Azimuth, energy, Q, and temperature: Variations on P wave amplitudes in the United States

Abstract

Mean relative P wave (1 s) amplitude characteristics are determined for standardized seismic stations in the United States. Data are compiled from three azimuths: to the north, explosions in Russia; to the northwest, earthquakes along the Asian‐Pacific arc; and to the south‐southeast, earthquakes in South America. Characteristic receiver amplitudes are found to be azimuthally dependent. High‐amplitude areas occur in the central United States, and lowest amplitudes occur along the Rocky Mountain front range and the Rio Grande rift zone. Amplification effects of low‐velocity sediments are significant in the central United States. Azimuthal amplitude variations in the eastern United States are consistent with an anomalous attenuating or defocusing body in the lower mantle beneath Colombia, South America. Nonazimuthal amplitude studies are tied to the azimuthal data set to increase station amplitude coverage. The regional and local stability of amplitudes is analyzed. Amplitudes are stable with an average variation of a factor of about 1.3 around the regional mean. For seismic stations situated on hard bedrock, the squared amplitude variations are related to energy variations in the P wave fronts. Acknowledging the likely importance of elastic focusing/defocusing and scattering effects at individual stations, variations of anelastic attenuation are derived that are consistent with the observed energy‐amplitude variations. Assuming that the variations of anelastic attenuation occur in the upper 400 km of the earth, high‐amplitude stations may be unattenuated, low‐amplitude stations are consistent with Q ∼ 110, and a mean amplitude implies Q ∼ 220–240. The range of variation of Q is mapped into temperature variations, assuming a thermally activated Q mechanism and an olivine activation energy for the upper mantle. The apparent Q variations are consistent with temperature variations of 135°–287°C between high‐ and low‐amplitude seismic station sites. Geophysical and physiographic correlations with the P wave amplitude variations are discussed. Accounting for the motion of the North American plate, low P wave amplitudes along the Rocky Mountain front may be related to the uplift of the Colorado Plateau. Low amplitudes extending from northern Texas to New England lie along the approximate boundary of the continental craton.

Spatial variations of P wave attenuation in the mantle beneath North America

“…The first‐order pattern of high t * P in TNA and low t * P in SNA is consistent with previous studies of short‐period amplitude anomalies [ Cleary , 1967; Booth et al , 1974; Der et al , 1982; Butler , 1984], spectral ratio variations [ Solomon and Toksöz , 1970; Der and McElfresh , 1976, 1977; Der et al , 1982], Lg coda waves [ Baqer and Mitchell , 1998], and regional Rayleigh waves [ Mitchell , 1975]. The contrast in t * P of 0.2 s between SNA and TNA is consistent with that inferred by Der et al [1982] who studied P wave spectra at frequencies up to 4 Hz.…”

“…A large number of studies have provided constraints on the attenuation structure in North America, including analyses of short‐period amplitude anomalies [ Cleary , 1967; Booth et al , 1974; Butler and Ruff , 1980; Der et al , 1982; Butler , 1984], spectral ratio variations [ Solomon and Toksöz , 1970; Der and McElfresh , 1976, 1977; Der et al , 1982], Lg coda waves [ Baqer and Mitchell , 1998], and ScS multiples [ Lay and Wallace , 1988]. The studies show a common first‐order pattern of a highly attenuating TNA and low‐attenuation SNA.…”

Investigation for anisotropic attenuation in the Pacific

“…5 is easily derived (e.g. Butler 1984). For instance, under the assumptions stated, the fluctuations in Fig.…”

Effects of lateral velocity heterogeneity under the Nevada Test Site on short-periodP wave amplitudes and travel times

“…This type of correlation is generally an indication of focussing-defocussing effects (e.g., CHANG and VON SEGGERN, 1980;BUTLER, 1983). Thermally activated variations in attenuation, on the other hand, would predict a correlation of slow times with low amplitudes (cf., BUTLER, 1984). …”