Shear-wave velocity measurements to over 2km depth in the Seattle basin, Washington State, USA, as characterized with the krSPAC microtremor array method

William J Stephenson, Michael Asten, Jack Odum

Research output: Contribution to conferenceAbstract


We acquired microtremor Rayleigh-wave arraydata at 11 sites in the Seattle basin, Washington State, USA, and applied the wavenumber normalized SPatial Auto-Correlation method (krSPAC) to obtain shear wave velocity (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 describe wavenumber and nominal station separation, respectively) prior to Vs modeling. The requirement for array symmetry is eased through this transformation, permitting flexibility in array deployments at challenging sites such as in urban areas. We deployed seven-sensor nested asymmetric triangular arrays, with nominal interstation separations that varied from approximately 300 m to 2000 m. We model Vs to 3300 m depth at one site, with an average modeled depth of 1200 m among all sites.

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 depths between 0 and 250 m. Whereas noise tomography methods commonly require months of recording time, the krSPAC approach in this study required less than six hours of recording time per site. At each site, we resolve a geologic boundary where Vs increases above 900 m/s, which is interpreted as the base of the unconsolidated (Quaternary) deposits in the Seattle basin. Using this boundary as the reference horizon for linear-elastic transfer function calculations, we estimate ground motion amplifications of a factor of up to 2 from the overlying Quaternary sediments between 0.3 and 7 Hz. Our amplification estimates are consistent with previously-published simple spectral ratio amplification spectra from earthquakes recorded in the basin.
Original languageEnglish
Number of pages1
Publication statusPublished - 2 Sep 2018
EventGeneral Assembly of the European Seismological Commision, 2018 - Mediterranean Conference Centre, Valletta, Malta
Duration: 2 Sep 20187 Sep 2018
Conference number: 36th


ConferenceGeneral Assembly of the European Seismological Commision, 2018
Abbreviated titleESC2018
Internet address

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