Ground Motion Amplification of Soils in the Upper Mississippi Embayment

The results of this study were compared with several attenuation relationships developed for the Central and Eastern U.S.   The comparisons with Atkinson and Boore (1997) and Toro et al. (1997) are shown below for several spectral accelerations.   The comparisons were made for a moment magnitude of 6.5 since this study calculated spectral accelerations at 5 distances for this magnitude.   The Atkinson and Boore (1997) relationship uses a two-corner frequency, w² model.   Since Atkinson and Boore (1997) used the quarter-wavelength approximation (Joyner, et al., 1981) to estimate the amplification of a soil site relative to a rock site, it does not include the effects of nonlinear soil behavior.   Toro et al. (1997) use a one-corner frequency, w² model to calculate ground motions.   The attenuation relationship developed by Toro et al. (1997) for the Midcontinent region is shown here.   The EPRI (1993) amplification factors were used to adjust rock spectral accelerations for soil sites.

• Peak Ground Acceleration
  

• 0.2 Second Spectral Accelerations
  

• 1 Second Spectral Accelerations
  

The peak ground acceleration (PGA) computed by this study were compared with the attenuation relationships developed by Atkinson and Boore (1997) and Toro et al. (1997) for PGA.   Figure 1 compares the results for rock sites.   The difference between the Atkinson and Boore (1997) attenuation relationship and the results of this study are largely due to differences in the stochastic modeling of rock motions.

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Figure 1 Comparison of attenuation relationships and calculated rock PGA for a moment magnitude of 6.5.

The attenuation relationships for soil were compared with the results of the Uplands (Pleistocene-age) and Lowlands (Holocene-age) profiles.   The results are shown in Figures 2 and 3.   The linear case assumes the depth of the soil column is 1000 m and constrains the soil column to behave linearly at all depths and under all strain levels.   The results of the linear case are compared with the results of the nonlinear cases for three embayment depths: 100, 600, and 1000 meters.

The effects of nonlinear soil behavior reduce PGA for all hypocentral distances.   However, as expected, the effect is more pronounced at short distances where the induced shear strains are expected to be larger than at larger distances.   At distances less than 100 km, the attenuation relationships overestimate PGA due to the significant nonlinear soil behavior that occurs.   The underestimation of PGA at distances greater than 200 km may be due to differences in geometric attenuation models between the model used in this study and that used by others.

Figure 2 Comparison of attenuation relationships and calculated PGA for the Uplands profile for a moment magnitude of 6.5.

Figure 3 Comparison of attenuation relationships and calculated PGA for the Lowlands profile for a moment magnitude of 6.5.

Figure 4 compares the results of this study with the attenuation relationships developed by Atkinson and Boore (1997) and Toro et al. (1997) for rock sites at a period of 0.2 sec.   The results of this study are comparable with the published attenuation relationships.

Figure 4 Comparison of attenuation relationships and calculated rock spectral accelerations for a period of 0.2 second and a moment magnitude of 6.5.

The attenuation relationships for soil were compared with the results of the Uplands (Pleistocene-age) and Lowlands (Holocene-age) profiles.   The results are shown in Figures 5 and 6.   The effects of nonlinear soil behavior reduce spectral accelerations at short periods for all hypocentral distances since nonlinearity controls behavior at short periods.   The results of the linear analyses are comparable to the published attenuation relationships.

Figure 5 Comparison of attenuation relationships and calculated spectral accelerations for the Uplands profiles for a period of 0.2 sec.

Figure 6 Comparison of attenuation relationships and calculated spectral accelerations for the Lowlands profiles for a period of 0.2 sec.

Figure 7 compares the results of this study with the attenuation relationships developed by Atkinson and Boore (1997) and Toro et al. (1997) for rock sites at a period of 1 sec.   The results of this study are comparable with the attenuation relationships developed by Toro et al. (1997).

Figure 7 Comparison of attenuation relationships and calculated rock spectral accelerations for a period of 1 second at a moment magnitude of 6.5.

The results for the soil profiles are shown in Figures 8 and 9.   The difference between the linear profile and the nonlinear profiles is negligible at a period of 1 second.   At longer periods, amplification controls site response and the effect of nonlinear soil behavior is negligible.   The 100-meter profiles have a fundamental resonance near 1 second that is responsible for the larger spectral accelerations for this profile.   The attenuation relationship developed by Toro et al. (1997) adequately estimates spectral accelerations for the Uplands profile but may underestimate spectral accelerations for the Lowlands profile.   This is due to the low-velocity deposits in the Lowlands profile amplify rock motions more significantly than the Uplands profile.

Figure 8 Comparison of attenuation relationships and calculated spectral accelerations for the Uplands profiles for a period of 1 sec.

Figure 9 Comparison of attenuation relationships and calculated spectral accelerations for the Lowlands profiles for a period of 1 sec.

References

• Atkinson, G.M. and D.M. Boore (1997), "Some Comparisons Between Recent Ground-Motion Relations," Seismological Research Letters, Vol. 68, No. 1, pp. 24-40.

• EPRI (1993), Guidelines for Determining Design Basis Ground Motions, Palo Alto, CA, Electric Power Research Institute, Vol. 1, EPRI TR-102293.

• Joyner, W.B., R.E. Warrick, and T. Fumal (1981), "The Effect of Quaternary Alluvium on Strong Ground Motion in the Coyote Lake, California, Earthquake of 1979," Bulletin of the Seismological Society of America, Vol. 71, No. 4, pp. 1333-1349.

• Toro, G.R., N.A. Abrahamson, and J.F. Schneider (1997), "Model of Strong Ground Motions from Earthquakes in Central and Eastern North America: Best Estimates and Uncertainties," Seismological Research Letters, Vol. 68, No.1, pp. 41-57.

Contents

Summary

• Site Classification Using Remote Sensing Imagery

• Development of Reference Shear Wave Velocity Profiles

• Stochastic Approach for Modeling Rock Motions

• Site Response Analyses

• Attenuation Relationships