CHARACTERISTICS OF GROUND MOTION RESPONSE SPECTRA FROM RECENT LARGE EARTHQUAKES AND THEIR COMPARISON WITH IEEE STANDARD 693 ABSTRACT

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1 CHARACTERISTICS OF GROUND MOTION RESPONSE SPECTRA FROM RECENT LARGE EARTHQUAKES AND THEIR COMPARISON WITH IEEE STANDARD 693 F. Su 1, J. G. Anderson 2 and Y. Zeng 3 ABSTRACT We have examined ground motion response spectra from the recent large earthquakes in California, Japan, Turkey and Taiwan. We have studied the distance, site, and magnitude effect on ground motion response spectra and compared these spectra with the required response spectrum (RRS) specified by the IEEE Standard 693, Recommended Practice for Seismic Design of Substations. The shapes of the observed response spectra show strong magnitude and site dependency, but weak distance dependency. A larger earthquake generates much more long-period seismic energy than a smaller earthquake. A soft soil site shows higher amplification of the long-period seismic signal than a rock or stiff soil site. These combined effects of a large earthquake acting on a soft soil site are to significantly increase seismic energy at long periods. Thus, the normalized spectra from a large earthquake recorded on a soft soil site have the highest likelihood of exceeding the IEEE 693 spectrum at long periods. By comparing observed acceleration response spectra with the required response spectrum given in IEEE 693 for high seismic performance level, we found the stations that exceed the absolute level of IEEE 693 spectrum are generally located: 1) very close to the fault rupture trace; 2) in a location that experienced a strong directivity effect; 3) on the hanging wall adjacent to the primary fault trace; or 4) at a site that experiences large site amplification. In the low frequency range (f<1 Hz), the exceedance is usually associated with a larger magnitude earthquake at soft soil sites at near source distance (D<50 km). For larger earthquakes (M>7.5), exceedance at low frequencies may be common. Introduction Ground motion, and the corresponding response spectrum, is significantly influenced by the size of the earthquake, the travel path between source and receiver, and the local site effect. Sorting out the influences of these various factors is challenging. Past studies on response 1 Research Associate Professor, Dept. of Geophysics, Colorado School of Mines, Golden, CO Professor, Seismological Laboratory., University of Nevada-Reno, Reno, NV Research Geophysicist, US Geological Survey, Golden, CO

2 spectra have been limited by the number of strong motion data available. Most studies (e.g. Seed et al., 1976; Mohraz, 1976; Seed and Idriss, 1979; Singh, 1985; Atkinson and Boore, 1990; Crouse and McGuire, 1996; Sabetta and Pugliese, 1996) were based on combined data sets that come from different sizes of earthquakes and recorded from different regions. Thus, the isolation of individual factors from various influences is limited by the nature of the data. Recently, as a result of the great improvements in strong motion instrumentation and deployment, as well as the occurrence of several large destructive earthquakes in California, Japan, Turkey and Taiwan, the number of strong motion recordings increased dramatically. Just two of the earthquakes, the Northridge, California earthquake (1994, Mw=6.7) and the Chi-Chi, Taiwan earthquake (1999, Ms=7.6), provided over a thousand strong motion records. They are also excellent strong motion data sets in terms of data quality, the range of recorded ground motion, and the range of travel distances and site conditions. These new data provide a great opportunity to study strong ground motion and examine various factors influencing their response spectra. In this paper, we have investigated the influence on response spectra from earthquake magnitude, propagation path, and local site effects. Our intention is to study the influence of each of these factors separately. This requires event specific analyses, separating data of each event into different groups, and making statistical analysis accordingly. Large numbers of ground-motion data made this possible. We have also compared our results with the recent IEEE (the Institute of Electrical and Electronics Engineers) standards. Data We have collected strong motion data from the Central Weather Bureau of Taiwan, which operates the strong motion array in Taiwan, the Guerrero strong motion accelerographs array jointly operated by the Seismological Laboratory at the University of Nevada, Reno and the Instituto de Ingenieria, UNAM, and from the uniformly processed strong motion database at the Pacific Earthquake Engineering Research Center (web site: and through Dr. Walter Silva of Pacific Engineering and Analysis who has also built this database. To study individual factors, such as earthquake magnitude, travel distance and site response independently, strong motion data from different events were analyzed separately and data from each event were divided into groups according to their source-site distances and station site conditions. The source-site distance (D), defined as the closest distance between the station and source, is divided into four ranges, D<20 km, 20<D<50 km, 50<D<100 km, and D>100 km. Both Chi-Chi and Northridge earthquakes have been extensively recorded, and the large numbers of ground-motion data made event specific statistical analysis possible. Only horizontal components were used in this analysis. 1. Distance Effect on Response Spectra The effect of distance on response spectra can be examined by comparing response spectra from the same site category at different distances. Figure 1 compares average acceleration spectra in four different distance ranges for the Chi-Chi earthquake. From the 2

