The A.D. 900-930 Seattle-Fault-Zone Earthquake with a Wider Coseismic Rupture Patch and Postseismic Submergence: Inferences from New Sedimentary Evidence

Abstract

Stratigraphic evidence for coseismic uplift, a tsunami, and a sandy debris flow linked to the A.D. 900-930 Seattle-fault-zone earthquake and subsequent submergence is present at Gorst at the terminus of Sinclair Inlet, Washington. This study indicates that at least 3 m of uplift preceded a tsunami followed by a sandy debris flow. Though the Seattle and Tacoma fault zones ruptured within the error of ages of the tsunami deposit, model simulations indicate the Seattle fault generates an order of magnitude larger tsunami in the vicinity of the field area than the Tacoma fault and is the more likely generator of the tsunami. Simulations indicate amplitude from a Seattle-fault-zone-generated wave train was 4-5 m, and stratigraphy shows the tsunami left a sandy deposit. Soft-sediment deformation narrows the time window for the debris flow to the hours to months after the tsunami. Slope failure in unconsolidated glacial sands likely initiated a debris flow that surged at least 2 km down Gorst Creek valley and left a more than 40-cm-thick deposit along the creek and in the wetlands. Submergence of a Thuja plicata (western red cedar) swamp into the intertidal zone indicates at least 1.5 m of submergence in the last 1100 years. This submergence indicates uplift on nearby marine terraces associated with the Seattle fault zone was likely higher, and the A.D. 900-930 earthquake was potentially larger, than previously recognized.

Diverse rupture modes for surface-deforming upper plate earthquakes in the southern Puget Lowland of Washington State

“…The spatial extent of evidence for uplift and subsidence (>600 km 2 ) and the >1 m of deformation at 15 sites in the Tacoma fault zone imply a large earthquake or earthquakes within the millennial earthquake series ( Fig. 1C; Bucknam et al, 1992;Sherrod, 2001;Sherrod et al, 2004a;Arcos, 2012). If, as assumed in our OxCal analysis of Tacoma fault zones ages (Fig.…”

“…As much as 7 m of coseismic uplift at Restoration Point and >2 m of shoreline uplift along at least 40 km of the central Seattle fault zone imply a magnitude of 7-7.5 for this earthquake (Bucknam et al, 1992;Sherrod et al, 2000;ten Brink et al, 2006;Kelsey et al, 2008). Widespread tsunami, liquefaction, and landslide deposits are consistent with an earthquake of this size (Atwater and Moore, 1992;Blakely et al, 2009;Arcos, 2012;Martin and Bourgeois, 2012). At sites that record thousands of years of relatively continuous sedimentation, evidence attributed to the Restoration Point earthquake is more distinct and/or suggests greater deformation or stronger shaking than evidence for any other earthquake (Sherrod et al, 2000;Nelson et al, 2003a;Karlin et al, 2004;Kelsey et al, 2008).…”

“…1C), the present heights of many shorelines may be minimum values for coseismic uplift (Arcos, 2012).…”

“…We synthesize these new interpretations with prior studies of uplifted, subsided, or warped shorelines (Bucknam et al, 1992;Sherrod et al, 2000;Sherrod, 2001;Kelsey et al, 2008;Arcos, 2012), fault scarp and fold scarp stratigraphy (Nelson et al, 2002(Nelson et al, , 2003aWitter et al, 2008;Blakely et al, 2009), wetland stratigraphy adjacent to scarps (Hughes, 2005;Polenz et al, 2011), strong-shaking-induced liquefaction and landslide deposits ( Fig. 1C; Abella, 1992, 1996;Pringle et al, 1998;Sherrod, 2001;Karlin et al, 2004;Martin and Bourgeois, 2012;Arcos, 2012), and the deposits of tsunamis generated by fault rupture (Atwater and Moore, 1992;Bourgeois and Johnson, 2001;Arcos, 2012;Martin and Bourgeois, 2012). A key part of our synthesis is an evaluation of the more than 260 14 C ages obtained during paleoseismic studies in the southern lowland.…”

“…This includes the Seattle fault zone, the Tacoma fault zone and Tacoma basin, and the Saddle Mountain deformation zone ( Fig. 1C; Atwater and Moore, 1992;Bucknam et al, 1992;Bucknam and Biasi, 1994;Atwater, 1999;Sherrod et al, 2000;Hughes, 2005;Kelsey et al, 2008;Blakely et al, 2009;Arcos, 2012).…”