Nonlinear elasticity is evidenced by the variation of the elastic properties of a system under slight strain and their recovery after loading. These characteristics have been previously observed at the scale of the laboratory sample and at the Earth’s crust immediately after strong earthquakes. In this study, we observe that the nonlinear elastic response can be observed in a similar manner in civil engineering structures. Herein, we study a Japanese building under permanent monitoring since 1998 and during the 2011 Mw 9.0 Tohoku seismic sequence. First, we observe that under low strain, the resonance frequency of the building decreases continuously until a plateau value is reached, characterizing the continuous damage process with repeated dynamic loading forces conditioning the structure. After each earthquake, intraevent recovery of the resonance frequency is observed, according to a log‐time slope, which changes, presumably, according to the health of the structure. A strong variation of the resonance frequency is also observed after the Tohoku earthquake, characterizing the damage state of the building, but this is followed by long‐term recovery (over about six months) of the elastic properties. By analyzing the site response, we conclude on the moderate contribution of the nonlinear soil‐structure interaction to the slow dynamics observed after the Tohoku earthquake, and we assume that the degree of heterogeneities (cracks) present in the structural elements of the building governs the nature of the recovery. Analysis of these variations might represent an easy and suitable way of monitoring structural health.

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