Effect of Site Conditions on Earthquake Intensity

A study is reported of over 100 strong motion earthquake accelerograms at seven sites based on interpretation of integrated and balanced strong motion acceleration records and 5% damped pseudo-velocity response spectra from these records; pertinent site properties computed from reported relationships; epicentral distances and earthquake magnitudes; and wave theory applied to bounded elastic media having constant or linearly variant properties. It is concluded that no strong preferential site period was amplified on a maximum pseudo-velocity response spectrum during strong motion earthquake; spectra velocities at soft layer sites average twice the base spectrum values; normal amplification near the earthquake mechanism period is approximately 3.2 times that of the base spectrum; an equation for the maximum velocity of the Helena earthquake is not recommended for design use because of the large number of unknowns. Further study is recommended into the influence of foundation rigidity and structural mass and stiffness related to base motion of the structures and the effective depth of impedence measurement; correlation, from standard soil mechanics methods, of shear strength to shear impedences; and transient random disturbances propagating up through energy absorbent elastic materials.