Transient and Long-Term Changes in Seismic Response of the Natural Resources Building,Olympia, Washington,due to Earthquake Shaking

The Natural Resources Building (NRB) in Olympia, Washington, was shaken by three earthquakes (Mw = 5.8, 6.8, and 5.0) between 1999 and 2001. Building motions were recorded on digital accelerographs, providing important digital recordings of repeated strong shaking in a building. The NRB has 5-stories above grade with 3 sub-grade levels and a ductile steel-frame elongated in the E-W direction. The upper two floors extend significantly beyond the lower 3 on the southern and eastern sides. N-S motions dominate the fundamental modal vibrations of the building system. In the 1999 Satsop M5.8 earthquake, the frequency of this fundamental system mode was 1.3 Hz during motions of 10% g. The frequency dropped to 0.7 Hz during the 2001 M6.8 Nisqually strong motions. Moreover, the Nisqually recordings reveal both numerous high-frequency transients of up to 0.18 g, several of which are visible on widely spaced sensors, and long-term tilts of some of the sensors. The weaker 2001 M5.0 Satsop earthquake motions showed the frequency remained depressed at less than 1 Hz for the eastern side of the structure, although the western side had recovered to 1.3 Hz. An ambient noise survey in 2008 showed the fundamental frequency of N/S vibrations remains about 1.0 Hz for the eastern side of the building and 1.3 Hz for the western side. These results suggest that in the Nisqually earthquake, the east side of the NRB suffered a permanent reduction in fundamental mode frequency of 37% due to loss of system stiffness by undetermined mechanism.