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Particle-velocity attenuation relations: San Fernando earthquake of February 9, 1971

OFFICE OF EARTHQUAKE STUDIES U.S. GEOLOGICAL SURVEY, DENVER, COLORADO 80225

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 log₁₀. The attenuation of Modified Mercalli intensity with distance and the horizontal particle velocity are: log₁₀ = 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 log₁₀v^H = 3.68 – 1.25 log₁₀ and log₁₀v^V = 2.91 – 1.03 log₁₀, respectively. Other relations for different spectral components are also presented.

The ratios of horizontal to vertical pseudo-spectral velocities (v^H/v^V) are 3.5 for a 3-sec-period wave at = 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 m_b = 7.2 shows a difference of one order of magnitude in the level of motion at T = 0.3 sec, and the attenuation for m_b = 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, m_b = 6.2 (NEIS). The absorption coefficient for a T = 0.1-sec wave is = 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 ^–1 in this frequency range.

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