Impacts of an M9 Cascadia Subduction Zone Earthquake and Seattle Basin on Performance of RC Core Wall Buildings

Abstract

The performance of tall reinforced concrete core building archetypes in Seattle, Washington, was evaluated for 30 simulated scenarios of a magnitude 9 (M9) Cascadia subduction zone interface earthquake. Compared with typical risk-adjusted maximum considered earthquake (MCER) motions, the median spectral accelerations of the simulated motions were higher (15% at 2 s) and the spectral shapes were more damaging because the Seattle basin amplifies ground-motion components in the period range of 1.5–6 s. The National Seismic Hazard Maps do not explicitly take into account this effect. The significant durations were much longer (∼115 s) than typical design motions because the earthquake magnitude is large. The performance of 32 building archetypes (ranging from 4 to 40 stories) was evaluated for designs that barely met the minimum code requirements as well as for more rigorous designs that were typical of current tall building practice in Seattle. Even though the return period of the M9 earthquake is only 500 years, the maximum story drifts for the M9 motions were on average 11% larger and more variable than those for the MCER design motions that neglect basin effects. Under an M9 event, the collapse probability for the code-minimum archetypes averaged 27% for code minimum-designed archetypes. In contrast, the collapse probability for the archetypes designed according to current tall-building practice in Seattle were lower and averaged 14%. These collapse probabilities for an M9 earthquake, which has a return period of about 500 years, exceeded the target 10% collapse probability in the MCER, which has a longer return period.