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As in most earthquakes, the patterns of damage that occurred during the Northridge event showed irregular distributions. Generally, the region closest to the earthquake—the northern San Fernando Valley which was directly above the ruptured fault—was shaken most severely and sustained the greatest amount of damage. However, there were also isolated pockets of damage in Sherman Oaks, West Hollywood, Santa Monica, and other distant locations. There are a variety of reasons to explain the more distant damages, such as differences in building construction, effects of ground deformation, liquefaction, and local site amplification. Local site amplifications, or site effects, can result in significant differences in structural damage within the same general area. Understanding and characterizing the site effects are important since areas that demonstrate adverse site effects probably will have amplified ground shaking during future earthquakes.
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Damage patterns may vary greatly within small areas, and major damages may occur at sites far from the earthquake source. Sometimes this may be due to different types of building construction but, in many cases, the geological characteristics of the site have a large influence on the intensity of the ground shaking. This is the reason that we often see a heavily damaged building at one place while a building of similar construction a block or two away may be completely unaffected. |
USGS scientists initiated several studies to quantify the differences in site responses throughout the Los Angeles region. The initial task was analyzing large volumes of aftershock data (more than 1,300 seismograms from 90 sites, including data from the California Division of Mines and Geology (CDMG) strong motion/instrumentation program) to study site-response issues. The unprecedented database covered variations in site response over widely varying distances, from one city block to an entire sedimentary basin such as the San Fernando Valley. The bulk of the data were collected during portable instrument deployments that targeted many specific areas of severe damage and compared them with minimally damaged or undamaged areas. The deployments also targeted a variety of types of surface rocks, including consolidated and unconsolidated sediments.
Site-response factors at each site were determined for a frequency range of 0.5 to 20 hertz. Generally lower factors (less amplification) were observed for sites in the mountains on firmer rock sites. Higher factors (larger amplification) were observed in the San Fernando and Los Angeles basins, which primarily contain alluvial deposits. There was a good correlation between high factors and the localized areas of severe damage at sites including Sherman Oaks, Northridge, and at the Interstate (I) 10 highway collapses near Santa Monica, and the I-5/State Highway (SH) 14 interchange collapses near Sylmar (see p.52).
| The site-response factor is the numerical quantity that multiplies the amplitude of a reference wave motion to match the observed ground motion at the site. |
Site-response factors throughout the Los Angeles region demonstrate how shaking varies depending upon the types of geologic materials present. The yellow and orange colors indicate the “softer” materials of sedimentary basins, and the red and green colors represent the harder rocks of hills and mountains. The circles indicate the amplification of earthquake energy in the 2-6 hertz range—the range for which 3- to 5-story buildings are particularly susceptible to the effects of earthquake shaking. Green circles are results from the main shocks of four large earthquakes—the 1971 San Fernando, 1987 Whittier Narrows, 1989 Sierra Madre, and 1994 Northridge events. The blue circles are results from the aftershocks of the Northridge quake. The site-response factor varies as the diameter of the circles, showing that amplifications range up to more than 7 times the reference value of 1.0 for a site on rock. Damage data from the Northridge event (red squares) show the correlation between site-response factors and severe building damage.
In addition to the aftershock data, several hundred records from the main shocks of the 1994 Northridge, 1971 San Fernando, 1987 Whittier Narrows, and 1989 Sierra Madre earthquakes were also used to extend the coverage of site-response estimates. There was generally good agreement between site amplifications determined from the strong-motion data of the main shocks and amplifications from weak-motion data from aftershocks. This agreement validates the practice of using small aftershocks to predict the site response to strong motions.
The I-10 freeway at La Cienega Blvd. collapsed despite its distance and direction from the Northridge
earthquake epicenter. The collapse illustrates the importance of local amplifications of earthquake energy.
Some of the largest site-amplification factors (up to 7.6) of the 90 sites investigated were obtained in the Sherman Oaks area. However, the site response there was also highly variable on a block-by-block basis. The heavily damaged section of Sherman Oaks is in one of the lowest-lying parts of the San Fernando Valley, contains areas of shallow ground water, and is transected by the Los Angeles River channel. The surficial geology is characterized by unconsolidated sands and gravels. The highly variable pattern in site response may be explained by old meanders in the Los Angeles River that are likely to have produced a complicated pattern of buried, unconsolidated, sandy deposits.
