A continent-wide map of 1-Hz Lg coda Q variation across Eurasia and its relation to lithospheric evolution

摘要

We present new maps of Lg coda Q (Q o and its frequency dependence) at 1-Hz that cover virtually all of Eurasia. Q o is relatively high, up to 650 or more, in the east European, Siberian, Indian, and Kazakh platforms but is surprisingly low (300–500) in the Arabian platform and western portion of the Siberian platform as well as in Great Britain and the northwestern portion of mainland Europe. It is generally low throughout the Tethysides orogenic belt, but there too it displays substantial regional variations (150–400). Most Q o anomalies appear to be related to the tectonic history of the Eurasian crust. The four regions with lowest values coincide with four of Eurasia's most active concentrations of earthquake activity. Observed Eurasian Q o values are consistent with a previously determined empirical plot of global values in which Q o in any region is proportional to the time elapsed since the most recent episode of tectonic or orogenic activity there. Comparisons of the new Q o map with maps of Rayleigh wave velocity, temperature, subducted lithosphere, and seismicity lead us to infer that Lg coda wave attenuation is most easily explained by energy lost in moving fluids through permeable rock or in being scattered from fluid-enhanced zones. The fluids are likely to have originated by hydrothermal release from subducting lithosphere or other upper mantle heat sources. Anomalies in Q o and other lithospheric properties in some cases extend well outside the Tethysides belt and other regions where upper mantle heat sources are known to occur. Possible explanations include earlier episodes of subduction activity, crustal deformation due to collision of the Afro-Arabian and Indian plates, or broader than expected back-arc activity beneath Eurasia.