State-of-the-Art for Assessing Earthquake Hazards in the United States. Report 10. Attenuation of High-Frequency Seismic Waves in the Central Mississippi Valley.
SAINT LOUIS UNIV MO DEPT OF EARTH AND ATMOSPHERIC SCIENCES
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This study was concerned with the attenuation of high-frequency earthquake waves in the central Mississippi valley. The data were obtained from seismographs which measured the vertical component of ground motion. Recording was on analog magnetic tape and on 16-mm photographic film. Most attention was devoted to a study of Lg waves, which are higher mode surface waves that produce the largest ground motion. At frequencies of 1 to 10 Hz the specific dissipation factor Q of the Lg waves was found to be 1500. The results of the present study indicate that in the central United Sates high-frequency waves will produce significant ground motions at relatively large distances, which is not the case in California or other tectonic areas. In the frequency domain the Fourier displacement spectra of the Lg motion showed the typical flat level at the lower frequencies and the rapid fall-off of amplitude with increasing frequency at the high frequencies. In the time domain, on the other hand, the level of ground displacement in the source region was essentially constant between frequencies of 1 to 10 Hz. This fact, together with the relatively low attenuation of the Lg waves, indicates that waves of frequencies as high as 10 Hz may have relatively large amplitudes and accelerations at epicentral distances as large as hundreds of kilometers for central United States earthquakes.