Another area of significant damage that is distant from the epicentral region is the site of the I-10 freeway collapse. Two sections of I-10 separated by about 1 kilometer, sustained damage. Both structures were designed and built in the 1960’s and survived the 1971 San Fernando and 1987 Whittier Narrows earthquakes. They were scheduled for retrofit to bring them into compliance with current earthquake design standards when the Northridge earthquake occurred. Site-response calculations show that the highest amplification factors are nearest the bridge collapse, with values decreasing away from this location. To the east-southeast, site-response levels are also high. The trend in large-amplification factors follows an east-west lineation in structural damage for about 4 kilometers. This lineament may coincide with parts of the channels of Ballona Creek and the pre-1825 course of the Los Angeles River.
Horizontal amplifications of shaking at the I-10 bridge collapses in Santa Monica were 3-5 times greater than
at sites farther from the collapses. The large amplifications were probably related to the presence of “soft” sediments
of the old river channel of the Los Angeles River.
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We need not wait until the next major earthquake to further define areas of severe damage potential. There are general correlations between the measured site response and rock or soil types, and good correlations between site response and damage. However, the pattern of site response is characterized by high variability over distances of less than a kilometer. Variations of a factor of 2 were observed over 200 meters, even for the same geologic unit. |
| Clues from Historical Place Names One of the areas that was found to have large site-response factors due to amplifications in soft sediments, was at the collapse of the I-10 freeway at La Cienega Blvd. At present it is difficult to see but, prior to 1825, the Los Angeles River flowed through this location and the surficial geology is composed of sands and gravels from the old river channel. The place name, La Cienega, was given to the area in the 1700’s, and in Spanish means “the swamp.” |
In a simplified view, “softer” geologic conditions at a locality cause larger amplifications of the seismic wave. Thus, the softer sediments in a valley usually exhibit stronger shaking than the hard-rock sites in the mountains. One way to characterize this quality of the ground is to measure the velocity of seismic shear waves at a site. Slower velocities imply softer ground and, therefore, larger site amplifications. In order to made reasonable estimates of these velocities, scientists make measurements in boreholes drilled to depths of about 100 meters. The USGS drilled holes at 12 sites where significant strong ground motions were recorded or where heavy damage occurred. These site-specific borehole data are being used with the strong-motion and damage data to investigate the effects on the geological properties of the sites. Ultimately, this will lead to recommendations for the use of surficial geologic maps to predict site responses in future earthquakes.
In addition to the borehole measurements, USGS scientists made 11 high-resolution S-wave refraction profiles in conjunction with the portable seismograph station sites. These profiles were designed to obtain site information related to heavily damaged regions in Northridge and Sherman Oaks. Also, profiles were acquired in the northern San Fernando Valley to examine the differences in S-wave velocities among various geologic deposits mapped on the ground surface. Based on data from the upper 30 meters on the S-wave velocity structure, scientists grouped these sites into the four categories shown in the table.
| Site Location | S-wave Velocity (meters per second) | Depth Range (meters) | Site Response |
|---|---|---|---|
| Sherman Oaks |
150 |
0-10 |
High |
| Mid-valley |
250 |
0-10 |
Moderate to Low |
| North valley |
400 |
0-20 |
Low |
| “Rock” |
80 |
0-15 |
Moderate |
USGS scientists confirmed an important correlation between S-wave velocity and ground-motion amplification. The best-fit line for main-shock data for four large southern California earthquakes (blue squares) compares favorably with relations for aftershock data from other earthquakes such as the 1989 Loma Prieta earthquake in northern California. The correlation offers scientists and engineers a tool for predicting site response based on aftershock data.
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Zones of low shear-wave (S-wave) velocities can generally be associated with amplified ground shaking that caused greater damage. However, simple identification of low S-wave velocities may not be adequate for mapping areas of potentially hazardous site responses. Geological structures more than 100 meters below the surface can be important contributors to site response. |
One of the highest accelerations ever recorded during an earthquake occurred at the Cedar Hill Nursery in Tarzana located about 6 kilometers south of the epicenter. The strong-motion record showed a peak acceleration of 1.78g (g is the acceleration due to gravity) and sustained large amplitudes near 1g for about 7-8 seconds. However, much smaller ground accelerations were observed at Encino Reservoir and on Ventura Blvd., each less than 2 kilometers from the Cedar Hill Nursery site. The USGS deployed an array of 21 seismographs to record aftershocks at the site to identify the factors that caused the large motions and to identify reasons for the large differences in ground motions at these three closely spaced sites.
