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Nonlinear Suppression of High‐Frequency S Waves by the Near‐Field Velocity Pulse With Reference to the 2002 Denali Earthquake
Author(s) -
Sleep Norman H.,
Liu Tianze
Publication year - 2020
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.983
H-Index - 232
eISSN - 2169-9356
pISSN - 2169-9313
DOI - 10.1029/2019jb018386
Subject(s) - geology , pulse (music) , seismology , seismic wave , aftershock , viscoelasticity , physics , geophysics , optics , detector , thermodynamics
The near‐field velocity pulse of large strike‐slip earthquakes brings near‐fault stiff crystalline rock into failure. The uppermost ~2 km beneath Pump Station 10 (PS10) likely failed nonlinearly during the 2002 Denali earthquake. High‐frequency S waves traversed this region during and immediately after failure with only weak waves reaching the surface. In contrast, high‐frequency S waves were briefly weak at station LUC only during the strong near‐field velocity pulse of the 1992 Landers earthquake. The observed horizontal spectra during the near‐field velocity pulse at PS10 decreased exponentially with frequency with a low apparently constant Q of ~20. This observation is incompatible with simple elastic‐plastic and nonlinear viscoelastic rheologies that do not preferentially attenuate high frequencies. Candidate rheologies involve heterogeneous crystalline rock masses where nonlinear domains act in parallel. Maxwell (spring and dash pot) elements in parallel produce apparently constant Q over a range of frequencies. This rheology may arise from pseudolinear inelastic interaction of weak stresses from high‐frequency S waves with strong low‐frequency stresses from the near‐field velocity pulse. Alternatively, inelastic deformation associated with the high‐frequency waves may interact nonlinearly with low‐frequency deformation associated with healing of damage associated with the near‐field velocity pulse. The latter process is attractive for the PS10 signal which remained weak after the near‐field velocity pulse had passed. We unsuccessfully examined aftershock records for healing of damage within the uppermost crystalline rock.