Regional Variation of

Abstract

Records from broadband digital stations have allowed us to map regional variations of Lg coda Q across almost the entire United States. Using a stacked ratio method we obtained estimates of Q 0 (Lg coda Q at 1 Hz) and its frequency dependence, η, for 218 event-station pairs. Those sets of estimates were inverted using a back-projection method to obtain tomographic images showing regional variations of Q 0 and η. Q 0 is lowest (250–300) in the California coastal regions and the western part of the Basin and Range province, and highest (650–750) in the northern Appalachians and a portion of the Central Lowlands. Intermediate values occur in the Colorado Plateau (300–500), the Columbia Plateau (300–400), the Rocky Mountains (450–550), the Great Plains (500–650), the Gulf Coastal Plain and the southern portion of Atlantic Coastal Plain (400–500), and the portions of the Central Lowlands surrounding the high-Q region (500–550). The pattern of Q 0 variations suggests that the United States can be divided into two large Q provinces. One province spans the area from the Rocky Mountains to the Atlantic coast, is technically stable, and exhibits relatively high Q 0. The other extends westward from the approximate western margin of the Rocky Mountains to the Pacific coast, is tectonically active, and exhibits low Q 0. The transition from high to low Lg coda Q in the western United States lies further to the west than does an upper mantle transition for Q and electrical resistivity found in earlier studies. The difference in Q 0 between the western and eastern United States can be attributed to a greater amount of interstitial crustal fluids in the west. Regions of moderately reduced Q within the stable platform often occur where there are accumulations of Mesozoic and younger sediments. Reduced Q 0 in the southeastern United States may not be due to anelasticity but may rather be explained by a gradational velocity increase at the crust-mantle boundary that causes shear energy to leak into the mantle.