University of Southern California

 Civil Engineering Department

Strong Motion Research Group

Selected Activities in Response to the 1994 Northridge Earthquake


This web page summarizes the activities of the USC Earthquake Engineering-Strong Motion Group following the 1994 Northridge earthquake.  These activities include gathering and processing of the accelerograms of the main shock and aftershocks recorded by the Los Angeles and Vicinity Strong Motion Network which this group operates, and numerous studies of the recorded ground motion and its relation to observed damage.
 

  1. Background
    1. About the USC Strong Motion Group
    2. The Los Angeles and Vicinity Strong Motion Network
    3. The Strong Motion Data Processing Laboratory at USC
  2. 1994 Northridge Earthquake Recording at USC Strong Motion Stations
  3. Analyses of Strong Ground Motion from the Northridge Earthquake
    1. Strong Ground Motion Database
    2. Ground Motion Contour Maps
    3. Nonparametric Attenuation Functions for Peak Acceleration
  4. Correlation of Strong Ground Motion and Damage
    1. Red-tagged Buildings, Breaks in the Water Pipes and Outbreaks of Fires
    2. Important New Observation
  5. Current Research
  6. References


1. Background


1.1 About the USC Strong Motion Group

The Earthquake Engineering-Strong Motion Research Group at USC was established in 1976 when Prof. M.D. Trifunac joined the Civil Engineering faculty. The group's research activities are focused on studies of earthquake strong ground shaking for engineering applications and on its effects on man made structures. This involves representation of the earthquake sources, modeling of the propagation of seismic waves from the source to the site, characterization of the effects of geology and local soils on the characteristics of ground shaking (amplitudes, duration, spectral characteristics…), strong motion recording and data processing, identification of structural properties from ambient and forced vibration tests, analyses of structural response to various realistic representations of strong ground shaking, and analysis of actually recorded structural response. The group operates the Los Angeles and Vicinity Strong Motion Network, installed in 1979/80, which contributed a significant number of strong motion records of the Northridge earthquake. It also maintains and develops equipment and processing methods for ambient vibration testing of full scale structures. The following is a summary of the activities of this group in response to the Northridge earthquake, in particular, (1) of the collection and processing of accelerograms recorded by the Los Angeles Strong Motion Network, and (2) of the studies of the characteristics of strong ground motion from this earthquake, and of the causal relationships between various characteristics of strong ground motion and the documented damage to buildings and to lifelines.


1.2 The Los Angeles and Vicinity Strong Motion Network

The Los Angeles and Vicinity Strong Motion Network has 80 stations spread throughout the metropolitan area, each recording three components of ground acceleration in "free-field" conditions. The recording instruments are SMA-1 accelerographs. The array was planned and installed between 1978 and 1980 with financial support from the National Science Foundation. It became fully operational in the spring of 1980. The purpose of the array has been to record strong ground motion in the metropolitan area, to help understand and quantify the spatial distribution of strong shaking, the attenuation of strong motion amplitudes with distance, the effects of the geological structure and local soils on the strong motion amplitudes, and the relationship between strong ground shaking and damage to structures. Figure A shows a map of the stations. The Strong Motion Recording Laboratory at USC was established in 1978 to support the activities of the array. The array maintenance is currently supported by a grant from the National Science Foundation (PI: Todorovska).

 The array represents a full-scale laboratory, of the size of a metropolitan area. Damaging urban earthquakes are rare events with significant consequences (earthquakes like the ML=6.4 Northridge can occur on the average about every 20 years, and the damage is of the order of billions of dollars). They also happen unexpectedly, both in time and in space. Therefore, repeated recording of significant ground motions during the life of an array is a significant success. The Los Angeles Strong Motion Network has been very fortunate in that respect. During the 18 years of operation, the array recorded many earthquakes of magnitude 3 and grater, including the Whittier-Narrows (1 October, 1987, M=5.9, 68 records), Upland (1 March, 1990, M=5.2, 51 records), Sierra Madre (28 June, 1991, M=5.8, 65 records), Landers (28 June, 1992, M=7.5, 61 records) and Northridge (17 January, 1994, M=6.7, 65 records). The processed data is distributed by the National Geophysical Data Center  of the National Oceanic and Atmospheric Administration, U.S. Department of Commerce, Boulder, Colorado (NGDC/NOAA) or can be downloaded from the internet (http://www.usc.edu/dept/civil_eng/Earthquake_eng/StrongMotionData_OnLine.html).


