Attenuation of seismic waves in the crust and the upper mantle has been studied in three global rift systems: the Baikal rift system (Eurasia), the North Tanzanian divergence zone (Africa) and the Basin and Range Province (North America). Using the records of direct and coda waves of regional earthquakes, the single scattering theory [Aki, Chouet, 1975], the hybrid model from [Zeng, 1991] and the approach described in [Wennerberg, 1993], we estimated the seismic quality factor (QC), frequency parameter (n), attenuation coefficient (δ), and total attenuation (QT). In addition, we evaluated the contributions of two components into total attenuation: intrinsic attenuation (Qi), and scattering attenuation (Qsc). Values of QC are strongly dependent on the frequency within the range of 0.2-16 Hz, as well as on the length of the coda processing window. The observed increase of QC with larger lengths of the coda processing window can be interpreted as a decrease in attenuation with increasing depth. Having compared the depth variations in the attenuation coefficient (δ) and the frequency (n) with the velocity structures of the studied regions, we conclude that seismic wave attenuation changes at the velocity boundaries in the medium. Moreover, the comparison results show that the estimated variations in the attenuation parameters with increasing depth are considerably dependent on utilized velocity models of the medium. Lateral variations in attenuation of seismic waves correlate with the geological and geophysical characteristics of the regions, and attenuation is primarily dependent on the regional seismic activity and regional heat flow. The geological inhomogeneities of the medium and the age of crust consolidation are secondary factors. Our estimations of intrinsic attenuation (Qi) and scattering attenuation (Qsc) show that in all the three studied regions, intrinsic attenuation is the major contributor to total attenuation. Our study shows that the characteristics of seismic wave attenuation in the three different rift systems are consistent with each other, and this may suggest that the lithosphere in the zones of these different rift systems has been modified to similar levels.Key words: Baikal rift system; East African rift system; North American Basin and Range Province ; attenuation of seismic waves; seismic quality factor; seismicity; coda waves; intrinsic attenuation; scattering attenuation; heat flow GEODYNAMICS & TECTONOPHYSICS P U B L I S H E D B Y T H E I N S T I T U T E O F T H E E A R T H ' S C R U S T S I B E R I A N B R A N C H O F R U S S I A N A C A D E M Y O F S C I E N C E
Abstract:The paper presents the results of deep seismic studies on Geophysical Reference Profile 1-SB (Sredneargunsk -Ust-Karenga -Taksimo -Vitim) in East Transbaikalia, Russia. The 1200 km long profile crosses the major tectonic structures of the Central Asian fold belt: the Argun median massif, the Selenga-Stanovoy and Transbaikalia folded regions, and the Baikal rift zone. Its northwestern fragment extends into the Angara-Lena monocline of the Siberian platform. The southeastern (Transbaikalia) and northwestern (Baikal-Patom) fragments of the profile are based on the spot (differential) seismic sounding technique using explosions and 40-tonne vibrators. The southeastern (Transbaikalia) fragment shows small crustal thickness values (~40 km), an almost horizontal position of the Moho, and high velocities of longitudinal waves (~8.4 km/sec) beneath the Moho. The analysis of parallelism graphs and the dynamic expression of the wave refracted from the Moho suggests a less than 5-10 km thick layer of high velocities and low gradients below Moho. The database on the territory of Transbaikalia includes ~200 wave arrival times from large earthquakes, which were refracted at the Moho at distances of ~200-1400 km. As part of the tomographic interpretation, using additional DSS data on the Moho, the territory of Transbaikalia has been mapped to show the patterns of the threshold velocity values at the Moho. The seismic data was used to contour an area with high velocity values in the mantle in the central part of the Mongolia-Okhotsk orogenic belt and the neighboring fold structures of Transbaikalia. According to the analysis of the seismic and geologic data on the study area, the mantle layer with high velocity values in the Mongolian-Okhotsk orogenic belt may be represented by the eclogitic rock plates.Key words: deep seismic sounding (DSS); velocity of longitudinal waves; hodographs from explosions and earthquakes; Moho; Mongolia-Okhotsk orogenic belt For citation : Soloviev V.M., Chechelnitsky V.V., Salnikov A.S., Seleznev V.S., Liseikin A.V., Galyova N.A., 2017. Specific velocity structure of the upper mantle in the Transbaikalia segment of the Mongolia-Okhotsk orogenic belt. GEODYNAMICS & TECTONOPHYSICS P U B L I S H E D B Y T H E I N S T I T U T E O F T H E E A R T H ' S C R U S T S I B E R I A N B R A N C H O F R U S S I A N A C A D E M Y O F S C I E N C E
The seismic quality factor (Qc), the frequency parameter (n), and the attenuation coefficient (δ) of seismic waves in the lithosphere of the southwestern flank of the Baikal rift system are obtained from records of S coda waves from regional earthquakes. There is a strong dependence of the quality factor on the frequency and coda window length: for central frequencies of 0.3 and 12.0 Hz and a coda window length W = 20 s, Qc varies from 47 ± 51 to 1036 ± 190, and for the same frequencies and W = 60 s, it varies from 83 ± 46 to 1596 ± 573. The values of the quality factor and the attenuation coefficient are calculated for the whole southwestern flank of the Baikal rift system and for individual tectonic blocks: the stable Siberian Platform, Khamar-Daban and East Sayan uplifts, and South Baikal and Tunka basins. A comparison of the obtained values of Qc with data on the age, tectonics, and seismic activity level of the structures shows that the quality factor depends on the recent tectonic activity of the structures, and to a lesser extent, on the age of the crust.
International audienceThe seismic effective quality factor (QC) and its frequently dependences or the frequency parameter (n) and attenuation coefficient (δ) for the Earth’s crust and upper mantle of the North Tanzanian divergence zone (East African rift system) were estimated from an analysis of the earthquake coda waves recorded in the SEISMO-TANZ’07 French-Tanzanian seismic experiment. The QC values increase and the n and δ values decrease with increasing frequency and length of the lapse time window. This behavior of the attenuation parameters may be evidence that the degree of heterogeneity of the lithosphere decreases with depth. Comparison of the depth variations in the attenuation coefficient δ and the frequency parameter n with the velocity structure of the region shows that there is a distinct change in the behavior of seismic wave attenuation at velocity discontinuities. The obtained attenuation parameters were compared with the same parameters obtained in our previous studies for other continental rift systems—the Baikal rift system (Eurasia) and the Basin and Range Province (North America)
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