Abstract
Earthquake prehistory of the southern Puget Lowland, in the north-south compressive regime of the migrating Cascadia forearc, reflects diverse earthquake rupture modes with variable recurrence. Stratigraphy and Bayesian analyses of previously reported and new 14C ages in trenches and cores along backthrust scarps in the Seattle fault zone restrict a large earthquake to 1040–910 cal yr B.P. (2σ), an interval that includes the time of the M 7–7.5 Restoration Point earthquake. A newly identified surface-rupturing earthquake along the Waterman Point backthrust dates to 940–380 cal yr B.P., bringing the number of earthquakes in the Seattle fault zone in the past 3500 yr to 4 or 5. Whether scarps record earthquakes of moderate (M 5.5–6.0) or large (M 6.5–7.0) magnitude, backthrusts of the Seattle fault zone may slip during moderate to large earthquakes every few hundred years for periods of 1000–2000 yr, and then not slip for periods of at least several thousands of years. Four new fault scarp trenches in the Tacoma fault zone show evidence of late Holocene folding and faulting about the time of a large earthquake or earthquakes inferred from widespread coseismic subsidence ca. 1000 cal yr B.P.; 12 ages from 8 sites in the Tacoma fault zone limit the earthquakes to 1050–980 cal yr B.P. Evidence is too sparse to determine whether a large earthquake was closely predated or postdated by other earthquakes in the Tacoma basin, but the scarp of the Tacoma fault was formed by multiple earthquakes. In the northeast-striking Saddle Mountain deformation zone, along the western limit of the Seattle and Tacoma fault zones, analysis of previous ages limits earthquakes to 1200–310 cal yr B.P. The prehistory clarifies earthquake clustering in the central Puget Lowland, but cannot resolve potential structural links among the three Holocene fault zones.
Highlights
The seismic hazard of active convergent margins derives from great earthquakes on the megathrust, and from shallow faulting in the actively deforming forearc
Much of the north- to northeastdirected strain in the southern Puget Lowland is accommodated by deformation in the Seattle fault zone, the Tacoma fault zone, the Saddle Mountain deformation zone, and along the Olympia fault (Figs. 1B, 1C)
In this densely forested, heavily glaciated region, the most successful paleoseismology studies begin with aeromagnetic mapping and seismic reflection surveys to identify potentially active fault zones, move to searches for fault or fold scarps and deformed shorelines on lidar imagery, and follow up with studies of Holocene stratigraphy beneath scarps and shorelines to date prehistoric earthquake deformation (Barnett et al, 2010; e.g., Blakely et al, 2002; Johnson et al, 2004a; Sherrod et al, 2004a, 2008; Kelsey et al, 2012)
Summary
The seismic hazard of active convergent margins derives from great earthquakes on the megathrust, and from shallow faulting in the actively deforming forearc. Unlike the Inland Sea of Japan, the historical record of seismicity in the Puget Lowland is short, and information about past large, surface-deforming earthquakes must come from paleoseismology. In this densely forested, heavily glaciated region, the most successful paleoseismology studies begin with aeromagnetic mapping and seismic reflection surveys to identify potentially active fault zones, move to searches for fault or fold scarps and deformed shorelines on lidar (light detection and ranging; i.e., airborne laser swath mapping) imagery, and follow up with studies of Holocene stratigraphy beneath scarps and shorelines to date prehistoric earthquake deformation (Barnett et al, 2010; e.g., Blakely et al, 2002; Johnson et al, 2004a; Sherrod et al, 2004a, 2008; Kelsey et al, 2012)
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