1.3 The Strong Motion Data Processing Laboratory at USC

This laboratory was established in 1976. Its activities involve developing standard and specialized software for automatic digitization and processing of strong motion accelerograms. The LeAuto software package is used for digitization of accelerograms recorded on film using flat-bed scanners and PC computers. The standard data processing software package (Volume 1, 2 and 3 programs; Lee and Trifunac, 1990; Trifunac et al, 1998d) consists of programs that perform baseline and instrument correction, filtering of high-frequency noise, integration in time of the recorded accelerations to obtain velocity and displacement, and further computation of Fourier and Response spectra. The USC group has also developed specialized software for nonstandard processing (e.g., for coupled transducer-galvanometer recording systems and for force-balanced accelerometers; Novikova and Trifunac, 1991, 1992), and software for detailed instrument calibration and corrections for misalignment and cross-axis sensitivity (Todorovska et al., 1996; Todorovska, 1998; Todorovska et al., 1998). Various versions of the data processing software developed by the USC group have been and continue to be used by government organizations, universities and industry in the US and in many countries in the world.



2. 1994 Northridge Earthquake Recording at USC Strong Motion Stations

The Northridge earthquake of 17 January 1994 (ML=6.4) was recorded by 65 stations of the Los Angeles Strong Motion Array, at epicentral distances between 2 km and 85 km, and at various azimuths from the source. Eleven of the stations were at distances closer than 20 km, and 25 were at distances closer than 30 km from the epicenter. The recorded peak accelerations approached 1g at several stations. All but few of the stations are in the densely populated metropolitan area, and one is at Potrero Canyon where large ground movements were observed. The recordings are invaluable for studies of the fault mechanism, spatial distribution of ground shaking, nonlinear soil response and ground failure, and interpretation of damage to structures and lifelines.

The details of gathering and processing of the Northridge main event data are described in Trifunac et al. (1994) and Todorovska (1997). Funding for the data gathering and processing was provided by the US Geological Survey and the National Science Foundation (PI: Todorovska). Volume 1, 2 and 3 data (corrected acceleration, velocity, displacement and Fourier and Response Spectra) are available on the web

ftp://cwis.usc.edu/pub/todorovs/northridge/ - new address for the original ftp site (main event only)
http://www.usc.edu/dept/civil_eng/Earthquake_eng/North_M5/Data_sumary.html  - new data site (includes aftershocks), July 99

or can be ordered from the National Geophysical Data Center of the National Oceanographic and Atmospheric Administration (NGDC/NOAA) in Boulder, Colorado.

Figure 1 shows the location of the recording stations, identified by the USC station numbers. Table 1 lists the stations, their location, geographical coordinates, epicentral distance, duration of the record and uncorrected peak acceleration for the three components of motion, determined from digitized data. The records were digitized by a PC system with a flat-bed scanner and processed with standard data processing software (Lee and Trifunac, 1990). Stations USC 03 and 53 were the closest strong motion stations to the epicenter, and USC 91 was the closest strong motion station to the collapsed La Cienega-Venice and Fairfax-Washington under-crossings of Interstate 10.

The USC stations are installed mostly in one storey buildings (exception is USC 96). The site condition parameters (geologic site condition parameter, local soil condition parameter and the A, B, C and D classification, based on the average shear wave velocity in the top 30 m of soil) for the recording stations are discussed in Trifunac et al. (1994) and Trifunac and Todorovska (1996).



3. Analyses of Strong Ground Motion from the Northridge Earthquake


3.1 Strong Ground Motion Database

The Northridge earthquake is the best recorded urban earthquake so far, in terms of strong motion recordings available for analysis. Ground motion was recorded by more than 200 strong motion accelerographs stations.

