  The analysis of strong motion recordings in structures is crucial to
understand the damaging process during earthquakes. A very precise
time-frequency representation, the reassigned smoothed pseudo-Wigner-Ville
method, allowed us to follow the variation of the Millikan Library (California)
and the Grenoble City Hall building (France) resonance frequencies during
earthquakes. Under strong motions, a quick frequency drop, attributed to damage
of the soil-structure system, followed by a slower increase is found. However,
in the case of weak earthquakes, we show that frequency variations come from
the ground motion spectrum and cannot be interpreted in terms of change of the
soil-structure system.