The Cedar Hill Nursery main-shock recording was made atop a hill about 15 meters high, 500 meters long, and 130 meters wide. By comparing aftershock ground motions recorded at the top and base of the hill, scientists observed that the top of the hill shook more strongly than the base. Specifically, the top-to-base amplification ratio was about 2 for motions parallel to the hill (approximately east-west, which is the direction of the 1.78g main-shock peak acceleration). For motions perpendicular to the long axis of the hill, the top-to-base amplification ratio was as large as 4.5 for 3.2-hertz motions. Assuming that the hill soils responded linearly during the main shock, the large east-west main-shock motion was amplified by about a factor of 2 due to the topography.
Instruments at the Cedar Hill Nursery in Tarzana recorded significant amplifications of seismic energy that were far greater than those of nearby sites. The amplifications, partly related to the local topography, are about twice what would be expected for similar sites in the Los Angeles region.
The main-shock accelerations at Cedar Hill Nursery were about 3.5 and 7 times larger than those at the nearby Ventura Blvd. and Encino Reservoir sites, respectively. This discrepancy cannot be fully explained by the differing site geologies and S-wave velocities. Even after considering the surficial geology and the effects of topography, the main-shock motions at Cedar Hill Nursery are about twice what would be expected. These observations show that variations in site geology, topography, and other deeper geological factors caused a factor of 7 difference in accelerations between Tarzana and a site 2 kilometers distant at Encino Reservoir. Such differences make it difficult to make a meaningful contour map of peak acceleration (observed or predicted) for the Los Angeles region without using site geology, at the very least, to guide the placement of contours.
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Minor hills can cause substantial topographic amplification. Even though our eyes are drawn to the tallest peaks in a hilly region, for ground motions, the strongest topographic effects may be associated with the smaller wrinkles on the sides of major peaks. Topographic amplifications may occur on most of the hills in the Los Angeles region. These amplifications could be damaging or harmless, depending upon the relative resonance frequencies of the hills and the structures built upon them. |
How the Sedimentary Basin Affects Ground Motion in the San Fernando Valley
The effects of sedimentary basins (such as the San Fernando Valley) on seismic waves are more extensive than amplifications and resonances caused by soft alluvium near the surface. Complicated interactions between the structure of the basin and the traveling seismic waves can increase the amplitude and duration of shaking during an earthquake. These interactions can focus the waves from the bottom of the basin, thereby concentrating the intensity of strong shaking in small regions at the surface, while diminishing intensity at other sites. Additionally, the edges of basins can effectively trap incoming seismic waves, thereby increasing the duration of shaking in the basin.
Using small arrays of seismometers over distances of 150-500 meters, scientists determined the velocity and direction of propagation of seismic waves from aftershocks. Even though these data are from small aftershocks, identifying characteristics of the seismic waves is directly applicable to the strong shaking caused by the main shock. A significant finding at one station in Northridge was a large seismic-wave arrival about 10 seconds after the direct S-wave—the principal shear wave that travels directly from the earthquake source to the station. The analysis indicates that this arrival had a relatively slow velocity of about 600 meters per second. This velocity suggests a wave traveling nearly horizontally in the uppermost geological materials—essentially a surface wave. Because of the geometry between the hypocenter and the station, this surface wave cannot have been generated in a standard way by propagation through horizontal layers. Instead, the wave must have been caused by trapping waves near the northern edge of the San Fernando Valley. Analyses of wave amplitudes show that the basin surface wave had amplitudes at 2-4 hertz, as large or larger than the direct S-wave, and increased the duration of shaking for several seconds. Interestingly, scientists found no evidence for large surface waves reflected at the southern edge of the valley. This absence of reflections may be related to the gradual ramplike shape of that edge.
