Earthquakes and tsunamis in the Puget Sound area from

liquefaction evidence

presented by Lauren Savage

Snohomish River Delta

Snohomish River Delta

Snohomish River Delta

Snohomish Sedimentary Environments

In succession from deeper to shallower:

subtidal channelslower intertidal flats and point barssupratidal marshlower delta plain and levees

Subtidal

Subtidal environments currently submerged, so direct observation is impossible.

Mostly sand and gravel.Source of sandy dikes and sand

volcanoes that will be mentioned later.

Lower Intertidal

Location: near delta front and along channel point bars.

Contains sand, silt, and mud. Unvegetated.

Upper Intertidal

Location: above lower intertidal flats, upper point bar and lower cutbank

Contains olive-grey mud.Vegetation: Carex lyngbyei (sedge),

Triglochin maritima (arrowgrass), Scirpus maritima (bulrush)

Upper Intertidal Vegetation

Carex lyngbyei Triglochin maritima Scirpus maritima

Supratidal

Location: lower delta areaMore vegetation and driftwood than

Upper Intertidal.Vegetation: Deschampsia caespitosa

(tufted hair grass), Typha latifolia (broadleaf cattail), in addition to that found in Upper Intertidal region.

Upper 30 cm weathered and oxidized.

Delta Plain

Contains immature soils on a peaty mud substrate.

Vegetation: grasses, shrubs, trees.

Evidence of Seismic Activity

AD 130-530: Event A1 (Tsunami) AD 420-640: Event A2 (Tsunami)AD 850-980: Event B (Tsunami,

Liquefaction, Subsidence)AD 910-990: Event C (Liquefaction)AD 1040-1400: Event D (Subsidence) AD 1430-1640: Event E (Liquefaction)

Evidence of Seismic Activity

Events A1 and A2 - Tsunami

Events A1 and A2

A1 is a thin layer of medium-grained sand, and is present at one site.

A2 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.

Events A1 and A2

A1 - possible tsunami deposit unusual grain size deposited within supratidal environment

A2 - possible tsunami deposit fines and disappears inland deposited in intertidal and supratidal

environments sand deposited in thin sheet

Event B - Tsunami, Liquefaction, Subsidence

Event B

Sand-Clay Couplet 25-50 cm above A2 deposited on vegetated olive-grey mud layer of fine- to medium-grained sand

from a few mm to 5 cm thickfines upstreamthicker over low topography, thinner over

high

Event B

Layer of grey clay5 cm thickthicker over low topographyno thickness variation between sitesmedium light grey silty claysharp contact

Event B

Sand layer - tsunami deposit thin, widespread layer thins upstream and disappears graded and top laminated (rapid

deposition from suspension) unique deposit since AD 700, so it must

have been from a rare event presence of subtidal diatoms

Event B

Liquefaction sand dikes (mm-m wide) some dikes cut through sand-clay

couplet, meaning they are younger others form sand volcanoes or spread

laterally at the sand-clay couplet, meaning they occurred at the same time

Liquification

Liquification

Event B

Subsidence no change in lithology above and below

sand-clay couplet, but change in plant fossils

plant fossil succession from deeper to shallowerCarexTriglochinScirpus acutus

Upper Intertidal Vegetation

Carex lyngbyei Triglochin maritima Scirpus maritima

Event B - Subsidence At one site, the Triglochin is found below the

couplet, and Carex found above. Triglochin doesn’t reappear for another 50-75 cm.

At another site, spruce grows before the couplet, disappears after the couplet, and reappears 30-100 cm later.

Of 28 sites, 15 showed abrupt subduction of variable amounts.

Likely caused by compaction and liquefaction rather than tectonic subsidence.

Events C & E - Liquefaction

Events C & E

Dikes that cut through the sand-clay couplet are younger and are not associated with event B.

Sand lenses (from sand volcanoes) occur at two different horizons, corresponding with events C and E.

Event D - Subsidence

Event D

Vegetation change: as in event B, plant species disappear at event D, only to reappear again 50-100 cm later.

Stratigraphic change: from olive-colored, plant-rich sediment to grey, plant-poor sediment

Possible Earthquake Sources

Possible Earthquake Sources

Events A1 and A2 (AD 130-640) could be the result of

tectonics or of a landslide

no other evidence for an earthquake, but there is a large undated landslide to NW

Possible Earthquake Sources

Event B (AD 850-980) could correlate with

an earthquake on the Seattle fault (AD 900-930), which is known to have produced a tsunami

Possible Earthquake Sources

Event C (AD 910-990) post-dates large Seattle Fault

earthquake no evidence of faulting to NW evidence for earthquake in

Puget Lowland could also be plate-boundary

earthquake or deep intraplate earthquake

Possible Earthquake Sources

Event D (AD 1040-1400): currently only one possible Puget

Lowland earthquake at AD 1200Event E (AD 1430-1640):

little evidence for corresponding earthquake

Bibliography

Overmeier, S.F., 1998, Seismic Liquifaction Features: Examples from Paleoseismic Investigations in the Continental United States: U.S. Geological Survey Open-File Report 94-488.

Bourgeois, 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.

Images: http://bulletin.geoscienceworld.org/cgi/content/full/113/4/482/F04 http://maps.google.com/ http://dnr.metrokc.gov/WTD/carnation/images/homepage/snohomish-river.jpg http://www.geocities.com/cpusrv/Debs-UCUT.jpg http://aknhp.uaa.alaska.edu/aksedges/images/cover2.jpg http://www.em.ca/garden/native/triglochin_maritima1.JPG http://tematico.princast.es/mediambi/siapa/web/espacios/espacios/rn/eo/eo_limonium_th.jpg http://pubs.usgs.gov/of/1998/of98-488/thumbs.html