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Approximate method to estimate the short-period strong motion spectra on bedrock

J. A. BLUME EARTHQUAKE ENGINEERING CENTER DEPARTMENT OF CIVIL ENGINEERING STANFORD UNIVERSITY, STANFORD, CALIFORNIA 94305

Abstract

The methodology to estimate the strong motion Fourier amplitude spectra in a short-period range (T 1 to 2 sec) on a bedrock level is discussed in this paper. The basic idea is that the synthetic strong motion Fourier spectrum calculated from smoothed rupture velocity model (Savage, 1972) is approximately similar to that of low-pass-filtered strong earthquake ground motion at a site in a period range T 1 to 2 sec: . is an observed Fourier spectrum on a bedrock level and A() is a low-pass filter. As a low-pass filter, the following relation,

is assumed. In order to estimate the characteristic coefficients {n} and {a}, the Tokachi-Oki earthquake (1968), the Parkfield earthquake (1966), and the Matsushiro earthquake swarm (1966) were analyzed. The results obtained indicate that: (1) the coefficient {n} is nearly two for three earthquakes, and {a} is nearly one for the Tokachi-Oki earthquake, eight for the Parkfield earthquake, and four for the Matsushiro earthquake swarm, respectively; (2) the coefficient {a} is related with stress drop as (a = 0.07.). Using this relationship between {a} and , the coefficients {a} of past large earthquakes were estimated.

The Fourier amplitude spectra on a bedrock level are also estimated using an inverse filtering method of .

Footnotes

^* Visiting Postdoctoral Research Fellow from Central Research Institute of Electric Power Industry, Chiba Prefecture, Japan.