage to the Bay Bridge and other locations.
Inside Candlestick Park, fewer than half of the more than 62,000 fans had reached their seats by the time of the quake, and the load on the structure of the stadium was lower than maximum. There had also been a seismic-strengthening project previously completed on the upper deck concrete windscreen that may have prevented large numbers of injuries in the event of serious damage or even a catastrophic collapse. Fans reported that the stadium moved in an articulated manner as the earthquake wave passed through it, that the light standards swayed by many feet, and that the concrete upper deck windscreen moved in a wave-like manner over a distance of several feet. When electrical power to the stadium was lost, someone drove a police car onto the field, where an officer used the car's public address system to advise that the game had been postponed. After the shaking subsided, many of the players on both teams immediately searched for, and gathered, family and friends from the stands before evacuating the facility.
The October 18, 1989 edition of NBC's Today that covered the earthquake ran until noon Eastern Time. Bryant Gumbel, Jane Pauley and Deborah Norville anchored from Chicago, with reports done by Bob Jamieson and Don Oliver in San Francisco, and George Lewis in Oakland. Jim Miklasewski and Bob Hager covered disaster response from Washington. NBC Sports commentators Bob Costas and Jimmy Cefalo discussed the effect the temblor would have on the 1989 World Series.
The World Series was delayed while the Bay Area started the recovery process. While the teams' stadiums had suffered only minor damage, it took several days for power and transmission links at Candlestick Park to be repaired. After ten days, Game 3 was held in San Francisco on October 27 and Game 4 the following afternoon as the A's swept the Giants, four games to none.
It is likely that the World Series game saved many lives, as Bay Area residents who would have normally been on the freeways were at home ready to watch the game when the earthquake hit. It is a rough estimate that thousands of people may have otherwise been on the Cypress Structure during the 5:00 p.m. rush-hour, as the structure was said to have carried 195,000 vehicles a day.

In popular culture

The earthquake is featured in films and television shows:
Some earthquake and disaster-themed television documentaries feature the earthquake include: