loma prieta quake 89

The October 17, 1989 Loma Prieta Earthquake - October 1989

Loma Prieta FactsDate of the quake: October 17, 1989Time: 5:04pmMagnitude: 7.1Deaths: 68Injuries: 3,757Homes Damaged: 23,408Homes Destroyed: 1,018Businesses Damaged: 3,530Businesses Destroyed: 366Estimated Dollar Loss: $6 billion to $7 billionAftershocks: More that 7,000 between magnitudes

1.0 and 5.4 by Oct. 1, 1990

THE OCTOBER 17, 1989 LOMA PRIETA EARTHQUAKE - OCTOBER 1989

Introduction

At 5:04 P.M., Tuesday, October 17, 1989, as over 62,000 fans filled Candlestick Park for the third game of the World Series and the San Francisco Bay Area commute moved into its heaviest flow, a Richter magnitude 7.1 earthquake struck. It was an emergency planner's worst-case scenario. The 20-second earthquake was centred about 60 miles south of San Francisco, and was felt as far away as San Diego and western Nevada. Scientists had predicted an earthquake would hit on this section of the San Andreas Fault and considered it one of the Bay Area's most dangerous stretches of the fault.Among the most catastrophic seismic-induced events were the collapse of the elevated Cypress Street section of Interstate 880 in Oakland, the collapse of a section of the roadbed of the San Francisco-Oakland Bay Bridge, multiple building collapses in San Francisco's Marina district, and the collapse of several structures in the town of Santa Cruz at the Pacific Garden Mall and in other areas around the epicentral region. It is possible that surface waves, which are a slower rolling motion and the last type of wave to arrive, stimulated heavy motion in soft, water-saturated soils around the Bay's margin, resulting in much of the dramatic damage in parts of San Francisco and Oakland. This was somewhat like the amplified earthquake waves that destroyed sections of Mexico City hundreds of miles from that earthquake's epicenter.Damage and business interruption estimates reached as high as $10 billion, with direct damage estimated at $6.8 billion. $2 billion of that amount is for San Francisco alone and Santa Cruz officials estimated that damage to that county will top $1 billion. Areas outside of Santa Cruz, including the towns of Watsonville, Hollister, and Los Gatos, also suffered heavy damage. President Bush declared a disaster area for the seven hardest-hit counties, from Monterey and San Benito in the south to Marin and Solano in the north.Over 62 people died, a remarkably low number given the time and size of the earthquake. Most casualties were caused by the collapse of the Cypress Street section. At least 3,700 people were reported injured and over 12,000 were displaced. Over 18,000 homes were damaged and 963 were destroyed. Over 2,500 other buildings were damaged and 147 were destroyed.


Seismicity and GeologyThe epicenter of the Richter magnitude 7.1 earthquake was located about 10 miles northeast of Santa Cruz along a segment of the San Andreas Fault, near Loma Prieta in the Santa Cruz Mountains. The focal depth has been placed at 11 miles. This is unusually deep, as typical California earthquake focal depths are 4 to 6 miles.

AftershocksA magnitude 5.2 aftershock occurred approximately 2.5 minutes after the main shock, and thousands of aftershocks have been recorded since. In the week following the earthquake, a total of 300 of these have been magnitude 2.5 or greater and 20 have been greater than 4.0. The aftershock zone stretches across 25 miles from just north of Los Gatos near Highway 17, to south of Watsonville near Highway 101. This zone corresponds with the areas of greatest damage. The zone ranges from about 2 to 11 miles in depth and is believed to be the length of rupture associated with the main shock. Surface EffectsSurface displacements with offsets of up to 3 or 4 feet along a zone about 20 miles long would normally be expected to accompany an earthquake of this magnitude. Instead many cracks have been found over several discontinuous and indistinct zones. There are several possible explanations for this lack of clear surface expression. The earthquake was unusually deep, making it difficult for the bedrock rupture to propagate to the ground surface. The combination of rugged topography, thick soil, and forest cover could also make surface breaks less distinct. The State Commission report of the 1906 earthquake described

very similar surface rupture characteristics along the Santa Cruz Mountains portion of the San Andreas Fault.Historical Seismicity The San Andreas Fault trends northwesterly and extends more than 800 miles from the Gulf of

California to Cape Mendocino north of San Francisco. It has been the source of many large earthquakes including an 1838 earthquake located on the peninsula south of San Francisco (magnitude in excess of 7.0), an 1865 earthquake northeast of Santa Cruz, and the famous 1906 San Francisco Earthquake with magnitude 8.3.

