1. Local Site Effects Seismic Site Response Analysis CEE 531/ESS 465

2. Local Site Effects Ground surface motions are affected by local site conditions Site effects can influence: Amplitude - may amplify or de-amplify motion Frequency content - may shift to higher or lower  Duration - may extend duration of strong shaking

3. Important Aspects of Earthquake Motion Earthquake damage is influenced by ground motion: Amplitude Frequency content Duration Low amplitude High amplitude

4. Important Aspects of Earthquake Motion Earthquake damage is influenced by ground motion: Amplitude Frequency content Duration Low frequency motion High frequency motion  FAS 

5. Important Aspects of Earthquake Motion Earthquake damage is influenced by ground motion: Amplitude Frequency content Duration Short duration Long duration

6. Site Response Problem: Predict the response of a soil deposit due to earthquake excitation Source Path Site

7. Site Response Soil response depends on: Type of soil Thickness of soil Stiffness of soil Results: Some soil deposits amplify bedrock motion Some soil deposits de-amplify bedrock motion Some soils do both Bedrock

8. Site Response 1985 Mexico City Earthquake M = 8.1 Over 200 miles away Young lake deposits University Communications Building 30 m soft clay Rock

9. Site Response 1985 Mexico City Earthquake M = 8.1 Over 200 miles away Rock – 0.03g Soft clay – 0.15g Soft clay amplified bedrock motions by factor of 5 Soft clay amplified motions at periods of ~2 sec. even more

10. Site Response 1989 Loma Prieta Earthquake M = 7.1 Over 60 miles away Oakland Yerba Buena Island Treasure Island San Francisco Yerba Buena Island Treasure Island Rock Soft soil

11. Site Response 1989 Loma Prieta Earthquake M = 7.1 Over 60 miles away Rock – 0.06g Soft soil – 0.15g Rock Soft soil Soft soil amplified bedrock motions by factor of 2-3 Amplification is even larger in response spectrum at some periods

12. Evidence 1957 San Francisco Earthquake (M = 5.3) Rock Dense sand Soft clay

13. Evidence Compilation of data from multiple events Rock Stiff soils Loma Prieta Mexico City Calculations Peak acceleration on rock (g) Peak acceleration (g) Peak acceleration For low loading levels, strong amplification can occur For high loading levels, deamplification can occur When? Why?

14. Evidence Compilation of data from multiple events Soft to medium clay and sand Deep cohesionless soils Stiff soils Rock S a /a max Short-period Long-period

15. Summary Local site conditions can strongly influence: Amplitude Frequency content Duration Shown by: Measured surface motions Compilations of data on local site effects Measured amplification functions Theoretical analyses