The notion that the Cascadia subduction zone (CSZ) has produced very large earthquakes in the past, and that it can be expected to produce very large earthquakes again, is now widely accepted in the seismological and engineering communities. Because no records of ground shaking or damage exist for historical CSZ earthquakes, it is difficult to evaluate their potential effects on bridges, buildings, embankments, and other structures. However, recent advances in engineering seismology now allow the numerical simulation of earthquakes, including fault rupture, the propagation of seismic waves from the fault to the site of interest, and amplification of the resulting rock motions by shallow soil and rock layers beneath the site.
Rock outcrop motions were simulated for three CSZ earthquake scenarios: magnitude 8.0, 8.5, and 9.0 earthquakes. The magnitude 8.0 earthquake was assumed to result from rupture of the portion of the CSZ adjacent to the northern part of the state; the larger magnitude earthquakes were associated with rupture on a portion of the CSZ extending along the entire length of the state. Thirty different simulations of each earthquake scenario were analyzed. For each, rock outcrop motions were computed at each of 13 locations within Washington state. Site response analyses were then performed for 15 soil profiles at the 13 locations.
The rock outcrop motions showed amplitudes, frequency contents, and durations that were significantly different than the ground motions that civil structures are commonly designed for in Washington state. Peak accelerations and spectral acceleration at T=0.3 sec were all considerably lower than the values on which most current design procedures are based. Spectral accelerations for T=1.0 sec were less than those on which current design procedures are based for Mw=8.0 earthquakes, but they were comparable for Mw=9.0 earthquakes and, at some sites, for Mw=8.5 earthquakes. CSZ ground motions have strong long-period (low frequency) components and thus should be more damaging to structures with long natural periods. Finally, the durations of CSZ ground motions are much longer than those of the motions on which current design procedures are based. This aspect of CSZ motions may be quite significant for reinforced concrete structures and potentially liquefiable soil deposits in which the accumulation of damage depends on the number of load or stress reversals that occur during earthquake shaking.