Figure 2 shows the locations of the stations used by the USC research group. The agencies that contributed these data are: United States Geological Survey, National Strong Motion Program (USGS, squares), Department of Water and Power of City of Los Angeles (DWP, diamonds), University of Southern California, Department of Civil Engineering array (USC, circles), and California Division of Mines and Geology/Strong Motion Instrumentation Program (CDMG, triangles).


3.2 Ground Motion Contour Maps

Contour maps of uncorrected peak ground acceleration (the largest horizontal and the vertical components) were published in Trifunac et al. (1994). These maps showed that the horizontal peak acceleration exceeded 0.5 g in San Fernando Valley, and the vertical accelerations were larger than 0.3g. Similar maps for peak ground velocities were published in Trifunac et al. (1996). The peak strain in the soil during earthquake shaking is proportional to the ratio , where vmax is the peak velocity of strong motion and  is the average shear wave velocity in the soil in the top 30 m. Maps of the peak strain factor, , were also published in Trifunac et al. (1996). The analysis showed that, during the Northridge earthquake, the peak strain factor reached 10-2.25 in the heavily shaken areas of San Fernando Valley and then diminished with distances to 10-3.75 in the Newport Beach area. These maps were later used in interpretation of the observed patterns of damage (e.g. red-tagged buildings and breaks in the underground water, gas and sewer pipes).

Later, baseline and instrument corrected acceleration data was used to calculate and map in more detail the peak amplitudes of acceleration, velocity and displacement in the radial (R), transverse (T) and vertical (V) directions (R and T directions are defined relative to a point in the "center" of the ruptured area) (Todorovska and Trifunac, 1997a). Figure 3, Figure 4 and Figure 5 show examples of the contour maps of the T component of acceleration, velocity and displacement. The shaded areas outline the areas where peak amplitude was positive (clockwise w.r.t. epicenter). The corresponding maps for the R and V components of motion can be found in Todorovska and Trifunac (1997a). These maps show that there is remarkable consistency of the amplitude and of the sign of the largest peaks of strong motion over extended areas, and strong dependence of the peak amplitudes on the geological structure. These maps also suggest that, to model attenuation of strong motion amplitudes reliably, the empirical equations for prediction of peak amplitudes of strong motion must include dependence on the geology, at the site and along the path.

Contour maps of PSV response spectrum amplitudes (for the SRSS combination of two horizontal components of motion) for oscillator periods T=0.04, 0.34, 0.90 and 2.80 s were published in Todorovska and Trifunac (1997b). Figures 6 and Figure 7 show two of these maps, for periods T=0.11 and 2.8 s. The PSV amplitudes also show strong dependence on the three-dimensional geologic structure of Los Angeles basin. Figure 8 shows smoothed contours of the logarithm of the horizontal strain factor, , from Trifunac et al. (1996).


3.3 Nonparametric Attenuation Functions for Peak Acceleration and Nonlinear Soil Response

The relatively high density of strong motion stations made it possible to develop non-parametric attenuation functions specific for the Northridge earthquake. Trifunac and Todorovska (1996) presented a family of such functions for peak acceleration, for "soft" and for "hard" soil conditions, and for vertical and horizontal components of motion (total of four functions, Figure B). The results showed that, within 20 km from the epicenter, seven (possibly 10) stations with "soft" soil conditions recorded smaller horizontal peak accelerations than expected from the overall trend, while no such trend was observed for the vertical motions. At the "soft" sites with reduced peak horizontal accelerations, the peak strains were estimated to be larger than 0.001. The widespread nonlinear response of soil in the same general area (sliding, liquefaction, cracked pipes and pavement) suggest that the observed reduction of peak accelerations was most probably due to nonlinear soil response.



4. Correlation of Strong Ground Motion and Damage


4.1 Red-tagged Buildings, Breaks in the Water Pipes and Outbreaks of Fires

The data on damaged (red-tagged) buildings reported by the Los Angles Department of Building Safety and on breaks in the water pipes reported by Los Angeles Department of Water and Power gave an excellent opportunity to correlate these with the strong motion amplitudes and reported intensities inferred from recordings and felt reports, respectively. Correlation of density of red-tagged buildings in typical residential areas of San Fernando Valley Los Angeles and Santa Monica with peak horizontal velocity and Modified Mercalli Intensity was analyzed in Trifunac and Todorovska (1997b). The same was repeated for the pipe breaks, but the peak strain factor was used instead of peak velocity to characterize the ground shaking. The functional relationships between these two measures of shaking and of damage were published in Trifunac and Todorovska (1997c). Recently, analyses of the causative relationship between the number of fire outbreaks per household and peak ground strain and peak velocity were published (Trifunac and Todorovska, 1998c).


