Wave gradiometry for USArray: Rayleigh waves

摘要

Wave gradiometry (WG) is a new array data processing technique to extract phase velocity, wave directionality, geometrical spreading, and radiation pattern from spatial gradients of waveforms. A weighted inversion method and a reducing velocity method are introduced to compute spatial gradients accurately for irregular arrays. Numerical experiments are conducted to test techniques and to evaluate the parameters determined from the WG method. We apply this method to USArray data for the western United States. In this study, Rayleigh waves from nine earthquakes with varying azimuths are analyzed. The stability of this method is shown by the similarity between the results from two nearly collocated earthquakes from the Kurile Islands. The error check shows the WG results are stable for ambient noise level as high as 10%. Phase velocities determined by WG and two station (TS) methods are statistically consistent, while these determined from beam forming method are systematically higher for wavelength larger than one quarter of the array diameter. Our results show that, first, the average phase velocities of Rayleigh waves range from 3.8 to 4.1 km/s for periods from 60 s to 150 s. This is consistent with average earth models. The prominent feature on the phase velocity map is that the Basin and Range province is dominated by velocity lows while the west coast of the United States and the north and northeastern Snake River plain are dominated by velocity highs. The Snake River plain appears to be a primary tectonic boundary. Second, azimuthal variations represent the accumulated wave directionality changes along the raypath. A velocity contrast of 0.25 km/s across the oceanic-continental lithosphere boundary along the west coast of the United States is needed to explain the negative azimuth variations. Third, geometrical spreading is slightly anticorrelated with phase velocity, which may suggest that amplitude variations in radial directions are subject to surface wave focusing and defocusing. Fourth, similar to the wave directionality, radiation pattern variations also exhibit strong path dependence. Further theoretical and experimental studies will be conducted to understand the two amplitude parameters: geometrical spreading and radiation pattern and their relations with the local geophysical properties.