The analysis of strong and weak motion recordings in existing structures is critical for understanding both the damaging process during earthquakes and their structural behavior. The time-frequency representation is one of the existing methods to get information on the frequency variations in buildings, which may indicate either damage or degradation. In this article, we use standard time-frequency methods with amplitude normalization and reassignment method in order to observe smaller variations in earthquake recordings in buildings. The method used in this article is first validated on strong recordings from the R. Millikan Library (Pasadena, California) and later applied to weak earthquakes recorded in the Grenoble City Hall (France). This method detects torsion activation and very small frequency variations under weak motion. We show that these variations are probably due to variations of the input motion and cannot be attributed to variations in the parameters of the structural system.