The Loma Prieta Earthquake essentially repeats the 1865 event and is the first major rupture along the San Andreas since 1906.

The length of fault rupture generally extended from the southern end of the 1906 break and thus relieved strain, which had accumulated since before that time.

The lack of activity on this section of the fault, as well as the occurrence of several magnitude 5.0+ events in the area over the last two years, had prompted geologists to forecast this event.

The earthquake caused thousands of landslides along steep slopes, from hills in the epicentral area to at least as far north as the Pacific Coast just south of San Francisco.

Several residential developments in the Santa Cruz Mountains were badly damaged by these slides.

On Highway 17 two lanes were blocked west of the summit by a large slide. Large fissures opened in roadways throughout the Bay Area due to settlement and/or lurching.

The October 17, 1989 Loma Prieta Earthquake - October 1989 This rupture of a 30-mile-long segment of the San Andreas Fault has not altered the assessment

that there is a 50% chance for one or more magnitude 7.0 earthquakes in the San Francisco Bay Area in the next 30 years.

The probability of a repeat of the 1906 magnitude 8.3 earthquake is still significant.

Damage to Buildings The Loma Prieta Earthquake and its subsequent aftershocks resulted in widespread damage to a

variety of commercial structures. A large geographical area was affected, as is typical for an earthquake of this magnitude.

In total, building structures experienced damage over an area of approximately 3,000 square miles.

Although damage was widespread, it was also quite sporadic. As would be expected, areas closest to the epicenter experienced the most concentrated damage. Farther away, heavy damage was generally limited to buildings of very poor construction founded on soft soils that failed or amplified the earthquake ground motions. This is similar to the effects noted in the 1985 Mexico City Earthquake.

Earthquake effects also tended to be highly directional. Most damage occurred within a narrow band that extends northwest to southeast, approximately paralleling the San Andreas Fault. Thus many communities along the margins of San Francisco Bay escaped serious damage.

Unreinforced Masonry Buildings As has been observed in past California earthquakes, the most concentrated and severe damage

to building structures occurred in unreinforced masonry (URM) bearing-wall buildings. URM buildings, constructed of wood-frame roof and floor systems supported by thick

unreinforced brick walls, were commonly constructed throughout California until the 1930s, when new building regulations taking into consideration the need to withstand earthquakes prevented further buildings of this style.

As a result, these older URM buildings are typically found in the crowded central business districts of older California cities.

The remote location of the epicentre of this earthquake allowed the San Francisco Bay Area to survive with relatively few instances of structural collapse. Except for buildings near the epicenter, most cases of severe damage occurred in older buildings with little ability to withstand earthquakes and in areas of extremely weak soils.

The fact that many inadequate structures in the region experienced little damage indicates that ground motion in most areas was not severe.

Even so, most businesses experienced at least a week's business interruption and some capital loss. Many businesses must relocate to new facilities until their buildings are repaired or replaced.

In future stronger shocks or in earthquakes located closer to the major population centers, much more extensive damage and commercial loss are likely.

A major and encompassing effect of the Loma Prieta Earthquake is to transportation. Several major highways, overpasses, and ground thoroughfares were damaged and rendered useless, some for only a short time, others for as much as a few years.

Fire Following Earthquake San Francisco had 22 structural fires and over 500 reported incidents during the seven hours

from the time the earthquake struck until midnight. During this period over 300 off-duty firemen responded to a general recall, approximately

doubling the available fire-fighting personnel.

Conclusion Little of the damage caused in this earthquake was unexpected. Typically, unreinforced masonry structures, older tilt-up buildings, and poorly designed wood-

frame houses in the epicenter region were heavily damaged, while damage farther away from the epicenter was to high-risk structures on soft and saturated soils.

