Nonlinear suppression of high-frequency S waves by the near-field velocity pulse with reference to the 2002 Denali earthquake

Norman H. Sleep, & Tianze Liu

Accepted October 26, 2019, SCEC Contribution #9959

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.

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

Citation
Sleep, N. H., & Liu, T. (2019). Nonlinear suppression of high-frequency S waves by the near-field velocity pulse with reference to the 2002 Denali earthquake. Journal of Geophysical Research - Solid Earth, (accepted).


Related Projects & Working Groups
GM, Ground Motions, Fault and Rupture Mechanics (FARM)