Figure 13. Acceleration data and resulting spectrograms for MIK from 22 February 2003. M 5.4 Big Bear earthquake occurs at 12:19 UTC. Maximum acceleration = 19.3 cm/sec² north–south, 10.9 cm/sec² east–west. Spectrograms are set around the four natural frequencies. (a) 12:00–13:00 UTC, 30-sec-window spectrograms, with slices shifted by 15 sec. The natural frequencies shorten considerably during the mainshock shaking and aftershocks. Analysis of long-term behavior, especially that of the second mode east–west, is limited by poor resolution due to short FFT length. (b) 11:30–14:30 UTC, 5-min window spectrograms, with slices shifted by 1 min. After shaking has dissipated, over this time window it is clear there is no perceptible long-term shortening of the fundamental frequencies due to the earthquake. The event at 168 min is a M 4.1 aftershock.