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

Abstract

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 (∼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.