Holocene earthquakes and right-lateral slip on the left-lateral Darrington–Devils Mountain fault zone, northern Puget Sound, Washington

Abstract

Sources of seismic hazard in the PugetSound region of northwestern Washington include deep earthquakes associated with the Cascadia subduction zone, and shallow earthquakes associated with some of the numerous crustal (upper-plate) faults that crisscross the region. Our paleoseismic investigations on one of the more prominent crustal faults, the Darrington-Devils Mountain fault zone, included trenching of fault scarps developed on latest Pleistocene glacial sediments and analysis of cores from an adjacent wetland near Lake Creek, 14 km southeast of Mount Vernon, Washington. Trench excavations revealed evidence of a single earthquake, radiocarbon dated to ca. 2 ka, but extensive burrowing and root mixing of sediments within 50-100 cm of the ground surface may have destroyed evidence of other earthquakes. Cores in a small wetland adjacent to our trench site provided stratigraphic evidence (formation of a laterally extensive, prograding wedge of hillslope colluvium) of an earthquake ca. 2 ka, which we interpret to be the same earthquake documented in the trenches. A similar colluvial wedge lower in the wetland section provides possible evidence for a second earthquake dated to ca. 8 ka. Three-dimensional trenching techniques revealed evidence for 2.2 ± 1.1 m of right-lateral offset of a glacial outwash channel margin, and 45-70 cm of north-side-up vertical separation across the fault zone. These offsets indicate a net slip vector of 2.3 ± 1.1 m, plunging 14° west on a 286°-striking, 90°-dipping fault plane. The dominant right-lateral sense of slip is supported by the presence of numerous Riedel R shears preserved in two of our trenches, and probable right-lateral offset of a distinctive bedrock fault zone in a third trench. Holocene northside-up, right-lateral oblique slip is opposite the south-side-up, left-lateral oblique sense of slip inferred from geologic mapping of Eocene and older rocks along the fault zone. The cause of this slip reversal is unknown but may be related to clockwise rotation of the Darrington-Devils Mountain fault zone into a position more favorable to right-lateral slip in the modern N-S compressional stress fi eld.

Quaternary Rupture of a Crustal Fault beneath Victoria, British Columbia, Canada

“…Within a zone up to 1 km wide, we observe near vertical faults, variable scarp facing directions, laterally discontinuous surface scarps, and field evidence for strike-slip and reverse faulting. These characteristics are typical of strike slip systems and are similar to features observed along active oblique-reverse faults in the adjacent Pacific Northwest (e.g., Johnson et al, 2001;Sherrod et al, 2008Sherrod et al, , 2016Kelsey et al, 2012;Personius et al, 2014;Blakely et al, 2014).…”

“…Yet, trenching, coring, and geophysical studies indicate multiple Quaternary ruptures of adjacent fault systems in Washington state, USA, including the Southern Whidbey Island fault, the Utsalady Point fault, and the DarringtonDevil's Mountain fault ( Fig. 1) (Johnson et al, 1996(Johnson et al, , 2001Sherrod et al, 2008;Personius et al, 2014). Quaternary seismic activity is also recognized 10-20 km offshore of the Leech River fault along a structure in the Juan de Fuca Strait (Barrie and Greene, 2015) (Figs.…”

“…JF-Juan de Fuca plate; PA-Pacific plate. (B) Population centers (Balk et al, 2006) relative to mapped active faults in black USGS, 2010;Kelsey et al, 2012;Personius et al, 2014;Barrie and Greene, 2015). The Leech River fault (LRF) is shown as dashed line.…”

Holocene Surface Rupture History of an Active Forearc Fault Redefines Seismic Hazard in Southwestern British Columbia, Canada

“…Geologic mapping indicates that the faults in the trenches are part of a 100‐ to 500‐m‐wide steep Quaternary‐active fault zone that extends for ∼30 km eastward from the trench sites to the shoreline near Colwood (Figure b; Graham, ; Morell et al, ). Marine geophysical surveys by Barrie and Greene () confirm that this fault system continues eastward for an additional ∼50 km offshore beneath the Juan de Fuca Strait to connect with the Holocene‐active Darrington‐Devil's Mountain fault zone of Washington State (Personius et al, ; Figure a). Collectively, these observations suggest that the active fault system in the trenches extends to the east for ∼130 km.…”

“…(a) Regional setting showing simplified trace of forearc faults (in red) identified as Quaternary‐active (Barrie & Greene, ; Nelson et al, ; Personius et al, ; USGS Quaternary fault and fold database for the United States, ) with respect to cities in British Columbia (BC), Canada, and Washington (WA), United States. Faults beneath the Pacific Ocean are not shown.…”