Variability of crustal attenuation in the northeastern United States from Lg waves

High-quality, digital seismograms from eight pairs of collocated earthquakes in the northeastern United States were analyzed to determine accurate source spectrum corner frequencies. This was accomplished by applying the empirical Green's function method to regional Pg and Lg (or Sg) phases recorded by vertical component seismographs of the U.S. National Seismographic Network (USNSN) and the Lamont-Doherty Cooperative Seismographic Network (LCSN) stations. The frequency band used was 0.5-16 Hz for USNSN and 1-30 Hz for LCSN records. The source spectrum corner frequencies for the eight larger earthquakes of the event pairs (magnitudes between m(b)(Lg) = 2.5 - 4.1) range from about 4.3 to 16.3 Hz. Based on the corner frequencies obtained independently from the empirical Green's function analysis, Sg or Lg wave displacement amplitude spectra up to 30 Hz were used to determine the crustal average Q factors along 87 event-station paths. These paths crossed diverse tectonic features in the northeastern United States and were in the epicentral distance range of 41 to 1394 km. We found that within the northeastern United States, the crustal average Q(Lg) we obtained was frequency dependent and showed spatial variability which correlated fairly well with the major tectonic features in the region. Our attenuation measurements indicated low Lg attenuation in the Adirondack Mountains with exposed Precambrian Grenville basement with Q(Lg) = 905 f(0.40), high Lg attenuation in the central Appalachian Province with Q(Lg) = 561-586 f(0.46-0.47), and an intermediate Lg attenuation in northern New England Appalachians with Q = 705 f(0.41).