Features of soil behavior in the near-fault zones during the 2011 Tohoku mega-thrust earthquake Mw 9

Soil behavior is studied during the Tohoku earthquake, where abnormally high accelerations > 1 g were recorded. Based on vertical array records, models of soil behavior are constructed at 28 sites in northern Honshu (Tohoku region). They are compared with previously studied models of soil behavior in southern Tohoku and Kanto regions, where shock waves were identified as possible causes of the recorded high accelerations. Shear moduli did not reduce during strong motion at many sites, and the behavior of softer and denser soils was similar to a large extent. The nonlinearity of soil response during the Tohoku earthquake was weaker than that observed earlier during the 1995 Kobe and 2000 Tottori earthquakes (Mw ~6.7–6.8). Instead, a widespread soil hardening was found, most expressed at stations recorded the highest PGAs. To explain the observed features of soil behavior, two possible mechanisms are suggested, such as, 1) shock wave fronts generated by rupture propagation along the fault plane induce soil hardening and high PGAs; 2) soil compaction and hardening is a soil response to long-lasting dynamic loadings during the earthquake. Most likely we may expect similar effects of soil hardening and generation of high PGAs during other mega-thrust earthquakes in future.