Shear-wave velocity (Vs) to over 2 km depth as characterized through the krSPAC microtremor array method: Case study from the Seattle basin, Washington

William J. Stephenson, Michael W. Asten, Jack Odum

Research output: Contribution to conferenceAbstract


We acquired microtremor array data at 11 sites in the Seattle basin, Washington State, and applied the wavenumber-normalized SPAC method (krSPAC) to obtain Vs to over 2000 m depth. In the krSPAC approach, we transform observed coherency-versus-frequency spectra to coherency-versus-kr (where k and r are wavenumber and station separation, respectively) prior to Vs modeling. Through this transformation, the requirement for array symmetry is eased. We deployed seven-sensor nested irregular triangular arrays, with nominal interstation separations that varied from ~300 m to 2000 m. Comparison of krSPAC Vs results to a previous interpretation from ambient noise tomography suggests a broadly comparable Vs structure in the 250 to 1000 m depth range and improved resolution at shallower depth. At each site, we interpret a high-velocity Quaternary boundary where Vs increases above 900 m/s. Using this boundary as the reference horizon, we calculate ground motion amplification of a factor of up to 2 from the overlying Quaternary sediments between 0.3 and 7 Hz.

Original languageEnglish
Number of pages5
Publication statusPublished - 1 Jan 2019
EventSociety of Exploration Geophysicists International Exposition and Annual Meeting, 2018 - Anaheim, United States of America
Duration: 14 Oct 201819 Oct 2018
Conference number: 88th


ConferenceSociety of Exploration Geophysicists International Exposition and Annual Meeting, 2018
Abbreviated titleSEG 2018
CountryUnited States of America
Internet address

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