Nonlinear Attenuation at PS10 During the 2002 Denali Earthquake Associated With Interaction of High-Frequency S Waves With the Near-Field Velocity Pulse

Date: 4/24/2019

Time: 06:00 PM

Room: Grand Ballroom

Seismic station PS10 recorded reliable records of strong seismic waves ~3 km from the main fault of the 2002 Denali earthquake. The near-field velocity pulse reached 1.8 m s^-1. The peak ground acceleration of 3.5 m s^-2 was kinematically associated with the velocity pulse. However, high-frequency S waves were weak, ~1 m s^-2. Two nonlinear attenuation mechanisms diminished high-frequency S wave amplitudes. (1) For context, the S waves attenuated within the uppermost layer of gravel. Well-known scaling relationships are relevant. The Coulomb ratio of dynamic resolved horizontal shear traction on horizontal planes from S waves approximately equals the normalized resolved horizontal acceleration. The acceleration in g’s remained around the effective coefficient of friction, ~0.35, for a circularly polarized third-cycle during the near-field pulse. This strong shaking likely reduced the shear modulus and resonance frequently of the shallow gravel, providing indirect evidence for shallow nonlinear behavior. (2) The near-field velocity pulse brought the upmost uppermost hard rock perhaps down to ~2 km depth beneath PS10 into nonlinear failure. High-frequency S waves passing through this failing region attenuated nonlinearly. Site response formalism is inapplicable as the effect occurred along much of the S wave’s paths. The process is mathematically similar to strong linear attenuation of high-frequency S waves. Observed horizontal spectra show this decay with frequency with a quality factor Q of ~20. These observations are compatible with a nonlinear rheology where the inelastic strain rate within the hard rock increases gradually with deviatoric stress. In contrast, high-frequency waves with stress antithetical to the in the near-field velocity pulse should propagate through the hard rock with an ideal plastic rheology.

Presenting Author: Norman H. Sleep

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