The strain in the array is mainly in the plane (Waves below similar to 1 Hz)

We compare geodetic and single-station methods of measuring dynamic deforma tions and characterize their causes in the frequency bands 0.5-1.0 Hz and 4 .0-8.0 Hz. The geodetic approach utilizes data from small-aperture seismic arrays, applying techniques from geodesy. It requires relatively few assump tions and a priori information. The single-station method uses ground veloc ities recorded at isolated or single stations and assumes all the deformati on is due to plane-wave propagation. It also requires knowledge of the azim uth and horizontal velocity of waves arriving at the recording station. Dat a employed come from a small-aperture, dense seismic array deployed in Geyo kcha, Turkmenistan, and include seismograms recorded by broadband STS2 and short-period L28 sensors. Poor agreement between geodetic and single-statio n estimates in the 4.0-8.0 Hz passband indicates that the displacement held may vary nonlinearly with distance over distances of similar to 50 m. STS2 geodetic estimates provide a robust standard in the 0.5-1.0 Hz passband be cause they appear to be computationally stable and require fewer assumption s than single-station estimates. The agreement between STS2 geodetic estima tes and single-station L28 estimates is surprisingly good for the S-wave an d early surface waves, suggesting that the single-station analysis should b e useful with commonly available data. These results indicate that, in the 0.5 to 1.0 Hz passband, the primary source of dynamic deformation is plane- wave propagation along great-circle source-receiver paths. For later arrivi ng energy, the effects of scattering become important. The local structure beneath the array exerts a strong control on the geometry of the dynamic de formation, implying that it may be difficult to infer source characteristic s of modern or paleoearthquakes from indicators of dynamic deformations. Ho wever, strong site control also suggests that the dynamic deformations may be predictable, which would be useful for engineering seismically resistant structures.