3 figure we can see that for the hard sites (Figure 1a), the shapes of the averaged response spectra are generally similar among different distance groups. The spectral acceleration gradually increases with the increasing frequency, reaches peak at about 1 Hz and slowly decreases towards their averaged PGA at high frequencies. The spectral amplitude of each distance group steadily decreases with increasing distance. For the medium and soft soil sites (Figure 1b and 1c), however, this attenuation pattern is not as uniform. While the spectral amplitude is steadily decreasing with increasing distance for hard sites, it is flatter for soft soil sites. Figure 2 compares average acceleration response spectra for different distance groups from the Northridge earthquake. The general features are similar to those shown in Figure 1, although differences in the attenuation due to site condition are less pronounced on Figure 2. Other earthquakes we studied display a range of behavior consistent with the distance dependence shown by these two earthquakes. Figure 1: Comparison of average acceleration spectra of the same site category at different distance ranges for the Chi-Chi earthquake. The distance ranges are D<20km, 20<D<50km, 50<D<100km, and D>100km for all three frames. Each line is obtained by averaging all the data in that site-distance group. Although some lines nearly tangent in places, none of the lines cross. Figure 2: Comparison of average acceleration spectra of the same site category at different distance ranges for the Northridge earthquake. The distance ranges are D<20km, 20<D<50km, and 50<D<100km for all four frames. 3

4 2. Local Site Effect on Response Spectra To examine local site effects on response spectra, we compared averaged acceleration response spectra of different site categories within the same distance range for each event. Figure 3 is the result from the Chi-Chi earthquake. It shows that the spectral response curves from different site conditions differ in both amplitude and spectra shape. In Figures 3a and 3b, which have data for distances within 50 km, the response spectra are higher for soft soil sites than for medium or hard sites at frequencies from about 0.1 Hz to 1 Hz, but the situation is opposite at higher frequencies. The crossover occurs at about 1 Hz. At lower frequencies, the apparent cause of soil amplitudes being larger than rock motions is large site amplification at the softer soil sites. At higher frequencies, however, soil motions become lower than rock motions. This characteristic is most easily explained by the nonlinear site response of softer sites. In Figure 3d, which has data from distances over 100 km, the soil motion is consistently higher than rock motion over the whole frequency range observed and there is no crossover as in Figures 3a and 3b. At this distance range we generally do not expect nonlinearity to occur. In Figure 3c for the distance range of 50 to 100 km, the soil motion is greater than the rock motion at lower frequencies and they become close to each other for frequencies over 1 Hz. This could be due to the result of decreasing soil nonlinearity within this distance range. Figure 4 shows the result from Northridge earthquake. It has a similar soil-rock crossover pattern as discussed for the Chi-Chi earthquake with the crossover also occurring at about 1 Hz (Figure 4a). Figure 3: Comparison of average acceleration spectra for different site categories within the same distance range for the Chi-Chi earthquake. Each line is obtained by averaging all the data in that sitedistance group. Different line style indicates different site conditions, as labeled. 4

5 Figure 4: Comparison of average acceleration spectra for different site categories within the same distance range for the Northridge earthquake. Different line style indicates different site conditions, as labeled. 3. Earthquake Magnitude Effect on Response Spectra A good example for looking at the magnitude effect on the shape of response spectra would be to compare response spectra from earthquakes of different size that occurred in the same source area and were recorded at the same station. Such an approach is closely represented in Figures 5 and 6. Figure 5 shows the epicenter of the Chi-Chi mainshock and an aftershock that occurred next to the mainshock epicenter. Both earthquakes were very well recorded. Figure 6 compared response spectra from these two events recorded by a station on a soft soil site (TCU116) and a hard site (TCU089). The response spectra were normalized by peak acceleration in order to focus on the comparison of the spectral shape. We can see the normalized spectra of the mainshock are much higher than that of the aftershock at lower frequencies, indicating that the mainshock generates proportionately more long-period energy than the aftershock. Figure 7 compares response spectra from six earthquakes with a range of magnitudes from 3 to 8 recorded in Guerrero, Mexico (earthquake data are from Figure 5: Map shows the epicenter of the Chi-Chi mainshock (star) and an aftershock (circle) and the location of a rock site (TCU089) and a soil site (TCU116). The ruptures fault trace is shown as a thick solid line. Figure 6: Comparison of response spectra from the Chi-Chi mainshock and the aftershock recorded at: (a) the soft soil site (TCU116), and (b) the rock site (TCU089). Response spectra were normalized to a common peak 5