The significance of this earthquake is the warning it delivered of the dangers faced by people, governments, and businesses in seismically active areas. With such attention, it is hoped that preparedness efforts will be similarly stimulated and will continue at a high level.

There is the concern, however, that many building owners in the Bay Area may conclude that the survival of their buildings in this somewhat distant, but strong earthquake certifies their structure

The October 17, 1989 Loma Prieta Earthquake - October 1989as earthquake safe. In many cases, nothing could be further from the truth. For example, thousands of unreinforced masonry buildings, pre-1975 tilt-ups, and poorly engineered structures survived the earthquake with no apparent damage. A larger quake or one that is closer to San Francisco, San Jose, or Oakland would have produced dramatically different results.

It is interesting to note that in October 1865, a strong earthquake struck virtually the same section of the San Andreas Fault (in the Santa Cruz Mountains) with similar effects to the same areas. This quake was then followed three years later, in October 1868, by one of the largest earthquakes to occur on the Hayward Fault near Oakland. Finally, the "big one" struck in 1906 with a magnitude of 8.3 on the San Andreas Fault, devastating Northern California and destroying much of San Francisco. It should be understood that the recent earthquake did not lower the probability of another major earthquake or the "big one" occurring in the next 30 years. Based on past history (1865, 1868) there is speculation that there actually may be greater seismic activity in the future.

Some of our observations and conclusions following this earthquake include: Outside the epicentral region, damage was sporadic and occurred mainly to older structures

with little seismic capacity and to structures with known vulnerabilities in areas of extremely weak soils.

Within the epicentral region, there were few surprises. Damage was widespread, particularly in Hollister, Santa Cruz, Watsonville, and Los Gatos. Most of the buildings affected were unreinforced masonry, poorly designed or constructed wood-frame houses, and structures designed to antiquated building codes.

Many of the buildings that were damaged in downtown San Francisco and Oakland were older, mid-rise structures, leading us to believe that frequency content of the ground motion coincided with the building frequency (period), resulting in amplified motions.

Many buildings outside the epicentral region that appear to be seismically inadequate, including unreinforced masonry buildings, pre-1975 tilt-ups, poorly designed wood-frame buildings, and buildings with known vulnerabilities appear to have come through the earthquake unscathed or with minor damage, leading us to believe that the ground motions in these areas were fairly low. These buildings should, however, be carefully reviewed and monitored for structural damage. These structures may have been weakened and aftershocks or ground compaction may cause significant damage. Specific areas include San Francisco's Chinatown and Tenderloin districts, San Jose, and Santa Clara Valley's high-tech industries.

In general, the newer buildings performed well; however, this earthquake did not provide the severe ground motions to adequately test them, outside the general epicentral area.

Many corporations have embarked on earthquake risk reduction programs and have strengthened some or all of their buildings. This earthquake illustrated the value of these programs.

This earthquake was a major disaster. While engineers and seismologists had expected such an event, society in general was not prepared for it. As in most disasters, however, people responded well helping put out fires, assisting the injured, and helping each other.

The primary cause of death was the collapse of the Cypress overpass. Excluding this tragedy, the number of deaths was very low considering the earthquake's magnitude, the population of the area affected, and the time at which it occurred.

The disaster potential in California is at least 10 times greater today than in the 1906 San Francisco Earthquake because of the population and economic development.

Although this earthquake is considered a major disaster, similar risks exist in such areas as Puget Sound, the region centered on New Madrid between St. Louis and Memphis, the region around Charleston, the Salt Lake City region, and others. Given the regional similarities in earthquake potential, building types, and the absence of earthquake preparedness programs, the possibility for an earthquake with consequences of comparable or much greater proportions in other parts of the country is realistic. While such disasters are feared by the U.S. earthquake engineering community, society in general has not accepted the fact that the risk exists.

Many buildings, structures, and infrastructures that have been designed and constructed with little resistance to earthquake motions are still in use in California and other seismically active regions of the United States. Private corporations and governments at all levels in these areas must make important political and economic decisions on reducing the risk from such structures in order to prevent the kinds of serious damage observed in this earthquake. Whether through retrofitting or new design and construction, resolution of the problem can be cost-effective and practical, if a long-range, consistent program is implemented.

