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A Decade of the Seattle Liquefaction Array

Completed in February 2012, the Seattle Liquefaction Array (UW.SLA) has now recorded more than a decade of continuous borehole acceleration and pore pressure data at a liquefaction-susceptible site in Seattle’s south of downtown (SODO) neighborhood. While the seismically quiescent Pacific Northwest has not produced any earthquakes with appreciable excess pore pressure generation at SLA during the past decade, the urban ambient seismic noise of industrial south Seattle provides a rich dataset for near-surface investigations. Quasi-static surface loading from trains passing by or parking only 5-30 m from SLA drives pore pressure transients as large as 1 kPa at 52 m depth. By computing downhole horizontal-component cross-correlations with overlapping 40-s windows, we can measure perturbations in near-surface velocity from ballistic shear waves ~1-50 Hz generated by the trains. Combining these two datasets, we estimate the nonlinear elastic response of the sediment to poroelastic stress changes, which is as large as +20% dv/v (averaged 0-60 m) per 300 Pa (measured at 52 m). We also apply ambient noise coda wave interferometry to study changes in near-surface velocity and attenuation over seasonal to decadal scales. In the coda of vertical component cross-correlations, we find a decade-long monotonic stiffening trend, with 1-10 Hz Rayleigh wave phase velocity increasing by ~4% from 2012 to 2022. The horizontal-component and cross-component correlations are more complex, showing opposing seasonal trends in dt/t at different windows in the coda and different frequencies, likely representing a depth-dependent phase lag in poroelastic and/or thermoelastic velocity changes.

Session: Network Seismology: Recent Developments, Challenges and Lessons Learned [Poster Session]

Type: Poster

Room: Exhibit Hall

Date: 5/1/2024

Presentation Time: 08:00 AM (local time)

Presenting Author: Ethan Williams

Student Presenter: No

Additional Authors