Snowhomish Delta Conclusions

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

DISCUSSION

This paleoseismologic investigation of the Snohomish River delta adds to a slowly emerging prehistoric record of strong earthquakes and ground shaking in Washington's Puget Lowland. This nascent record strongly suggests that in the last ~1200 years, the region has been subjected to stronger earthquakes and ground shaking than in historic times (since about 1870). Completing this record should constrain the number, sources, frequency, and magnitude of these large seismic events and should have significant impact on regional earthquake hazard assessments.

Important questions that a more complete paleoseismologic catalog can address include the following:

(1) During the Holocene, how many other crustal faults in the Puget Lowland besides the Seattle fault have ruptured? Evidence described for the Snohomish delta and in Bucknam and others (1992) and Sherrod (1998) argues for two or more crustal earthquakes at ~A.D. 900-950, and Snohomish data possibly indicate one or two younger crustal events. Given the nascent status of investigations, we can expect more such earthquakes to be added to this catalogue. The clustering of the two ~AD 900-950 events also raises the possibility that movement on one fault can increase tectonic load and trigger movement on other Puget Lowland crustal faults.

(2) With regard to local earthquakes producing liquefaction, the Snohomish data indicate that the ~A.D. 900 Seattle fault earthquake produced significant ground shaking almost 50 km away, to the north. How widespread are the effects of this strong earthquake, and what could be expected from a similar earthquake in the future? Moreover, both the 1949 (M 7.1) and 1965 (M 6.5) intra-plate earthquakes caused local liquefaction in the Puget Lowland, but not at the Snohomish delta. How frequently does this kind of earthquake occur, and can its effects in the geologic record be distinguished from effects of shallow crustal earthquakes or plate-boundary events?

(3) How much ground motion or deformation in the Puget Lowland is induced by large plate-boundary earthquakes? As yet, there is no conclusive paleoseismologic evidence in the Puget Lowland for the A.D. 1700 plate-boundary event. Our investigation of the Snohomish delta provides no compelling evidence for earthquake-induced effects in AD 1700. Correlation with older plate-boundary earthquakes is possible but inconclusive (Fig. 9).

The regional paleoseismologic record needed to answer these questions is presently being assembled with a diverse approach including analysis of ancient land-level changes (e.g., Bucknam and others, 1992; Sherrod, 1998); tsunami deposits (e.g., Atwater and Moore, 1992), landsliding (e.g., Jacoby and others, 1992; Schuster and others, 1992); liquefaction (e.g., Clague and others, 1997); and lacustrine turbidite deposition (Karlin and Abella, 1992, 1996; Karlin and others, 1996). Continuation of this diverse approach is essential to completion of this record and to reliable earthquake hazard assessment in the Puget Lowland.

CONCLUSIONS

A paleoseismologic investigation of the Snohomish delta has revealed evidence of at least three episodes of liquefaction, at least one abrupt subsidence event, and at least one tsunami since about A.D. 800. The most distinctive stratigraphic unit, produced during Event B, can be correlated over the entire field area and provides evidence for three earthquake-related phenomena: strong shaking leading to liquefaction, abrupt subsidence, and a tsunami. Radiocarbon ages indicate that Event B has an age of A.D. 800 to 980, similar to the age of a large prehistoric earthquake on the Seattle fault about 50 km to the south. The age coincidence, coupled with the presence of a tsunami sand on the Snohomish delta which has also been described at other localities, lead us to conclude that the Event B features were caused by the earthquake on the Seattle fault in A.D. 900-930.

Two younger sets of sand dikes that locally feed sand volcanoes are present stratigraphically above the Event B couplet at horizons representing Event C and Event E. Radiocarbon dates and other stratigraphic information suggest Event C has an age of about A.D. 910-990; we tentatively attribute this event to a shallow crustal earthquake in the southern Puget Lowland. Radiocarbon ages indicate that Event E is younger than A.D. 1450. The section above the Event B couplet also commonly includes a second sharp lithologic change from olive-gray, rhizome-rich mud to grayer, rhizome-poor mud, which may indicate a second subsidence event associated with an earthquake (Event D).

ACKNOWLEDGMENTS

We thank Brian Atwater, Bob Bucknam, Sue Cashman, John Clague, Alan Nelson, Brian Sherrod, and Robert Witter for constructive reviews and stimulating discussions. Field assistance was provided especially by Brian Atwater, Tatiana Pinegina, and Brian Sherrod. Others who accompanied and aided us in the field include Hans Abrahamson, Boyd Benson, Stein Bondevik, Judy Boughner, Sarah Brown, Edward Cranswick, Taber Hersum, Roger Lewis, Andy Moore, Juan Carlos Moya, Leslie Phillips, Linda Smith, Faith Taylor, and Jen Zweibel. Eileen Hemphill-Haley provided preliminary diatom analyses and advice. Lisa Hodges also examined several Snohomish diatom samples for her University of Washington senior thesis, under the direction of Mark Holmes and Brian Sherrod.

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