Seismic Site Response Analysis Local Site Effects CEE

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

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 w • Duration - may extend duration of strong shaking

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

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

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

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

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

Site Response 1985 Mexico City Earthquake M = 8. 1 Over 200 miles away Communications Building Young lake deposits University 30 m soft clay Rock

Site Response 1985 Mexico City Earthquake M = 8. 1 Over 200 miles away Rock – 0. 03 g Soft clay – 0. 15 g Soft clay amplified bedrock motions by factor of 5 Soft clay amplified motions at periods of ~2 sec. even more

Site Response 1989 Loma Prieta Earthquake M = 7. 1 Yerba Buena Island Over 60 miles away Treasure Island Oakland San Francisco Yerba Buena Island Rock Treasure Island Soft soil

Site Response 1989 Loma Prieta Earthquake M = 7. 1 Over 60 miles away Rock – 0. 06 g Soft soil – 0. 15 g Soft soil amplified bedrock motions by factor of 2 -3 Amplification is even larger in response spectrum at some periods Soft soil Rock

Evidence 1957 San Francisco Earthquake (M = 5. 3) Rock Dense sand Rock Soft clay Rock

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

Evidence Compilation of data from multiple events Sa/amax Short-period Soft to medium clay and sand Deep cohesionless soils Stiff soils Rock Long-period

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