Bulletin of the Seismological Society of America; February 2006; v. 96; no. 1;
p. 237-257; DOI: 10.1785/0120050052
© Seismological Society of America
Click on image to view larger version.
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.