This earthquake demonstrated the need to increase education, awareness, and preparedness at all levels of government and private industry. Mitigation programs should be accelerated to

The October 17, 1989 Loma Prieta Earthquake - October 1989include preparedness planning, response and recovery planning, engineering vulnerability studies, and retrofit of hazardous buildings and structures.

Fire fighters, police officers, and other emergency personnel responded extraordinarily well to the catastrophe. The only major San Francisco fire was in the Marina district, but when combined with the 34 other fires and over 500 responses, the department was taxed to its full capabilities. The Marina fire was difficult to contain because mains supplying water to the district burst during the earthquake. If more fires had been ignited by the earthquake, it would have been difficult for the fire department to contain them.

The local media also deserves special commendation for the calm, informative, and responsible reporting immediately following the earthquake and in the days after. By diligent work and measured reactions, the local media kept the public informed of both the breaking stories and the underlying causes of the disasters.

Power and industrial facilities generally fared well during the earthquake with very little permanent damage. Several power-generating or power transmission facilities lost power during the earthquake. The earthquake-induced failures that caused these power outages are typical of those that have been observed in most major earthquakes.

Despite the various problems, officials and experts are quick to point out that this earthquake could have been far worse. Engineers continue to improve building codes, particularly in California, to incorporate improved seismic design. However, there are still many seismically vulnerable structures that need to be dealt with. This is particularly true outside of California. In terms of response, years of planning and drilling enabled police and fire departments to respond quickly and efficiently to what could have been a safety planner's ultimate nightmare--a major earthquake during rush hour and the World Series.


An automobile lies crushed under the third story of this apartment building in the Marina District. The ground levels are no longer visible because of structural failure and sinking due to liquefaction.

Entrance and garage level of a Beach Street apartment complex in danger of collapse, Marina District.

Aerial view of collapsed sections of the Cypress viaduct of Interstate Highway 880.

Bent reinforcement bars in failed support column, Cypress viaduct.

Aerial view of large slides north of Fort Funston. House moved laterally off cement foundation.

Structural failure of twin bridges carrying Highway 1 across Struve Slough, near Watsonville.

Earthquake Probabilities for the San Francisco Bay AreaOn the basis of research conducted since the 1989 Loma Prieta earthquake, U.S. Geological Survey (USGS) and other scientists conclude that there is a 70% probability of at least one magnitude 6.7 or greater quake, capable of causing widespread damage, striking the San Francisco Bay region before 2030. Major quakes may occur in any part of this rapidly growing region. This emphasizes the urgency for all communities in the Bay region to continue preparing for earthquakes.


Major Quake Likely to Strike San Francisco Bay Region Between 2000 and

2030On the basis of research conducted since the 1989 Loma Prieta earthquake, U.S. Geological Survey (USGS) and other scientists conclude that there is a 70% probability of at least one magnitude 6.7 or greater quake, capable of causing widespread damage, striking the San Francisco Bay region before 2030. Major quakes may occur in any part of this rapidly growing region. This emphasizes the urgency for all communities in the Bay region to continue preparing for earthquakes.

THE NEXT BIG EARTHQUAKE IN THE SAN FRANCISCO BAY AREA MAY COME SOONER THAN YOU THINKA MAJOR EARTHQUAKE IS HIGHLY LIKELY SOON Many of us breathed a little easier after October 17, 1989. The Loma Prieta earthquake, 7.1 on the Richter scale, meant that the big one, talked about for decades, had finally happened. And, bad as it was, we had survived. There are two things wrong with that. First, Loma Prieta was not the big one. It was a moderately big one, certainly destructive to some

parts of the Bay Area, but nowhere near the size of the great San Francisco earthquake of 1906. Second, having an earthquake like Loma Prieta has little to do with the likelihood of having

another one on a different fault, somewhere else in the area. The inevitability of a damaging earthquake still confronts everybody in the Bay Area, and we still risk substantial damage. A new study, released in July 1990 by the United States Geological Survey, says that there is a 67 percent chance of another earthquake the size of Loma Prieta during the next 30 years and that the quake could strike at any time, including today. In other words, scientists think that a magnitude 7 or larger earthquake is now twice as likely to

happen as not to happen. This is a substantial increase, since in 1988, scientists thought the chance for such an earthquake was 50 percent (just as likely to occur as not to occur) within 30 years.

