Lg coda Q variation across Eurasia and its relation to crustal evolution

An extensive set of broadband and short-period seismograms from regional seismic events has been used to derive a tomographic image of broad-scale variations of Lg coda Q for most of Eurasia. The image indicates that Lg coda Q, at a frequency of 1 Hz, varies between about 200 and 1000. Lowest Q values (<350) all lie within the Tethysides orogenic belt that was formed from the collision of the African/Arabian and Indian plates with the Eurasian plate. Relatively low Q values are found in the Arabian Peninsula (350-500), a region of recent uplift and volcanism, and in the eastern portion of the Altaid belt (similar to 450), a region covered by thick sediments. High Lg coda Q values (>800) are found in four regions, the East European shield, the Siberian shield, the Indian shield, and the southwestern (and oldest) portion of the Altaid belt. The pattern of Lg coda Q variation across Eurasia suggests that crustal Q in any region is directly proportional to the length of time that has elapsed since the most recent episode of large-scale compressional tectonic activity or upper mantle heating that occurred there. We interpret the low Q as resulting largely from hydrothermal fluids, generated by tectonic activity or heating, that now reside in cracks and permeable portions of the Eurasian crust. Crustal Q increases with time, after that activity, as fluids are lost to the surface or absorbed by retrograde metamorphism.