Figure 11. As Figure 9. (a) For a 5-day period in January 2003, with a particularly intense wind storm (no rainfall). The natural frequencies of the Library dramatically shorten for the duration of the most intense windstorm, most notably in the east–west direction, but also significantly for the north–south and torsional modes. The smaller windstorm in the evening of January 6 appears slightly shifted in time from the response; this may be due to variations in the winds between the Library and the weather station at JPL. The mean frequencies are for the whole month of January. (b) For a 22- day period, late August to early September 2002. Typical daily variation during the summer months. Note correlation of rise in all three natural frequencies during hottest days. 26 August, vibration testing; 3 September, M 4.8 Yorba Linda Earthquake.