Nonlinear attenuation from the interaction between different types of seismic waves and interaction of seismic waves with shallow ambient tectonic stress

Strong seismic waves bring rock into frictional failure at the uppermost few hundred meters. Numerous small fractures slip with the cumulative effect of anelastic strain and nonlinear attenuation; these fractures should not distinguish between remote sources of stress. Still, frictional failure criteria are not evident especially when seismic waves change the normal traction on fractures. We identify three earthquakes as examples where consideration of interaction among dynamic stresses from different wave types and ambient tectonic stress provides theoretical predictions of nonlinear attenuation that are potentially testable with single station seismograms. For example, because Rayleigh waves produce shallow horizontal dynamic tension and compression, frictional failure should preferentially occur on the tensile half-cycle if no shallow tectonic stress is present and on the compressional half-cycle if the tectonic stress is already near thrust-faulting failure. We observed neither effect on records from the 2011 M-w 9.0 Great Tohoku earthquake. However, Rayleigh waves from this event appear to have brought rock beneath MYGH05 station into frictional failure at approximate to 10 m depth and thus suppressed high-frequency S waves. The tensile half-cycle of high-frequency P waves reduced normal traction on horizontal planes beneath station IWTH25 during the 2008 M-w 6.9 Iwate-Miyagi earthquake, weakening the rock in shear and suppressing high-frequency S waves. The near-field velocity pulse from the 1992 M-w 7.3 Landers earthquake brought the uppermost few hundred meters of granite beneath Lucerne station into frictional failure, suppressing high-frequency S waves. These moderately positive examples support the reality of nonlinear wave interaction, warranting study future strong ground motions.