Seismic Attenuation, Lithospheric Structure and Intracontinental Deformation in Alaska

Southern Alaska is part of the plate boundary region between the Pacific and North American plates and tectonic activity is consequently high there. Notably, deformation, seismicity and magmatism also occur much further inland, as much as ~1000 km from the plate boundary. Previous studies have shown that subcrustal velocity structure defines distinct lithospheric provinces that appear to be related to the distribution of intracontinental magmatism and deformation. To gain additional constraints on the seismic properties of the region and how they may relate to its tectonics, we characterize its teleseismic attenuation. We use data from deep teleseismic earthquakes recorded by the EarthScope Transportable Array and other temporary broadband deployments in Alaska and the Yukon Territory to measure P-wave attenuation across the region using a time-domain approach. Preliminary results are consistent with attenuation being dominated by sub-crustal structure and with expectations from the velocity-defined lithospheric domains. High-attenuation regions largely coincide with areas of Quaternary volcanism whereas the undeformed, seismically inactive area north of the Brooks Range exhibits low attenuation. There is good correspondence between intracontinental seismicity and the boundaries between high and low-attenuation blocks, especially along the Brooks Range and Richardson Mountains. An intriguing observation is that the high-attenuation inland regions exhibit t* values that are as large or larger than those measured above the arc and backarc, in line with a previous study by different authors. Our results support the ideas that teleseismic attenuation is dominated by subcrustal structure and that lithospheric heterogeneity localizes intracontinental deformation at the boundaries between strong and weak blocks. The observation of inland attenuation that matches or exceeds that found in the arc and backarc defies expectations and highlights the need for a better understanding of the effect of melt and volatile contents in mantle attenuation.