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 (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.¿ 1997 American Geophysical Union