This thesis describes an investigation of the attentuation ofstrong earthquake ground motion in the 0.4 to 16 Hz frequency bandduring the M = 6.4, February 9, 1971, San Fernando, California earthquake. It is found that Fourier amplitudes of ground acceleration decayaccording to a simple expression incorporating a geometric spreadingterm, and a material attenuation term with constant specificattentuation Q. The scatter in the amplitude data about an expectedlevel given by the simple decay expression is nearly constant withrespect to both frequency and focal distance. Fourier amplitudes ofacceleration corrected to a reference hypocentral distance agreewell with those determined by a two-parameter source model of theSan Fernando earthquake. Focusing of energy to the south by thesouthward propagating rupture is observed at frequencies below 8 Hz.The propagation of rupture was incoherent with respect to higher-frequency components.
The relationship between intensity of ground motion and site geology is examined. It is found that while, in general, sedimentary sites were accelerated more strongly than basement rock sites, no clear difference could be found between sedimentary sites classifiedas "soft" by Trifunac and Brady (1975) (generally recent alluvium) and those classified as having "medium" soil stiffness, generally consisting of older alluvium and sedimentary rock. The difference between amplitudes recorded on basement rock and sediments is more complex. Ingeneral, smoothed amplitude spectra from accelerograms recordedon basement rock are lower than smoothed amplitudes at corresponding sedimentary sites. However, basement site spectra show marked isolated peaks, as high as those from sedimentary sites at similar distances.This is attributed to the focusing effects of the irregular topography normally accompanying basement rock outcrops. In thefrequency band considered, it is concluded that for the purposes of a seismic design of structures no discrimination should be made between the intensity of ground motion expected on basement rock,sedimentary rock, and coarse-grained alluvium typical of Southern California.
The agreement between the recorded strong motion amplitudesand those predicted by a simple two-parameter source model suggeststhat the model can be used for the assessment of strong ground motionto be used in design procedures. A procedure for estimating designearthquakes using the source model and the amplitude decay expressionis presented.
