Earthquakes and tsunamis in the Puget Sound area from liquefaction evidence presented by Lauren Savage

Snohomish River Delta

Snohomish Sedimentary Environments In succession from deeper to shallower: zsubtidal channels zlower intertidal flats and point bars zsupratidal marsh zlower delta plain and levees

Subtidal zSubtidal environments currently submerged, so direct observation is impossible. zMostly sand and gravel. zSource of sandy dikes and sand volcanoes that will be mentioned later.

Lower Intertidal zLocation: near delta front and along channel point bars. zContains sand, silt, and mud. zUnvegetated.

Upper Intertidal zLocation: above lower intertidal flats, upper point bar and lower cutbank zContains olive-grey mud. zVegetation: Carex lyngbyei (sedge), Triglochin maritima (arrowgrass), Scirpus maritima (bulrush)

Upper Intertidal Vegetation Carex lyngbyei Triglochin maritima Scirpus maritima

Supratidal zLocation: lower delta area zMore vegetation and driftwood than Upper Intertidal. zVegetation: Deschampsia caespitosa (tufted hair grass), Typha latifolia (broadleaf cattail), in addition to that found in Upper Intertidal region. zUpper 30 cm weathered and oxidized.

Delta Plain zContains immature soils on a peaty mud substrate. zVegetation: grasses, shrubs, trees.

Evidence of Seismic Activity zAD : Event A1 (Tsunami) zAD : Event A2 (Tsunami) zAD : Event B (Tsunami, Liquefaction, Subsidence) zAD : Event C (Liquefaction) zAD : Event D (Subsidence) zAD : Event E (Liquefaction)

Evidence of Seismic Activity

Events A1 and A2 - Tsunami

Events A1 and A2 zA1 is a thin layer of medium-grained sand, and is present at one site. zA2 is a thin layer of course-grained to granule-rich sand, found cm below A1. It is present at 5 sites, and was traced 40 meters at one site and 50 meters at another.

Events A1 and A2 zA1 - possible tsunami deposit yunusual grain size ydeposited within supratidal environment zA2 - possible tsunami deposit yfines and disappears inland ydeposited in intertidal and supratidal environments ysand deposited in thin sheet

Event B - Tsunami, Liquefaction, Subsidence

Event B zSand-Clay Couplet y25-50 cm above A2 ydeposited on vegetated olive-grey mud ylayer of fine- to medium-grained sand xfrom a few mm to 5 cm thick xfines upstream xthicker over low topography, thinner over high

Event B yLayer of grey clay x5 cm thick xthicker over low topography xno thickness variation between sites xmedium light grey silty clay xsharp contact

Event B zSand layer - tsunami deposit ythin, widespread layer ythins upstream and disappears ygraded and top laminated (rapid deposition from suspension) yunique deposit since AD 700, so it must have been from a rare event ypresence of subtidal diatoms

Event B zLiquefaction ysand dikes (mm-m wide) ysome dikes cut through sand-clay couplet, meaning they are younger yothers form sand volcanoes or spread laterally at the sand-clay couplet, meaning they occurred at the same time

Liquification

Event B zSubsidence yno change in lithology above and below sand- clay couplet, but change in plant fossils yplant fossil succession from deeper to shallower xCarex xTriglochin xScirpus acutus

Upper Intertidal Vegetation Carex lyngbyei Triglochin maritima Scirpus maritima

Event B - Subsidence yAt one site, the Triglochin is found below the couplet, and Carex found above. Triglochin doesn’t reappear for another cm. yAt another site, spruce grows before the couplet, disappears after the couplet, and reappears cm later. yOf 28 sites, 15 showed abrupt subduction of variable amounts. yLikely caused by compaction and liquefaction rather than tectonic subsidence.

Events C & E - Liquefaction

Events C & E zDikes that cut through the sand-clay couplet are younger and are not associated with event B. zSand lenses (from sand volcanoes) occur at two different horizons, corresponding with events C and E.

Event D - Subsidence

Event D zVegetation change: as in event B, plant species disappear at event D, only to reappear again cm later. zStratigraphic change: from olive-colored, plant-rich sediment to grey, plant-poor sediment

Possible Earthquake Sources

zEvents A1 and A2 (AD ) ycould be the result of tectonics or of a landslide yno other evidence for an earthquake, but there is a large undated landslide to NW

Possible Earthquake Sources zEvent B (AD ) ycould correlate with an earthquake on the Seattle fault (AD ), which is known to have produced a tsunami

Possible Earthquake Sources zEvent C (AD ) ypost-dates large Seattle Fault earthquake yno evidence of faulting to NW yevidence for earthquake in Puget Lowland ycould also be plate-boundary earthquake or deep intraplate earthquake

Possible Earthquake Sources zEvent D (AD ): ycurrently only one possible Puget Lowland earthquake at AD 1200 zEvent E (AD ): ylittle evidence for corresponding earthquake

Bibliography zOvermeier, S.F., 1998, Seismic Liquifaction Features: Examples from Paleoseismic Investigations in the Continental United States: U.S. Geological Survey Open-File Report zBourgeois, J. and Johnson, S.Y., 2001, Geological evidence of earthquakes at the Snohomish delta, Washington, in the past 1200 yr: GSA Bulletin, April 2001; p zImages: yhttp://bulletin.geoscienceworld.org/cgi/content/full/113/4/482/F04 yhttp://maps.google.com/ yhttp://dnr.metrokc.gov/WTD/carnation/images/homepage/snohomish-river.jpg yhttp:// yhttp://aknhp.uaa.alaska.edu/aksedges/images/cover2.jpg yhttp:// yhttp://tematico.princast.es/mediambi/siapa/web/espacios/espacios/rn/eo/eo_limonium_th.jpg yhttp://pubs.usgs.gov/of/1998/of98-488/thumbs.html