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A comparison of direct S-wave and coda-wave site amplification determined from aftershocks of the Little Skull Mountain earthquake

Seismological Laboratory 174 University of Nevada, Reno Mackay School of Mines, Reno, Nevada 89557-0141

Abstract

Site amplifications of direct S waves and coda waves are studied and compared using high-quality, three-component digital data from aftershocks of the Little Skull Mountain, Neveda, earthquake. We use data from 12 stations installed on a variety of geological and topographic site conditions, distributed widely in space with different azimuths and epicentral distances. S-wave site amplifications are obtained in the frequency range from about 0.5 to over 30 Hz, while coda-wave site amplifications are obtained in a frequency range from about 1.5 to over 30 Hz. A thorough statistical analysis of these results was performed. We find that (1) for S waves, all three components at a station follow a similar frequency-dependent trend. The amplitudes of the two horizontal components match more closely, while the vertical component shows consistently lower amplification than the horizontal components at low frequencies. (2) For coda waves, all three components share both similar frequency-dependent trends and amplification level. (3) S-wave and coda-wave site amplification are consistent for stations with epicentral distance greater than about 10 km (which is about the average focal depth of the earthquakes we used). Within the epicentral distance of 10 km, however, some stations show discordant S-wave and coda-wave site amplifications. Possible factors are that the direct S waves are affected by particular wave propagation paths and that at short distance SV is partitioned with more energy on the horizontal components and less on the vertical components than at larger distances.