6 Anderson and Quaas, 1988). All these earthquakes were recorded on hard rock sites with S-P travel time of about 3 seconds. Thus, the recording site conditions and the travel paths are quite similar among all these records. We can see from the figure that the magnitude of the earthquake has a strong effect on the shape of the response spectra. The larger earthquake generates much more long-period energy than the smaller ones. In Figure 8 we have compared averaged acceleration spectra from the Chi- Chi, Kocaeli, Landers, Kobe, Loma Prieta, and Northridge earthquakes. Although these earthquakes are from different tectonic regions, we compare them together in order to gain the general picture of the response spectra from these recent earthquakes. Each curve in the figure is obtained by averaging all available data within 50 km of the event. Figure 7: Comparison of response spectra for earthquakes with magnitude 3 to 8 recorded on rock sites in Guerrero, Mexico. Response spectra were normalized to a common peak Figure 8a compared average acceleration spectral shape for soil sites and Figure 8b for rock sites. In general, the shape of the response spectra, in spite of regional differences, has characteristics similar to those discussed above. Larger sized earthquakes have proportionately more low frequency energy and less high frequency energy relative to smaller ones. (a) (b) Figure 8: Comparison of average acceleration spectra among Chi-Chi, Kocaeli, Landers, Kobe, Loma Prieta and Northridge earthquakes. Each curve represents one earthquake with the spectrum averaged from data within distance of 50 km of the earthquake. Response spectra were normalized to a common peak acceleration for the comparison. (a) For soil sites (the curve for Chi-Chi earthquake is from soft soil sites, others are from site D sites). (b) For rock sites (the curve for Chi-Chi earthquake is from hard sites, others are from site A sites). 6

7 Comparison of Our Results with Recent IEEE Standards Since individual response spectra of actual earthquakes are highly irregular, for the purpose of describing ground motion for design, a smoothed response spectrum representing possible variability in ground motion (a design spectrum) is used. The design spectrum is usually determined by enveloping those of individual response spectra at a particular level, such as a p th percentile of the spectra. An 84 percentile of the spectra is commonly used by engineers, which is approximately mean-plus-one standard deviation of the spectra. Figure 9: Plots of 84 percentile acceleration spectra (approximately mean-plus-one standard deviation of the spectra) of different site categories within the same distance range for Chi-Chi earthquake compared with the standard response spectrum (thin solid line) specified by the IEEE for the high seismic performance level. Response spectra were normalized to a common peak acceleration. Figure 9 plots 84 percentile acceleration spectra of different site categories for the Chi- Chi earthquake compared with the standard response spectrum specified by the IEEE-693 for high level of seismic qualification of electric substations (which include all electric equipments). Significant differences in spectral shape between the data from the Chi-Chi earthquake and the IEEE-693 standard occurred at frequencies between about 0.1 and 2 Hz for soft soil sites within distance of 50 km. The average response spectra from the Chi-Chi earthquake at these distances are significantly higher than the standard spectrum defined by IEEE-693. This suggests that the current IEEE standard is not protecting those electric equipments if catastrophic earthquakes such as the Chi-Chi event should happen and if the affected equipment is vulnerable at this frequency range. Figure 10 compared 84 percentile acceleration spectra of different site categories with distance of less than 20 km for Northridge earthquake. Comparison shows that 7

8 the standard response spectrum specified by the IEEE-693 has effectively represented the input from the Northridge earthquake. We have also compared individual response spectra of the recent strong motion acceleration recordings with the standard response of IEEE 693 for high seismic performance level. Every event we have examined (see Figure 8 for the listing of the events) has some cases that the recorded acceleration response spectra have exceeded the IEEE standard for high seismic performance level. We found that those stations that recorded exceedances are generally located at sites that are: 1) very close to the fault rupture trace; 2) near the edge of the fault that suffer strong directivity effect; 3) on the hanging wall adjacent to the primary fault trace; 4) at the site that suffers large site amplification. In the low frequency range, the exceedance is usually associated with a larger magnitude earthquake at soft soil sites at near source distance (D<50 km). Due to the magnitude effect on response spectra, it is to be expected that for the rare earthquakes with magnitude significantly larger than Chi-Chi (M7.6), exceedances at long periods will be more common. Figure 10: Plot of 84 percentile acceleration spectra of different site categories within distance of 20 km for Northridge earthquake, compared with the standard response spectrum (thin solid line) specified by the IEEE for the high seismic performance level. Response spectra were normalized to a common peak acceleration. Discussion and Conclusion In this article, we have investigated the influence of earthquake magnitude, propagation path, and local site effects on response spectra. Through event-specific analysis and comparing data among different site and distance groups, we are able to obtain a more clear and realistic perspective on the influence of these individual parameters on the response spectra. We found response spectra to be strongly magnitude dependent. By comparing response spectra of different sized earthquakes, we found there was a marked increase in spectral amplitudes at low frequencies as magnitude increases. This is consistent with the seismological observations and is expected based on well-established seismic scaling laws (Aki, 1967; Brune, 1970). At extremely long periods (e.g. T~20 seconds) the amplitude is expected to increase 1.5MW proportional to 10 in the far field spectra. In the engineering community, however, design spectra in the past have not generally taken advantage of the magnitude dependency. There is some implicit dependency on the maximum magnitude in the IBC 2000 code. However, the knowledge of the broadband spectra from large earthquakes will certainly help us to be more 8

