Deformed submarine terraces in Puget Sound, Pacific Northwest, indicate only one M >∼7.5 earthquake on the Seattle fault zone in the past 11,000 yr

Submerged marine terraces in Puget Sound, deformed across the Seattle fault zone (SFZ), indicate that only one earthquake as large as M ∼7.5 has occurred in at least the past 11 k.y. Previous paleoseismic studies documented an M ∼7.5 earthquake between 923 and 924 CE, which uplifted coastal marine terraces by as much as 8 m. We demonstrated that this earthquake was the only such event since ca. 11 ka by mapping and quantifying deformation of older marine terraces that are now submerged in Puget Sound. The submerged terraces, attributed to a late-glacial sea-level lowstand, record both glacial isostatic rebound and tectonic deformation. Vertical offset of the ca. 11 ka terraces within the SFZ is comparable to that of the marine terraces uplifted in 923 CE, implying no additional large (M >∼7.5) earthquake on the SFZ since ca. 11 ka. This result implies a longer recurrence interval than current hazard estimates, which assume recurrence of M >7.1 events every 5 k.y. Our mapping of SFZ deformation since ca. 11 ka also supports fault segmentation and contiguous block uplift between the Seattle and Tacoma fault zones.