An application is presented of a recently proposed system identification method for buildings that takes into account the effects of soil-structure interaction and the coupling of the horizontal and rocking motions of the foundation. The method gives the uncoupled structural fixed-base frequency f1 and rigid-body rocking frequency fR using data only from two horizontal sensors (at base and roof). The fixed-base frequency is estimated from the wave travel time through the structure, the apparent system frequency from Fourier analysis, and the rocking frequency from a relation between these three. The case study is Millikan Library in Pasadena, California. Results are shown for four earthquakes between 1970 and 2002. The method makes it possible to quantify the degree to which the observed changes (wandering) of its resonant frequencies have been due to changes in the structure alone. The results show that (1) both f1 and fR are amplitude dependent, (2) significant permanent reduction of frequency occurred over the years, ∼22% for f1 and 11% for the apparent frequency f1,app, mostly caused by the San Fernando earthquake of 1971, while (3) the changes of fR have been amplitude dependent and recoverable. (4) During the San Fernando earthquake, both f1 and fR dropped, respectively, by ∼24% and ∼18%, resulting in 21% drop of f1,app. (5) After this earthquake, the changes in the observed resonant frequencies, which are those of the system, have been due to a much larger degree (4–5 times) to changes of fR than to changes of f1. (6) The small permanent changes in f1 that appear to have occurred after the San Fernando earthquake cannot be deciphered with certainty because of the small number of earthquake records available for this analysis since 1971. Records from the period 1988 to 2002, when released, can be used in future to refine and verify these trends.