4.2 An Important New Observation

One of the more interesting discoveries, which resulted from the above mentioned studies, is the spatial separation of the areas with many pipe breaks (indicating large strain in the soil) and with many red-tagged buildings almost everywhere (Trifunac and Todorovska, 1998a). Overlapping is observed only in the regions with very large amplitudes of shaking (peak ground velocity, inferred from damage, exceeding about 150 cm/s). One explanation for this remarkable separation is that the buildings on soft soils, which experienced nonlinear strain levels, were damaged to a lesser degree possibly because the soil absorbed a significant portion of the incident wave energy. As a result, the total number of severely damaged (red-tagged) buildings in San Fernando Valley, Los Angeles and Santa Monica may have been reduced by a factor of two or more. This interpretation is consistent with the recorded peak accelerations of strong motion in the same area. This lead to the conclusion that the soft soils, responding non linearly, acted as a natural passive isolation mechanism and abated the damage to wood-frame single family dwellings during the Northridge earthquake. This is illustrated in Figure 9 for San Fernando Valley. The areas with red-tagged buildings are indicated by red shade and the locations of breaks in the water pipes by solid black points. It is seen that the areas with red-tagged buildings and with large concentration of pipe breaks overlap only in the areas with very large velocities of strong motion (~150 cm/s, circled by solid contours) suggesting that major damage to structures there was caused by combined action of inertial forces and large differential motion of foundations (Trifunac and Todorovska, 1998a). Correlation of the distribution of damage with generalized categories of surficial geology suggests relative reduction of damage to single family residential wood frame buildings only for sites on shallow (0-3 m) and recent (less then 1000 yrs old) Holocene deposits (Trifunac and Todorovska, 1998b).



5. Current Research

The studies of the Northridge earthquake continue and will be reported in future publications. The USC group  recently completed  processing of selected Northridge aftershock data at strong motion stations to study whether some of the site effects observed in the ground motion of the main event were repeated during the stronger aftershocks. This project is funded by the US Geological Survey (PI-s: Todorovska and Lee).  The URL address for this page is:

http://www.usc.edu/dept/civil_eng/Earthquake_eng/North_M5/Repeatability_of_Site_Effects.html

 

