Figure 2. Graphical interpretation of Table 1. Dashed lines are east–west natural frequencies, dashed-dotted lines are north–south-natural frequencies, all from forced vibration testing. Shaded area is the likely region of natural frequencies taking into consideration errors in measurement, caused by unknown shaker weight configuration and weather conditions for each test, and experimental error. Crosses indicate the actual time of a forced vibration measurement. Circles indicate the natural frequency estimated from the strong-motion recording of the event, with the number in italics giving the peak acceleration recorded for the event (cm/sec²).