Shallow and deep nonlinear attenuation of S waves beneath PS10 during the 2002 Denali mainshock

Norman H. Sleep, & Tianze Liu

Published July 27, 2019, SCEC Contribution #9277, 2019 SCEC Annual Meeting Poster #011 (PDF)

Nonlinear failure likely attenuated strong S waves beneath Pump Station 10 during the 2002 Denali Earthquake. High dynamic stresses caused failure within gravel that is ~100 m thick at the station. The well-known scaling relationship for vertical S waves provides insight. The Coulomb stress ratio for resolved horizontal shear traction of lithostatic stress is the resolved horizontal acceleration in g’s. The resolved horizontal acceleration in g’s (as observed) clips at the effective coefficient of friction, ~0.35, which is appropriate for gravel with the water table near the surface. Numerical modeling indicates the high-frequency component of the S waves perpendicular to the resolved acceleration should propagate to the surface, producing jitter in the observed record. However, such jitter is not observed and the overall high-frequency acceleration is weak. We suspect that high-frequency S waves that traversed the failing region within deeper crystalline rocks associated with near-field velocity pulse and thus the interrogated their effective rheology. Mechanically, both the strong long-period stresses of the near-field velocity pulse and the weak stresses of the high-frequency S waves inelastically deformed the rock. The observed horizontal spectra are compatible with a moderately nonlinear rheology where the near-field velocity pulse controlled the deviatoric stress invariant and the material behaved pseudo-linearly for the weak stresses from the S waves. Mathematically, the high-frequency S waves strongly attenuated with an apparent quality factor of ~20 as predicted for pseudo-linear rheology. An ideally plastic material transmits some high-frequency signal as noted in our numerical models of shallow S waves. We also infer that this effective rheology nonlinearly attenuated the near-field velocity pulse of the Denali Earthquake and thereby hindered the gross earthquake rupture. We were unable to detect transient seismic-wave velocity changes with damage within the crystalline rock, nor subsequent healing of such damage. Our inferences are relevant to formulating nonlinear numerical models of future well-instrumented events.

Key Words
strong ground motion; nonlinear seismology; near-field velocity pulse; Denali earthquake

Citation
Sleep, N. H., & Liu, T. (2019, 07). Shallow and deep nonlinear attenuation of S waves beneath PS10 during the 2002 Denali mainshock. Poster Presentation at 2019 SCEC Annual Meeting.

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