Love wave dispersion in central North America determined using absolute displacement seismograms from high-rate GPS

We use seismic array processing of high-rate GPS (HRGPS) displacement time series from the Great, 2004, M-w 9+, Sumatra-Andaman earthquake recorded at over 90 nonuniformly distributed HRGPS stations in central North America to determine the fundamental Love wave phase velocity dispersion curve there. These measurements were performed using frequency domain beam forming, which we show reduces the effects of GPS multipath and common mode noise on the measurement of surface wave phase velocity and azimuth. Our HRGPS based results for surface wave phase velocity agree well with those obtained from 28 broadband seismometers between periods of 20 to 300 s. By separating waves based on their apparent velocity, beam forming supports the simple model for relative displacement HRGPS time series as being the difference between the reference and kinematic station's absolute displacements. Beam forming also demonstrates that for differential GPS processing the infinite apparent velocity beam of an array is composed of the sum of common mode noise and the reference station's absolute displacements, multipath and noise. We show the infinite apparent velocity beam, which we call the generalized spatial filter, is similar to the spatial filter commonly used to remove common mode noise from HRGPS seismograms and can be used to produce absolute displacement time series for the kinematic stations with reduced common mode noise. Beam forming also suggests a filtering method, complimentary to sidereal filtering, to produce GPS multipath reduced HRGPS time series.