9 aware of the threat to structures from large earthquake ground motions and help implement corresponding measures in structural design to mitigate earthquake-related damages. We found a significant influence of local site effects on response spectra in both the Chi- Chi and Northridge earthquakes. By comparing average response spectra of different site categories within the same distance range for each event, we found soft soil sites showed amplification of factors of 2 to 3 at lower frequencies in comparison to about 50% at higher frequencies relative to rock sites. The characteristic of the crossover between response spectra from soil sites and rock sites is best explained by the nonlinear site response of softer sites. We have also compared our result with recent IEEE standards. The IEEE-693 spectrum for the high seismic performance level is exceeded by the average spectra at long periods for soft soil sites within distances of 50 km. The result is that it will be no surprise to see the low frequency part of the spectra exceeding the IEEE spectra in soil site categories in a significantly larger magnitude earthquake. Acknowledgements We thank Dr. Norm Abrahamson for his valuable suggestions to this work and Dr. Walter Silva for his help in data collection. This material is based upon work supported in part by the PEER grant SA2976JB, NSF grants CMS and CMS , and SCEC funding. References Abrahamson, N. A., and W.J. Silva (1997). Empirical response spectral attenuation relations for shallow crustal earthquakes, Seismological Research Letters. 68, Aki, K (1967). Scaling law of seismic spectrum, Journal of Geophy. Res. 72, Anderson, J. G and R. Quaas (1988). The Mexico earthquake of September 19, 1985 Effect of magnitude on the character of strong ground motion: An example from the Guerrero, Mexico strong motion network, Earthquake Spectra, Vol. 4, Atkinson, G. M. and D. M. Boore (1990). Recent trends in ground motion and spectral response relations for North America, Earthquake Spectra, Vol. 6, Boore, D.M., W.B. Joyner and T.E. Fumal (1997). Equations for estimating horizontal response spectra and peak acceleration from western North American earthquakes: a summary of recent work, Seism. Res. Lett. 68, Brune, J. N. (1970). Tectonic stress and the spectra of seismic shear waves from earthquakes, J. Geophys. Res. 75, Campbell, K. W. and Y. Bozorgnia (2003). Updated near-source ground-motion (attenuation) relations for the horizontal and vertical components of peak ground acceleration and acceleration response spectra, Bull. Seism. Soc. Am. 93, Crouse, C.B. and J.W. McGuire (1996). Site response studies for purpose of revising NEHRP seismic provisions, Earthquake Spectra, Vol. 12, Field, E.H. (2000). A modified ground motion attenuation relationship for southern California that accounts for detailed site classification and a basin-depth effects, Bull. Seism. Soc. Am. 90, s209- s221. 9

10 Mohraz, B., 1976, a study of earthquake response spectra for different geological conditions, Bull. Seism. Soc. Am. 66, Sabetta, F. and A. Pugliese (1996). Estimation of response spectra and simulation of nonstationary earthquake ground motions, Bull. Seism. Soc. Am. 86, Sadigh, K., C.Y. Chang, J.A. Egan, F. Makdisi, and R.R. Youngs (1997). Attenuation relationships for shallow crustal earthquakes based on California strong motion data, Seism. Res. Lett. 68, Seed, H.B. and Idriss, I. M., 1979, Influence of soil conditions on ground motions during earthquakes, Journal of the Soil Mechanics and Foundations Divisions, ASCE, 95, Seed, H.B., Ugas, C., and Lysmer, J., Site-dependent spectra for earthquake-resistant design, Bull. Seism. Soc. Am. 66, Singh, J. P. (1985). Earthquake ground motions: implications for designing structures and reconciling structural damage. Earthquake Spectra, Vol. 1,