Surface wave phase velocities of the Western United States from a two-station method

Abstract

We calculate two-station phase measurements using single-station measurements made on USArray Transportable Array data for surface waves at periods from 25 to 100 s. The phase measurements are inverted for baseline Love and Rayleigh wave phase velocity maps on a 0.5 • × 0.5 • grid. We make estimates of the arrival angle for each event at each station using a mini array method similar to beamforming, and apply this information to correct the geometry of the two-station measurements. These corrected measurements are inverted for an additional set of phase velocity maps. Arrival angles range from 0 • to ±15 • , and the associated corrections result in local changes of up to 4 per cent in the final phase velocity maps. We select our preferred models on the basis of the internal consistency of the measurements, finding that the arrival-angle corrections improve the two-station phase measurements, but that Love wave arrival-angle estimates may be contaminated by overtone interference. Our preferred models compare favourably with recent studies of the phase velocity of the Western United States. The corrected Rayleigh wave models achieve greater variance reduction than the baseline Rayleigh wave models, and the baseline Love wave models, which are more difficult to obtain, are robust and could be used in conjunction with the Rayleigh wave models to constrain radially anisotropic earth structure.

Arrival angle anomalies of Rayleigh waves observed at a broadband array: a systematic study based on earthquake data, full waveform simulations and noise correlations

“…At longer period the deviation is smaller, yet not negligible, as illustrated by events 4-6. A second observation is that, in agreement with Foster et al (2014a), events in approximately the same location and with similar source mechanisms have quite similar deviations at long periods. At short periods the deviations may still be quite different, as illustrated by events 4 and 5 which have deviations with approximately 15 • difference at 20 s period.…”

“…Even though the error on local phase velocities measured by two-station methods can be reduced by averaging over several earthquakes, the deviations are likely to imply a systematic overevaluation of velocities. Network design can at least partly accommodate for this difficulty, through correcting the arrival angle using beamforming based on neighbouring stations (Baumont et al 2002;Bourova et al 2005;Kaviani et al 2007;Foster et al 2014a). Global studies (e.g.…”

“…It is possible to correct two-station phase velocities using the effective arrival angle (backazimuth, angle measured clockwise from north) observed by beamforming rather than the great-circle assumption. This technique was successfully applied by Baumont et al (2002), Bourova et al (2005), Kaviani et al (2007) and more recently by Foster et al (2014a). Tanimoto & Prindle (2007) used a similar approach, but used average corrections for any given azimuth.…”

Interstation phase speed and amplitude measurements of surface waves with nonlinear waveform fitting: application to USArray

“…New multiple-station techniques for constraining seismic structure have also been developed for making the most of the USArray data; for example, eikonal tomography (Lin et al 2009), Helmholtz tomography (Lin & Ritzwoller 2011) and two-station phase estimations based on single-station measurements with the arrival-angle corrections using a miniarray method (Foster et al 2014b). The velocity models of these studies are consistent and the USArray data have dramatically enhanced horizontal resolution of tomographic models in the United States.…”

“…In the recent study of two-station method using USArray by Foster et al (2014b), the interstation phase information has been derived from the single-station phase measurements for each station, based on the work by Ekström et al (1997). We, instead, employ the direct comparisons of waveforms at two stations to extract the interstation phase speeds and amplitude ratios simultaneously.…”