Regionalization of crustal coda Q in the continental United States

Abstract

A contour map of crustal Q0 values for the entire United States is presented based on a scattering model to explain the coda waves of local and near regional earthquakes. These coda Q0 values are in good agreement with Q of Lg waves. Data are obtained from over 250 local earthquakes, with most magnitudes between 3.0 and 5.0, recorded by 25 WWSSN and 27 LRSM stations throughout the continental United States. These two sets of data provide a range of frequencies from 0.5 to 3.5 Hz. A frequency dependence of Q is observed in the range of frequencies considered. A power law dependence of the form Q = Q0(f/f0)η is assumed. The value of frequency dependence η, is found to be maximum in the tectonically active western United States and least in the stable regions of the central and south central United States. The lowest Q0 values are obtained in the western United States, with values ranging from 140 to 200 in the Coastal Plains. Average crustal Q0 values for the Basin and Range province vary from 200 to 300, increasing gradually to 400 in the Colorado Plateau region. Q0 values in the Columbia Plateau vary from 200 to 400. Q0 values increase very rapidly along the central and southern Rocky Mountains from 400 to 800. East of the Rocky Mountains Q0 values increase gradually in the Interior Plains to a maximum value of around 1300 in the Mississippi Embayment region. Farther east Q0 values decrease gradually to an almost constant value of 1000 along the Appalachian Mountains. Coastal regions of eastern and northeastern United States have a crustal Q0 value between 900 to 700. Regions of north central and Gulf Coastal Plains show low Q0 values, ranging from 600 to 400 and less. The source factor B(fp), a measure of the intensity of scattering, was obtained for several different regions of the continental United States. Regions of high Q0 values exhibit low intensity of scattering.