Figure 9. Deviation from the mean for the natural frequencies of Millikan Library, February 2003, which includes a major rainstorm. In the center of the plot, for each of the three natural frequencies, the horizontal red lines are the monthly average, the black lines are the daily average percent deviation from this mean, and the blue lines are the hourly percent deviation from mean. At the bottom of the figure, the black-bar data are the cumulative hourly rainfall (re-zeros at midnight). The red line is the maximum hourly temperature, and the green is the wind gust. On the top of the figure are each natural frequency amplitude for the hourly FFT peak. The rainfall coincides with a very sharp rise in natural frequencies in the east–west and torsional modes, followed by a slow return toward prerainfall levels. Dashed vertical lines represent the start of each new day (12 a.m. PST). Frequency spikes are due to instrument glitches (6, 21, and 27 February), vibration testing (10 February), and the Big Bear earthquake (22 February). FFT peaks fall at night and on weekends. No major increase in excitation amplitude occurs during rainfall events not associated with high winds.