Geologic evidence of earthquakes at the Snohomish delta, Washington, in the past 1200 yr

Abstract

Exposed channel banks along distributaries of the lower Snohomish delta in the Puget Lowland of Washington reveal evidence of at least three episodes of liquefaction, at least one event of abrupt subsidence, and at least one tsunami since ca. A.D. 800. The 45 measured stratigraphic sections consist mostly of 2-4 m of olivegray, intertidal mud containing abundant marsh plant rhizomes. The most distinctive stratigraphic unit is a couplet comprising a 0.5؊3-cm-thick, laminated, fining-upward, tsunami-laid sand bed overlain by 2؊10 cm of gray clay. We correlated the couplet, which is generally ϳ2 m below the modern marsh surface, across an ϳ20 km 2 area. Sand dikes and sand-filled cracks to 1 m wide, which terminate upward at the couplet, and sand volcanoes preserved at the level of the sand bed record liquefaction at the same time as couplet deposition. Differences in the type and abundance of marsh plant rhizomes across the couplet horizon, as well as the gray clay layer, suggest that compaction during this liquefaction led to abrupt, local lowering of the marsh surface by as much as 50-75 cm. Radiocarbon ages show that the tsunami and liquefaction date from ca. A.D. 800 to 980, similar to the age of a large earthquake on the Seattle fault, 50 km to the south. We have found evidence for at least two, and possibly as many as five, other earthquakes in the measured sections. At two or more stratigraphic levels above the couplet, sand dikes locally feed sand volcanoes. Radiocarbon ages and stratigraphic position

Significance of active growth faulting on marsh accretion processes in the lower Pearl River, Louisiana

“…Evidence of fault motion could be expressed in the geomorphology, surface sediments and shallow stratigraphy of this environment as follows: 1) geomorphologic change that includes formation of open water areas and expansion of water bodies into unique geometries within the marsh interior (White and Morton, 1997;Gagliano et al, 2003;Nichol et al, 2007); 2) quantifiable changes or patterns in fluvial geomorphology, including variations of river and distributary planforms, channel sinuosity, and gradients (Burnett and Schumm, 1983;Maroukian et al, 2008); 3) localized and accelerated wetland loss and rates of subsidence (Morton et al, 2002;Shinkle and Dokka, 2004); 4) quantifiable lateral discontinuities near fault surface traces in sediment accumulation rates over small spatial scales; and 5) subsurface indicators including lateral chronostratigraphic offsets of dated facies, and/or lithostratigraphic or biostratigraphic offsets of facies or fossil assemblages over short distances (400 m), and/or high-resolution seismic reflectors that display deformation in the form of offsets, unique terminations, and abrupt changes of reflector orientations and trends (e.g., Bourgeois and Johnson, 2001;Hayward et al, 2004;Ferranti et al, 2008;Nixon et al, 2009). Other studies focused on the geomorphology and stratigraphy of coastal Louisiana have noted similar physical relationships with fault motion (e.g., Holbrook and Schumm, 1999;Dokka et al, 2006;Morton et al, 2006).…”

Evidence for Late Holocene Earthquakes on the Utsalady Point Fault, Northern Puget Lowland, Washington

“…11). Strong ground motion associated with the most recent earthquake on the Utsalady Point fault may have also caused liquefaction dated to about A.D. 1430 to 1640 in the Snohomish River delta (Bourgeois and Johnson, 2001) (Fig. 1).…”

Habitat characterization of a tidal energy site using an ROV: Overcoming difficulties in a harsh environment

“…Paleoseismologic studies have shown that at least five to seven Holocene ground-rupturing earthquakes have occurred (Bucknam et al, 1992;Sherrod et al, 2000;Nelson et al, 2003;Kelsey et al, 2004). Evidence exists that show that earthquake associated deformation of the seafloor or associated landslides have produced tsunamis (Atwater and Moore, 1992;Bourgeois and Johnson, 2001;Williams and Hutchinson, 2000;Williams et al, 2005), which have a potential of impacting the habitats within Admiralty Inlet. Multibeam echosounder bathymetric image with interpreted potential benthic habitat types in the vicinity of the originally-proposed Snohomish tidal turbine sites offshore Admiralty Head, Whidbey Island, Washington.…”