Figure 14. Acceleration data and spectrograms from CBC east–west channels, 11:30–14:30 UTC. Five-minute window slices, each shifted by 1 min. There are two spectrograms for each east–west channel; the first is a broad frequency range from acceleration data showing all identified modal frequencies; the second is a frequency band around the first mode using displacement data, which accentuates the behavior of the only mode without nearby machine noise (see Fig. 7). M 5.4 Big Bear earthquake occurs at 12:19 UTC. Maximum acceleration = 14.8 cm/sec² at BL6, northwest Roof. During the event, the fundamental frequencies shorten considerably. After shaking, over this time window there is no perceptible long-term shortening of the fundamental frequencies due to the earthquake.