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Regionalized Rayleigh and Love wave attenuation coefficients have been measured across the Basin and Range province of western United States in the frequency range 0.02–0.2 Hz. The measurements were made by using the methods of Tsai and Aki (1969) and Yacoub and Mitchell (1977) adapted to work on any number of events simultaneously. Rayleigh wave Q values at low frequency approach values near 40, which are significantly lower than previous measurements in the western United States. Love wave Q values, on the other hand, are quite high at low frequencies. We suggest that interference between fundamental and higher modes may explain the Love wave observations. The Rayleigh and Love wave attenuation measurements have been inverted simultaneously for shear‐wave attenuation as a function of depth. A frequency‐independent Q model is consistent with both Rayleigh wave and short‐period Love wave attenuation data. The shear‐wave Q (Qβ) model is characterized by low Qβ in the lower crust (Qβ∼100) and Qβ decreasing in the upper mantle with lowest values (Qβ∼30) beneath 60 km depth. Forward modeling shows that a high‐Q lower crust or upper mantle lid is inconsistent with the data. Our interpretation of these results is that the lithosphere is poorly developed beneath the Basin and Range and that the partially molten asthenosphere may reach very shallow depths, possibly to the base of the crust. This interpretation of the Q model in conjunction with a number of geological and geophysical evidences suggests that attenuation mechanisms involving partial melt predominate in the lower crust and upper mantle of the Basin and Range.

Regional Moment EstimatesIn this paper, M o is estimated using spectral amplitudes of surface waves recorded at regional distances between 300 and 1300 kin. Waveform data are taken from broadband channels for a single station, WMQ. Figure 1 shows a map of the study area with contours of crustal thickness and epicenters plotted, where epicentral information may be found in paper 1. Data AnalysisIntermediate-period (5-50 s) surface waves are generally well recorded on broadband channels. Figure 2 shows plots of raw and low-pass-filtered waveforms and illustrates good signal-to-noise quality at low frequencies for one event. The presence of higher modes is also noted; other low-frequency phases, such as P,,t, can be well excited by sources in this study. Such energy poses some difficulties isolating fundamentalmode surface waves for spectral analysis, because time sepa-26,963

[1] Rayleigh wave excitation is studied for an explosion source model consisting of a superposition of isotropic (monopole), tensile failure, and tectonic release point sources. The body-force representation for shock-induced, deep-seated tensile failure is a compensated linear vector dipole CLVD, where the relative strength of the CLVD is given by an index K. Rayleigh wave amplitudes are reduced owing to destructive interference between an explosive monopole and a CLVD source with vertical axis of symmetry in extension (K > 1). The effect of tensile failure on M s is to enhance the explosion-like characteristics on a plot of m b -M s . This model suggests that the success of the m b -M s discriminant results from the fact that nuclear tests were conducted under containment practices for which tensile failure is ubiquitous, while the North Korean nuclear test of 9 October 2006 is a harbinger of poor m b -M s performance when tensile failure is completely suppressed.

[1] Classical explosion source theory relates isotropic seismic moment to the steady state level of the reduced displacement potential. The theoretical isotropic moment for an incompressible source region M t is proportional to cavity volume V c created by pressurization of materials around the point of energy release. Source medium damage due to nonlinear deformations caused by the explosion will also induce volume change V d and radiate seismic waves as volumetric, double-couple, and compensated linear vector dipole (CLVD) body force systems. A new source model is presented where K is a relative measure of moment M CLVD with respect to the net moment from volumetric sources V c and V d . K values from moment tensor inversions steadily decrease from ∼2.5 at lower yields to ∼1.0 for the highest-yield shots on Pahute Mesa. A value of 1.0 implies M CLVD = 0 and, by inference, small V d . We hypothesize that the extent to which damage adds (or subtracts) volumetric moment is controlled by material properties and dynamics of stress wave rebound, shock wave interactions with the free surface, gravitational unloading, and slapdown of spalled near-surface layers. This hypothesis is tested by comparing measurements of isotropic momentM I with estimates of M t based on V c scaling relationships and velocity-density models. The results support the hypothesis and the conclusion thatM I represents the "apparent explosion moment" since it has contributions from direct effects due to cavity formation and indirect effects due to material damage. Implications for yield estimation usingM I are discussed in general and for the North Korean tests.Citation: Patton, H. J., and S. R. Taylor (2011), The apparent explosion moment: Inferences of volumetric moment due to source medium damage by underground nuclear explosions,