Abstract

Accelerograms recorded at 61 stations during the San Fernando earthquake were integrated and used to determine the resultant horizontal and the vertical particle velocity as a function of distance. The attenuation of the average value of particle velocity, in the period range from 0.28 to 1.4 sec, is described by the relationship: log10v = 3.19 − 1.35 log10Δ. The attenuation of Modified Mercalli intensity with distance and the horizontal particle velocity are: log10Δ = 3.05 − 0.25 l and log10v = −0.93 + 0.29 l. Attenuation relations obtained in the frequency domain for the horizontal and vertical pseudo-spectral velocities of 3-sec-period waves are log10vH = 3.68 − 1.25 log10Δ and log10vV = 2.91 − 1.03 log10Δ, respectively. Other relations for different spectral components are also presented.

The ratios of horizontal to vertical pseudo-spectral velocities (vH/vV) are 3.5 for a 3-sec-period wave at Δ = 10 km, and 3.2 for a 1-sec-period wave in the distance range from 10 to 100 km. Comparison between the particle-velocity attenuation curve found in this study and that determined for a design earthquake for the central United States having mb = 7.2 shows a difference of one order of magnitude in the level of motion at T = 0.3 sec, and the attenuation for mb = 6.2 and T = 3.0 sec agrees fairly well with the observed particle-velocity mean attenuation curve derived from the integrated accelerogram of the San Fernando earthquake, mb = 6.2 (NEIS). The absorption coefficient for a T = 0.1-sec wave is σ = 0.0225/km. The horizontal pseudo-spectral velocity mean curves show a similar rate of attenuation for periods between 0.1 to 3.0 sec, suggesting a decay rate for the displacement-source spectrum of ω−1 in this frequency range.

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