Earthquake of January 17, 1994 in California - The Northridge Earthquake
George Pararas Carayannis

Introduction

On Monday January 17, 1994, a moderate earthquake struck the densely populated San Fernando Valley, about 32 km northwest of downtown Los Angeles. Extremely strong ground motions - among the strongest ever recorded - were responsible for most of the damage. The quake's intense shaking caused extensive ground liquefaction and triggered many fires. The Northridge earthquake - as it was named - was one of the worst natural disasters in U.S. history. It was the third destructive earthquake to strike California in the last 23 years, following the 1989 Loma Prieta earthquake (magnitude 7.1) and the 1971 San Fernando (Sylmar) earthquake (magnitude 6.6).

Date and Time of Origin: 4:30 A.M., Pacific Standard Time (1230 UTC), on Monday January 17, 1994.

Epicenter: The epicenter was at 34 13.00N and 118 32.00 W, near Northridge at the northern edge of the valley approximately 100 km to the west and south of the San Andreas fault.

Magnitude: Moment magnitude (Mw) 6.7

Focal Depth: 17.5 km.

Fault Plane: The quake occurred along a deeply buried blind thrust fault that is believed to be an eastern extension of the Oak Ridge fault system.

Aftershocks: Thousands of aftershocks occurred in the following days, weeks and months. Some of these had magnitudes of 4.0 to 5.0.

Felt Reports and Intensities: The Modified Mercalli Intensities ranged from VIII to XI. Accelerations in the range of 1.0 g were recorded over a large area. Landslides and ground failures occurred as far away as 90 kilometers from the epicenter. The earthquake was felt over an area of more than 200,000 square kilometers and as far away as 400 kilometers from the epicenter.

Death Toll and Damages

Although the quake had a moment magnitude (Mw) of only 6.7, it was very damaging because it struck a well developed area within the San Fernando Valley with a population of nearly 3 million. The number of fatalities were about the same as in 1971. A total of 57 people lost their lives and more than 1,500 people were seriously injured. Fortunately, because the earthquake occurred in the early morning and on a holiday, its effects were not as bad as they could have been. Most of the buildings that collapsed were empty at the time. Furthermore, after the 1971 earthquake, many businesses had retrofitted and strengthened some of the older buildings, thus significantly reducing potential losses.

Although the quake did not kill many people, the extent of damage to buildings and structures created economic chaos for the area. Extremely strong ground motions - among the strongest ever recorded - were responsible for most of the damage. Some of the larger aftershocks with magnitudes ranging from 4.0 to 5.0 caused additional damage.

Most of the severe destruction occurred within 16 km of the epicentral area, however significant damage to structures was reported as far away as 77 km. Thousands of landslides occurred over an area of about 10,000 square kilometers, primarily in sparsely populated areas north of the valley. The landslides destroyed homes, roads, and utility lines, and obstructed streams. They also triggered dense clouds of dust, which caused an outbreak of valley fever. Although building damage was widespread throughout the Valley, Santa Monica and Sherman Oaks were two of the hardest hit communities. In Santa Monica, located directly south of the epicentral area, there was extensive damage. About 12,500 structures were moderately to severely damaged and thousands of people were left temporarily homeless. Also, utilities were significantly affected. For several days after the earthquake, more than 48,500 homes had little or no water, while 9,000 homes and businesses were without electricity and 20,000 more were without gas.

Damage to Highways - Major freeway damage occurred up to 32 km from the epicentral region. Freeways collapsed at seven sites, while 170 bridges sustained varying degrees of damage. Collapses and other severe damage forced closing portions of 11 major roads to downtown Los Angeles. The Santa Monica Freeway (Interstate 10) was badly damaged. As with the 1971 earthquake, the interchange between San Fernando and Newhall (Interstate 5 and State Route 14), sustained heavy damage. The collapse of this interchange was one of the most expensive freeway failures.

Financial Losses - In terms of financial losses and property damage, this earthquake was one of the worst natural disasters in U.S. history. Damage and economic losses were estimated to be about $20 billion.

Past Earthquakes in the Region

San Fernando valley has been struck frequently in the past by moderate to large earthquakes. The February 9, 1971 earthquake (magnitude 6.6) about 32 km to the northeast, had been the last major destructive event. However, the 1994 Northridge earthquake caused far greater damage than the 1971 event which had released most of its energy in the San Gabriel mountains.

