Nonlinearity evaluation considering the uncertainty of S-wave velocity based on total stress analyses and diffuse field assumption: the Mw 9.1 great Tohoku Taiheiyo-Oki earthquake at 05:46 UTC on 11 March 2011

Abstract

In this study, the inversion modeling method variability of S-wave velocity structure results by inverting/forward-modeling waveforms and HVSR curves is investigated. For this purpose, eight KiK-net seismic stations using 636 foreshocks, mainshock of the 2011 Tohoku earthquake, and 4983 aftershocks obtained from KiK-net are used. The most reliable and representative inverted S-wave velocity structures are based on close fitting of the theoretical to the observed data of waveforms and HVSR curves. The inverted S-wave velocity structures derived from the total stress analyses are showing high degrees of similarity and consistency using both EW and NS components and evidenced by the isotropic HVSRs directionality related to the linear and nonlinear site effects. Conversely, stiffness variations due to nonlinearity, inferred from inverted S-wave velocity structures derived from the diffuse field assumption, significantly correspond with stiffness variations evaluated from the observed fundamental resonance frequencies. The corresponding percent of reduction and recovery reflect low variability in S-wave velocities that result from inversion modeling based on diffuse field assumption. Qualitatively, the presence of the stiffness degradation process due to linear and nonlinear site responses could be obviously detected using both inversion modeling methods based on total stress analyses and diffuse field assumption. Quantitatively, the precision of the nonlinearity evaluation is questionable due to significant uncertainties in the inverted S-wave velocity structures inferred from both inversion modeling methods. These uncertainties are strongly dependent on the theory of each inversion modeling method. The use of complementary methods with more prior information is suggested to resolve the nonuniqueness of the inverted S-wave velocities and hence overconfidence in the precision of the nonlinearity evaluation.