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6. References

  1. Lee, V.W., & M.D. Trifunac (1990). ‘Automatic digitization and processing of accelerograms using P.C.’, Report No. 90-03, Dept. of Civil Engrg, Univ. of Southern California, Los Angeles, California.
  2. Novikova, E.I., & M.D. Trifunac (1991). ‘Instrument correction for the coupled transducer-galvanometer systems’, Report No. 91-02, Dept. of Civil Engrg, Univ. of Southern California, Los Angeles, CA.
  3. Novikova, E.I., & M.D. Trifunac (1992). ‘Digital instrument response correction for the force balance accelerometer’, Earthquake Spectra, 8(3), 429-492.
  4. Todorovska, M.I. (1997). `Strong motion recordings of the 1994 Northridge, California, earthquake at stations of the Los Angeles Strong Motion Array', Proc. Northridge Earthquake Research Conf., 20-22 August, 1997, Los Angeles, CUREe, Richmond, CA, 1998, Vol. II,  pp. II.413-420.
  5. Todorovska, M.I. (1998). `Cross-axis sensitivity of accelerographs with pendulum like transducers: mathematical model and the inverse problem,' Earthquake Engrg & Struct. Dynamics, 27, 1031-1051.
  6. Todorovska, M.I., E.I. Novikova, M.D. Trifunac & S.S. Ivanovic (1998). `Advanced sensitivity calibration of  the Los Angeles Strong Motion Array,' Earthquake Engrg & Struct. Dynamics, 27, 1053-1068.
  7. Todorovska, M.I., & M.D. Trifunac (1997a). ‘Amplitudes, polarity and time of peaks of strong ground motion during the 1994 Northridge, California, earthquake’, Soil Dynamics & Earthquake Engrg, 16(4), 235-258.
  8. Todorovska, M.I., & M.D. Trifunac (1997b). ‘Distribution of pseudo spectral velocity during the Northridge, California, Earthquake of 17 January, 1994’, Soil Dynamics & Earthquake Engrg, 16(3), 173-192.
  9. Todorovska, M.I., E.I. Novikova, M.D. Trifunac & S.S. Ivanovic (1995). ‘Correction for misalignment and cross-axis sensitivity of strong earthquake motion recorded by SMA-1 accelerographs’, Report No. CE 95-06, Dept. of Civil Engrg, Univ. of Southern California, Los Angeles, California, pp. 324.
  10. Trifunac, M.D. (1997). ‘Differential earthquake motion of building foundations’, J. of Structural Engrg, ASCE, 123(4), 414-422.
  11. Trifunac, M.D., & M.I. Todorovska (1996). ‘Nonlinear soil response 1994 Northridge, California, earthquake’, J. of Geotech. Engrg, ASCE, 122(9), 725-735.
  12. Trifunac, M.D., & M.I. Todorovska (1997a). ‘Response spectra for differential motion of columns’, Earthquake Engrg. & Struct. Dynamics, 26(2), 251-268.
  13. Trifunac, M.D., & M.I. Todorovska (1997b). ‘Northridge, California, earthquake of 1994: density of red-tagged buildings versus peak horizontal velocity and intensity of shaking’, Soil Dynamics & Earthquake Engrg, 16(3), 209-222.
  14. Trifunac, M.D., & M.I. Todorovska (1997c). ‘Northridge, California, earthquake of 1994: density of pipe breaks and surface strains’, Soil Dynamics & Earthquake Engrg, 16(3), 193-207.
  15. Trifunac, M.D., & M.I. Todorovska (1998a). `Nonlinear soil response as a natural passive isolation mechanism – the 1994 Northridge, California, earthquake’, Soil Dynamics & Earthquake Engrg, 17(1), 41-51.
  16. Trifunac, M.D., & M.I. Todorovska (1998b). ‘Damage distribution during the 1994 Northridge, California, earthquake in relation to generalized categories of surficial geology’, Soil Dynamics & Earthquake Engrg, 17(4), 239-253.
  17. Trifunac, M.D., & M.I. Todorovska (1998c). ‘The Northridge, California, earthquake of 1994: fire ignition by strong shaking’, Soil Dynamics & Earthquake Engrg, 17(3), 165-175.
  18. Trifunac, M.D. and M.I. Todorovska (2000). `Can aftershock studies predict site amplification? - Northridge, California, earthquake of 17 January, 1996', (submitted for publication).
  19. Trifunac, M.D. and M.I. Todorovska (2000). `Long period microtremors, microseisms and earthquake damage: Northridge, California, earthquake of 17 January, 1994', (submitted for publication).
  20. Trifunac, M.D., M.I. Todorovska & V.W. Lee (1998d). ‘The Rinaldi strong motion accelerogram of the Northridge, California, earthquake of 17 January 1994’, Earthquake Spectra, 14(1), 225-239.
  21. Trifunac, M.D., M.I. Todorovska & S.S. Ivanovic (1994). ‘A note on distribution of uncorrected peak ground accelerations during the Northridge, California, earthquake of 17 January, 1994’, Soil Dynamics & Earthquake Engrg,13(3), 187-196.
  22. Trifunac, M.D., M.I. Todorovska & S.S. Ivanovic (1996). ‘Peak velocities and peak surface strains during the Northridge, California, earthquake of 17 January 1994’, Soil Dynamics & Earthquake Engrg, 15(5), 301-310.
  23. Trifunac, M.D., T.Y. Hao and M.I. Todorovska (1999). `On reoccurrence of site specific response', Soil Dynamics & Earthquake Engrg, 18(8), 569-592.

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