No abstract

SUMMARY Measurements of fundamental-mode (FM) surface waves along the minor arc are impacted by overtone interference. This interference is primarily due to major-arc overtones for Rayleigh waves and minor-arc overtones for Love waves. In both cases, interference contaminates measurements of phase and amplitude and can introduce bias in seismic images. Here, we use synthetic seismograms computed via normal mode summation to probe how interference can vary as a function of the surface wave group velocities, source mechanism and depth, which control the relative excitation of the FM and overtones, and period. By comparing seismograms that include all overtones to those that include only the FM, we can quantify the interference, i.e., how the presence of the overtones perturbs the FM phase and amplitude. We compare the strength of this interference to calculations of excitation of the overtones and FM. We show that these calculations explain well the varying strength of interference for different source mechanisms and depths. Notably, these calculations illuminate source depths where Love wave overtone excitation is quite low and therefore interference is unusually weak, and depths where Rayleigh wave FM excitation is low and therefore interference is unusually strong. Our analysis also reinforces the dependence of the interference on the FM and overtone group velocities. For Love waves, this results in weak minor-arc overtone interference at long periods and, for continental paths, short periods. For Rayleigh waves, the differing overtone and FM group velocities and the relative excitation of the overtones and FM explain rapid variations in the strength of Rayleigh wave major-arc overtone interference as a function of epicentral distance. We then show that real data are affected by the relative excitation of the FM and overtones. We find that errors in Rayleigh wave phase velocities determined at the EarthScope USArray stations are larger when the ratios of overtone to FM excitation are larger. We also find a dependence of phase velocity error on excitation ratio for Love waves, and we identify the presence of major-arc overtone interference in Love wave measurements. Our results highlight opportunities for more nuanced quality control of surface wave measurements. The relative excitation ratio of the overtones and FM may be a better criterion for event selection than source depth, allowing deeper events that well excite FMs to be included and shallower events with large overtone excitation to be excluded. This would allow the collection of more accurate measurements that will increase the precision of seismic images.

Albite (NaAlSi3O8) is an aluminosilicate mineral. Its crystal structure consists of 3-D framework of Al and Si tetrahedral units. We have used Density Functional Theory to investigate the high-pressure behavior of the crystal structure and how it affects the elasticity of albite. Our results indicate elastic softening between 6–8 GPa. This is observed in all the individual elastic stiffness components. Our analysis indicates that the softening is due to the response of the three-dimensional tetrahedral framework, in particular by the pressure dependent changes in the tetrahedral tilts. At pressure <6 GPa, the PAW-GGA can be described by a Birch-Murnaghan equation of state with  = 687.4 Å3,  = 51.7 GPa, and  = 4.7. The shear modulus and its pressure derivative are  = 33.7 GPa, and  = 2.9. At 1 bar, the azimuthal compressional and shear wave anisotropy  = 42.8%, and  = 50.1%. We also investigate the densification of albite to a mixture of jadeite and quartz. The transformation is likely to cause a discontinuity in density, compressional, and shear wave velocity across the crust and mantle. This could partially account for the Mohorovicic discontinuity in thickened continental crustal regions.

We present evidence that measurements of minor-arc fundamental-mode (FM) Rayleigh waves experience interference from major-arc overtones, resulting in travel-time and amplitude measurements oscillating along a ray path. The oscillations are present in synthetic seismograms generated in a 3-D Earth model via SPECFEM3D_GLOBE and in a 1-D Earth model by mode summation. The absence of oscillations in synthetics containing only the FM indicates that the oscillations originate from higher-mode interference. This interference is present across multiple measurement techniques, including multi-channel cross correlation, phase-matched filtering, and cluster analysis. Experiments with 1-D synthetics suggest that contamination from interference is largest at epicentral distances greater than around 120 degrees, where record sections of seismograms show the major-arc overtones intersecting the minor-arc FM Rayleigh wave. The short wavelength of the interference pattern means it is only observable with dense station spacing and high data quality, which may explain why it has not, to our knowledge, been previously recognized. We show the interference is visible in real data. Its overall impact on phase-velocity maps is probably minor due to many measurements from shorter path lengths less prone to interference bias. However, phase-velocity maps constructed only from measurements at epicentral distances prone to interference exhibit significant noise and poor agreement with maps from measurements that include all path lengths; the issue is especially problematic for approaches that use differential travel times between nearby stations. Accounting for interference may diminish noise in measurements and improve the accuracy of images of the upper mantle.