The Faults of San Fernando Valley

The 1994 Northridge earthquake occurred beneath the northern fringes of San Fernando Valley, on a deep buried blind thrust fault that is believed to be an eastern extension of the Oak Ridge fault system. The area's biggest and best-known geologic fault, is the San Andreas fault, about 100 km to the east and north of the Valley.

There are several other faults in San Fernando Valley. The San Fernando fault transverses the valley and has two major segments. The Sylmar segment which crosses the heavily populated Sylmar - San Fernando area, and the Tujunga portion which consists of only one or two parallel breaks and extends along the foothills of the San Gabriel Mountains, the Tujunga Valley to the east of San Fernando.

The Northridge Hills Fault is considered to be the most potentially dangerous fault in the area. It extends through the populated region of the valley from the vicinity of Chatsworth to Northridge where it changes into an eastward direction to the east central valley from where it disappears beneath deep alluvium deposits. Since 1971, this fault has become the focus of extensive studies because of its location across a major metropolitan region of California. As already mentioned, it is believed that the Northridge earthquake occurred on a similar thrust fault which may be a blind eastern extension of the Oak Ridge fault system.

Focal Mechanism

According to USGS studies, the upward thrust movement of the Northridge earthquake on the blind thrust fault, deformed an area of about 4,000 square kilometers into the shape of an asymmetric dome. The fault's rupture begun at a depth of about 17.5 kilometers and moved upward in a northwestward direction for about 8 seconds, at a speed of about 3 kilometers per second. The rupture stopped about 5 kilometers beneath the surface. There was no clear surface rupture. Fortunately, most of the quake's energy propagated to the north of San Fernando Valley, an area sparsely populated.

Extreme Accelerations

The 1994 Northridge event resulted in very strong ground motions characteristic of thrust-type of earthquakes such as those which occurred in San Fernando in 1971 and in Coalinga in 1983. However, the vertical and horizontal accelerations of the 1994 quake were among the strongest ever recorded. Local conditions and topography in the valley amplified the seismic energy. Accelerations in the range of 1.0 g occurred over a much larger area. On a small hill, just south of the epicenter in Tarzana, the highest acceleration of 1.78 g was recorded. The strong motions caused extensive ground liquefaction and landslides which damaged many structures in San Fernando Valley. The extremely strong ground motions of the Northridge earthquake are indicative of the significant threat that even moderate magnitude, thrust-fault events present, particularly in valleys.

Ground motions of an earthquake are greater in valleys - which are usually areas with the highest population density. Geologically, valleys represent basins where thick layers of sediments accumulate as a result of active erosion of mountainous regions. In valleys, the thick layers of sediments tend to amplify the seismic surface waves, thus resulting in greater horizontal and vertical accelerations and ground movements. For example, the 1994 Northridge earthquake was particularly destructive at Sherman Oaks and Santa Monica, areas of San Fernando valley characterized by soft sedimentary layers. The energy of the earthquake generated greater ground motions at these two locations.

In a valley, an earthquake's seismic waves do not rapidly dissipate but tend to bounce off the sides and floor of the basin, often overlapping and interfering with one another. At the perimeter of a valley basin, ground motions may not be exceptionally high because of "edge effects". In certain areas, the seismic surface waves may cancel each other, thus resulting in reduced ground movements. However in other areas of a valley, like ocean waves, the surface seismic waves may also superimpose on each other and thus increase the intensity and extent of ground movements

Epicenter and Horizontal Accelerations of the 1994 Northridge earthquake (after USGS).

Understanding and predicting the surface behavior of seismic waves in valleys is rather difficult. Frequently, there are many variables that are not fully understood or are very difficult to estimate. For example, changes in the location, duration and magnitude of an earthquake will also change the interference patterns of the seismic waves, making it often impossible to know, on a block-by-block basis, where the most susceptible areas of the valley will be or where the ground movements will be the strongest. In designing buildings or other structures, engineers and architects do not always have a clear picture of the seismic forces in action during an earthquake and may not take them adequately into consideration. This is why an adequate Uniform Building Code is needed.

Lessons Learned

What is significant about the 1994 Northridge earthquake is that it was of moderate size, yet it caused great destruction in San Fernando valley, a region believed to be well-prepared after the 1971 event. Most of the structures that were affected had been built during the past three decades and were considered to be reasonably earthquake resistant. However, the unusually strong ground motions of this earthquake caused serious damage to buildings within about 16 km of the epicentral area.

