A study of high-frequency strong ground motion from the San Fernando earthquake

Abstract

This thesis describes an investigation of the attenuation of strong earthquake ground motion in the 0. 4 to 16 Hz frequency band during the M=6.4, February 9, 1971, San Fernando, California earthquake. It is found that Fourier amplitudes of ground acceleration decay according to a simple expression incorporating a geometric spreading term, and a material attenuation term with constant specific attenuation Q. The scatter in the amplitude data about an expected level given by the simple decay expression is nearly constant with respect to both frequency and focal distance. Fourier amplitudes of acceleration corrected to a reference hypocentral distance agree well with those determined by a two-parameter source model of the San Fernando earthquake. Focusing of energy to the south by the southward 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 classified as "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. In general, smoothed amplitude spectra from accelerograms recorded on 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 the frequency band considered, it is concluded that for the purposes of aseismic 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 amplitudes and those predicted by a simple two-parameter source model suggests that the model can be used for the assessment of strong ground motion to be used in design procedures. A procedure for estimating design earthquakes using the source model and the amplitude decay expression is presented.