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Q_P-Q_S relations in a sedimentary basin using converted phases

DEPARTMENT OF GEOSCIENCES DEIKE BUILDING PENNSYLVANIA STATE UNIVERSITY, UNIVERSITY PARK, PENNSYLVANIA 16802

Abstract

Determining Q in near-surface sedimentary rocks is important both for earthquake seismology and hydrocarbon exploration. Measurements can be performed in the laboratory or in situ. We present an in situ estimation of the relation between Q_S and Q_P in a 1.4-km-thick Tertiary sedimentary basin in the Gazli region, Uzbekistan, USSR, using the spectral ratio method. The data consist of three-component accelerograms from aftershocks of the 19 March 1984 M_S = 7.0 Gazli earthquake. Theoretically, the spectral ratio between a low-frequency direct S phase and a high-frequency S-to-P converted phase from the basin / basement interface should fit a straight line in log-amplitude-frequency space. The slope of this straight line yields a relation between the average apparent Q_S and Q_P for the sedimentary section. Results for stations with well-constrained spectral ratio slopes give a Q_S between 10 and 25 for Q_P between 10 and 70. Those with smaller, poorly constrained slopes give relations with a higher Q_S / Q_P ratio. These relations are compared with laboratory and in situ measurements of Q_S and Q_P and some theoretical relations. Effects of scattering on the apparent Q values are also considered. The well-constrained relations are consistent with a theoretical Q_P-Q_S relation with Q_S = 20 to 25 for Q_P = 50 to 70, with laboratory measurements on either dry or fully saturated rocks, and some in situ measurements. The poorly constrained relations are incompatible with any theoretical relation or in situ measurement. The poor fit and large Q_S / Q_P ratio are attributed to near-surface, frequency-dependent amplification effects.

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