The strong ground motions associated with the vertical and horizontal accelerations of this quake surprised many scientists, engineers and government officials. Some buildings were completely destroyed while others, only a short distance away, were unaffected. The different types of failures provided very important information in the evaluation of seismic hazards.
Extensive ground failures in the valley occurred primarily in filled land areas, underlain by ground water and fine-grained sediments at depths of less than 10 meters. For example, a major belt of ground failures occurred in the Granada Hills-Mission Hills, an area with soft soils.

Vertical Accelerations of the 1994 Northridge earthquake (Modified after the USGS).

In general, ground failure was the principal cause of damage to single-family homes. Structures within ground-failure zones suffered almost three times more damage than structures outside the zones. Also damage to foundations was about six times greater in the ground-failure zones. The well-engineered buildings withstood the strong ground motions without structural damage. However, numerous other buildings and structures throughout the region failed. Their failures were indicative of significant design or construction deficiencies and defects.

Generally most of the steel-framed buildings designed for seismic resistance, performed well. However, more than 100 buildings had failures due to the fracturing of their steel frames. Many buildings with reinforced concrete columns sustained also considerable damage. Fills and dams in the Van Norman Complex, retrofitted after failures in the 1971 San Fernando earthquake, performed well. Overall, most of the highway structures performed well but, as with the 1971 earthquake, the Santa Monica Freeway (Interstate 10) and the interchange (Interstate 5 and State Route 14) were considerably damaged.
The severe damage which occurred in Sherman Oaks and Santa Monica was confined primarily along isolated pockets with a variety of local geological and topographic conditions that were thoroughly investigated since they are indicative of potential problems in future earthquakes.

The Northridge quake revealed the need for engineers and architects to design buildings and structures that would not collapse during an earthquake, but would also continue to be functioning and be usable afterwards - thus minimizing the economic disruption of the affected area. There were several lessons learned from the 1994 Northridge earthquake, the most important being that better emergency preparedness and response procedures need to be implemented and that California's Uniform Building Code needed revisions to provide for higher levels of earthquake resistance. Also, the Northridge quake emphasized the need for stricter criteria and guidelines in identifying the seismic risk within a region.

References and Additional Reading

Andrus, F.M., 1952, "Earthquake design requirements of the Uniform Building Code." In Proceedings of the Symposium on Earthquake Blast Effects on Structures. Berkeley, 1952. pp. 314-316.

Heaton, T.N., 1982, The 1971 San Fernando earthquake: A double event?: Seismological Society of America Bulletin, v. 72, no. 6, p. 2037-2062.

Hill, M. L. 1954. “Tectonics of Faulting in Southern California.” California Division of Mines and Geology Bulletin, No. 170: 5-14.

Langston, C.A., 1978, The February 9, 1971 San Fernando earthquake: A study of source finiteness in teleseismic body waves: Seismological Society of America, v. 68, no. 1, p. 1-29.

Plafker, George, and Galloway, J.P. eds., 1989, Lessons learned from the Loma Prieta, California, earthquake of October 17, 1989: U.S. Geological Survey Circular 1045, 48 p.

Richter, C.F., 1973, Historical seismicity of San Fernando earthquake area, in Benfer, H.A., Coffman, J.L., and Bernick, J.R., eds., San Fernando, California, earthquake of February 9, 1971: Washington, U.S. Department of Commerce, National oceanic and Atmospheric Administration, Environmental Research Laboratories, v. 3, p. 5-11

U.S. Geological Survey, 1971, The San Fernando, California, earthquake of February 9, 1971: Professional Paper 733, 254 p.

Yerkes, R. F. 1973, “Effects of San Fernando Earthquake as Related to Geology.” In San Fernando Earthquake of February 9, 1971, Vol. 3. Edited by N. A. Benfer, J. L. Coffman, and J. R. Bernick. Washington, D.C.: National Oceanic and Atmospheric Administration, U.S. Department of Commerce.

Ziony, J., and Tinsley, J., 1982, "Mapping the Earthquake Hazards of the Los Angeles Region", Earthquake Information Bulletin, Vol. 15, No. 4, Jul.-Aug., 1982, p. 135-141.

Ziony, J.I., ed., 1985, Evaluating earthquake hazards in the Los Angeles region-an earth-science perspective: U.S. Geological Survey Professional Paper 1360, 505 p.

Ward, P. L., and Page, R. A., 1989, The Loma Prieta Earthquake of October 17, 1989; what happened, what is expected, and what can be done. (United States Government Printing Office, 1989.)