Engineering Characterization of Earthquake Ground Motion Coherency and Amplitude Variability
- Author(s): Ancheta, Timothy D;
- Stewart, Jonathan P;
- Abrahamson, Norman A.
- et al.
Earthquake ground motions exhibit spatial variability manifest as random variations of Fourier amplitude and phase. These variations increase with frequency and distance between observations points (d), and introduce demands for lifeline systems and foundations. Spatially variable ground motions (SVGM) are quantified by: (1) apparent horizontal wave velocity (Vapp), which controls wave passage effects that shift Fourier phase; (2) lagged coherency, representing random phase variations; and (3) standard deviation terms representing Fourier amplitude variability. We examine empirical relations for the three SVGM sources through analysis of data from the Borrego Valley Differential Array (BVDA) in California and re-analysis of data from the LSST array in Taiwan, both having a number of stations at d < 120 m. We show that Vapp from the two arrays have medians of 2.1 and 2.6 km/s and natural log standard deviations of about 0.5. We show that previous models for lagged coherency and standard deviation from amplitude variability have bias, and propose revisions. We show that amplitude and coherency residuals from the baseline model are uncorrelated, although frequency-to-frequency residuals for both quantities are weakly correlated for small frequency offsets.