The new report also says that the next one will most likely strike farther north than Loma Prieta, somewhere between San Jose and Santa Rosa on either side of the Bay. The epicenter of the October 1989 quake was in a sparsely pop-ulated area. The next one, according to the study, will likely be centered in a more populated area. During the

Loma Prieta earthquake, shaking was so severe in the Santa Cruz Mountains that a van overturned, treetops snapped off, and many people were thrown to the ground.

Because the next one is expected to strike closer to an urban area, it will cause much more damage.

Fortunately, there is something we can do about it. By taking actions, such as those described in this booklet, we can drastically reduce the losses and we can make the Bay Area a safer place to live. Earthquake damage is particularly great in certain locations and in certain buildings. Most locations and most modern buildings are relatively safe. By identifying the greatest hazards, we can set priorities for using our limited resources most effectively to reduce them. The choice is ours.


ARE YOU PREPARED?1.Practice duck, cover, and hold drills at home with your family and at work.

Injuries and deaths during earthquakes are caused by falling objects and collapsing structures. Knowing how to protect yourself when the shaking starts may save your life.

Duck under a strong table or desk. Cover your head and face to protect them

from broken glass and falling objects. Hold onto the table or desk and be prepared

to move with it. Hold your position until the shaking stops.

Caption: (A) Duck; (B) Cover; (C) Hold;

Do not run outside during the shaking or use the stairways or elevators. Many people are killed just outside of buildings by falling bricks and other debris.

If you are driving when the earthquake strikes, move to the shoulder of the highway and away from bridges, overpasses, power lines, and large buildings as quickly as is safe. Stay in your car and wait for the shaking to stop.

If you are riding BART (Bay Area Rapid Transit), the train typically will stop. Remain calm and follow instructions from BART staff members who have been trained to handle

earthquake emergencies.


REPLACING OAKLAND’S CYPRESS FREEWAY also called the Nimitz Freeway

Forty-two people were killed in the collapse of the double-decked Cypress Freeway in Oakland, Calif., on Oct. 17, 1989, when the Loma Prieta Earthquake rocked the San Francisco Bay Area.

The quake measured 7.1 on the Richter scale, and although its epicenter was roughly 100 kilometers from Oakland, the damage was extensive throughout the Bay Area.

In a matter of seconds, more than 100 people were killed and 3,000 were injured in the third most lethal earthquake in U.S. history.

On the TV coverage, none of the scenes were more

dramatic or more tragic than the collapse of the Cypress Freeway, Interstate 880. In a report for Time magazine, Oct. 30, 1989, Ed Magnuson described it this way:

“To the north in Oakland, auto mechanic Richard Reynolds glanced at the traffic on the double-decker I-880 freeway across the street and urged a friend not to drive to night school until after the rush hour. Minutes later, Reynolds felt ‘a ripple.’ Then a neighbour screamed a warning. He ran out of his shop to find ‘the whole goddam ground lifting up.’ He grabbed a telephone pole as the sidewalk buckled beneath his feet and looked up at a horrifying sight. A mile-long section of the freeway’s upper deck began to heave, then collapsed onto the lower roadway, flattening cars as if they were beer cans. ‘It just slid. It didn’t fall. It just slid,’ said Reynolds. ‘You couldn’t see nothing but dust. The people came out of the dust.’ But not many.”

A Community Stands as the Freeway Collapses

Traffic jams.The destroyed section of I-880 between 7th and 18th streets in West Oakland had to be removed. This segment of interstate was a major route for motorists travelling to and from San Francisco, Berkeley, and the South Bay. Before the earthquake, more than 160,000 vehicles used this eight-lane structure every day. With this highway out of service, the I-880 traffic shifted to the remaining parts of the Oakland freeway network, causing I-980 and portions of I-580 to become heavily congested. It took nine years and $1.2 billion dollars to rebuild the Cypress Freeway. The new

freeway is not a double-decker, and it was constructed using the latest technology and very high standards for seismic design. West Oakland residents got what they wanted, and those involved learned a lot along the way.

loma prieta quake 89

Documents