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

2. Snohomish River Delta

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

6. 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.

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

8. 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)

9. Upper Intertidal Vegetation Carex lyngbyei Triglochin maritima Scirpus maritima

10. 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.

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

12. Evidence of Seismic Activity zAD 130-530: Event A1 (Tsunami) zAD 420-640: Event A2 (Tsunami) zAD 850-980: Event B (Tsunami, Liquefaction, Subsidence) zAD 910-990: Event C (Liquefaction) zAD 1040-1400: Event D (Subsidence) zAD 1430-1640: Event E (Liquefaction)

13. Evidence of Seismic Activity

14. Events A1 and A2 - Tsunami

15. 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 45-70 cm below A1. It is present at 5 sites, and was traced 40 meters at one site and 50 meters at another.

16. 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

17. Event B - Tsunami, Liquefaction, Subsidence

18. 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

19. 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

20. 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

21. 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

22. Liquification

24. 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

25. Upper Intertidal Vegetation Carex lyngbyei Triglochin maritima Scirpus maritima

26. Event B - Subsidence yAt one site, the Triglochin is found below the couplet, and Carex found above. Triglochin doesn’t reappear for another 50-75 cm. yAt another site, spruce grows before the couplet, disappears after the couplet, and reappears 30-100 cm later. yOf 28 sites, 15 showed abrupt subduction of variable amounts. yLikely caused by compaction and liquefaction rather than tectonic subsidence.

27. Events C & E - Liquefaction

28. 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.

29. Event D - Subsidence

30. Event D zVegetation change: as in event B, plant species disappear at event D, only to reappear again 50-100 cm later. zStratigraphic change: from olive-colored, plant-rich sediment to grey, plant-poor sediment

31. Possible Earthquake Sources

32. zEvents A1 and A2 (AD 130-640) 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

33. Possible Earthquake Sources zEvent B (AD 850-980) ycould correlate with an earthquake on the Seattle fault (AD 900- 930), which is known to have produced a tsunami

34. Possible Earthquake Sources zEvent C (AD 910-990) 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

35. Possible Earthquake Sources zEvent D (AD 1040-1400): ycurrently only one possible Puget Lowland earthquake at AD 1200 zEvent E (AD 1430-1640): ylittle evidence for corresponding earthquake

36. Bibliography zOvermeier, S.F., 1998, Seismic Liquifaction Features: Examples from Paleoseismic Investigations in the Continental United States: U.S. Geological Survey Open-File Report 94-488. 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. 482-494. 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://www.geocities.com/cpusrv/Debs-UCUT.jpg yhttp://aknhp.uaa.alaska.edu/aksedges/images/cover2.jpg yhttp://www.em.ca/garden/native/triglochin_maritima1.JPG 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