A Computer Simulation of “Focusing” and “Defocusing” Effects
Seismic waves traveling upward from depth may be redirected by subtle irregularities at geological interfaces. As waves pass from the deeper unit (green) across the curved interface, their velocity and direction change, then change again in the unit nearest the surface. In this computer simulation, this “focusing” effect produces about 1.5 times the shaking where the waves converge at the surface. “Defocusing” reduces the shaking to about three-quarters of the value for unaffected wave travel.
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Surface waves generated by S-wave interactions with basin geometry can increase the duration of shaking for frequencies up to 4 hertz. This phenomenon can apply to earthquakes located outside of sedimentary basins. |
The relatively large amplitudes south of the Northridge Hills fault found in the computer simulation correspond approximately to locations of extensive damage in Northridge. Many factors may have played a role in causing this area of concentrated damage, but USGS scientists suggest that focusing from basement topography contributed to the amplified shaking. One characteristic of this focusing is that the amplification is very dependent upon the location of the earthquake source. One would expect less focusing from earthquakes located along strike of the buried topography. Scientists observed such differences in amplification between the damaged and undamaged areas of Sherman Oaks that were dependent upon the direction from the damaged areas to the source of the aftershocks (see p.25). This suggests that focusing from a deep basin structure could also have contributed to the extensive damage at Sherman Oaks.
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The curved boundary between a sedimentary basin and underlying hard rock can act as a lens, causing amplification of peak acceleration by 2-3 times over distances of a few hundred meters at the surface. Such focusing likely played a major role in producing the pattern of damage in the San Fernando Valley observed for the Northridge earthquake. |
Surface soils can be classified in terms of the style of their response to earthquake shaking, and site-response studies seek to determine the degree of “linearity” or “nonlinearity” of the soil response. For a “linear” soil, the observed motions at the surface are amplified proportional to the input ground motion. For “nonlinear” conditions, the soil tends to damp out more of the energy of large amplitude ground motions. Thus, in a large earthquake, nonlinear behavior of a soil will cause less severe shaking than linear behavior. The 1994 edition of the NEHRP Recommended Provisions for the Development of Seismic Regulations for New Buildings, prepared by the Building Seismic Safety Council (BSSC) for FEMA, includes provisions for the effects of nonlinear soil response. At large accelerations (>0.4g), the NEHRP Provisions specify almost no amplification for a stiff-soil site relative to a rock site. At moderate accelerations of 0.3-0.4g for periods of 0.3 seconds (3.3 hertz), the provisions call for amplification factors of 1.1-1.2.
The Northridge earthquake provided an opportunity to assess the linearity of stiff-soil sites that experience high levels of ground acceleration. USGS studies examined the relative ground motions at soil and rock sites, with careful comparisons that minimized some of the complicating effects such as rupture directivity (see p. 12) and wave propagation through the complexities of the Earth’s crust. Scientists examined the rock/soil differences at a variety of frequencies, producing results that must be considered preliminary until more sites can be studied. However, they found significant amplification for stiff-soil sites for frequencies up to at least 4 hertz, even for input accelerations as high as 0.4g. Two soil sites in Sylmar and Simi Valley experienced intense ground accelerations (0.8-0.9g) that were much stronger than would be expected if there were significant nonlinear behavior of the soils.
Relative ground motions for rock and soil sites indicate that amplifications during the Northridge earthquake were much higher than those specified in the NEHRP Recommended Provisions for nonlinear soil response.
USGS scientists also found one good example of nonlinear behavior at the Joseph P. Jensen Filtration Plant near Sylmar (see p. 43). The data from this site show de-amplification of high-frequency ground motions at a soil site relative to a neighboring rock site. The nonlinear response of the soil near the administration building is consistent with the nearby occurrence of liquefaction during the earthquake. Thus, nonlinear behavior in soft soils can reduce the intensity of shaking at high frequencies.
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Northridge strong-motion data suggest that the short-period amplification factors as proposed in the 1994 NEHRP Recommended Provisions for the Development of Seismic Regulations for New Buildings need to be re-examined. Although there may be some degree of nonlinear soil response, USGS scientists found substantial amplifications (factors of 2 compared with 1.1-1.2 in the NEHRP Provisions) at stiff-soil sites that recorded large accelerations of 0.8-0.9g. |
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