Attenuation and Source Studies in Northern Eurasia
Semi-annual rept. 9 Jul 1991-8 Jan 1992
SAINT LOUIS UNIV MO DEPT OF EARTH AND ATMOSPHERIC SCIENCES
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Time series for Lg coda are being collected for numerous paths across a broad region of Eurasia, including the Barents shelf. To date we have collected seismograms generated by 30 earthquakes and 35 explosions and recorded at SRO, DWWSSN, and IRIS stations, as well as several stations of the former Soviet Union. A stacked spectral ratio method was applied to the coda of 85 Lg phases from those events to obtain values for Q and its frequency dependence at 1 Hz. Q values obtained so far vary between about 500 and 1000 and the frequency dependences range between 0.0 and 1.0. Attenuation coefficient values of fundamental-mode Rayleigh waves determined for four two-station paths across the Basin and Range province fluctuate between about -2.0 x 10-3 km-1 and 3.0 x 10 -3 km-1 at periods between 6 and 33 s. Particle motion plots indicate that many of those determinations are contaminated by arrivals from non-great circle paths and from waves generated at heterogeneities along the path. Attenuation coefficient determinations for the path MNV to ELK, which is approximately normal to structural trends in that region, are, however, relatively free from such contamination. Mean values for that path decrease rapidly with period from about 3.0 x 10-3 to about 0.7 x 10 km between 6 and 10 s and then decrease slowly to about 0.3 x 10-3 km-1 at 33 s. Standard deviations range between 0.2 x 10-3 and 0.3 x 10-3 km-1 for most of the period range, but increase to about 0.4 x 10 at periods between 6 and 8 s. These results indicate that careful screening, based upon criteria such as three- dimensional particle motion, are necessary if surface wave attenuation data of sufficient quality to use for inverting for crystal anelasticity in complex regions are to be obtained.