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Nonlinear Attenuation of S Waves by Frictional Failure at Shallow Depths

Norman H. Sleep, & Nori Nakata

Published May 30, 2017, SCEC Contribution #7204

Strong S waves produce dynamic stresses, which bring the shallow subsurface
into nonlinear anelastic failure. The construct of coulomb friction yields testable
predictions about this process for strong-motion records. Physically, the anelastic strain
rate increases rapidly with increasing dynamic stress, and dynamic stress is proportional
to the difference between total strain and anelastic strain. Nonlinear models of vertically
propagating S waves in layered media confirmed and illustrated analytical inferences.
The effective coefficient of friction bounds (clips amplitude) the resolved horizontal
acceleration normalized to the acceleration of gravity. There is a tendency for the
random signal from vertically propagating S waves to become transiently circularly
polarized at the maximum (clipped) resolved acceleration, as the acceleration component
perpendicular to the current acceleration adds weakly the resolved acceleration.
Frictional attenuation does not preferentially suppress high-frequency signal; it cannot
be modeled by increasing ordinary linear attenuation. In addition, an effect of shallow
cohesion is to allow brief pulses of strong high-frequency acceleration to reach the surface.
Frictional attenuation within deep overpressured aquifers suppresses shaking recorded
at the surface, but does not simply clip amplitude at a given resolved
acceleration. The anelastic strain rate increases slowly with stress within shallow muddy
sediments. The accelerations from reverberations within such layers can exceed 1g.

Sleep, N. H., & Nakata, N. (2017). Nonlinear Attenuation of S Waves by Frictional Failure at Shallow Depths. Bulletin of the Seismological Society of America, 107, 1828–1848. http://10.1785/0120160334

Related Projects & Working Groups
Extreme Ground Motions (EXGM), Fault and Rupture Mechanics (FARM), Seismology, Earthquake Geology, GMP