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Rapid resetting of an estuarine recorder of the 1964 Alaska earthquake

Brian F. Atwater^*,1, David K. Yamaguchi^,2, Stein Bondevik ^,3, Walter A. Barnhardt^#,4, Lorin J. Amidon^**,5, Boyd E. Benson^##,6, Gudrun Skjerdal ^,7, John A. Shulene^###,8 and Futoshi Nanayama^***,9

¹ U.S. Geological Survey at Department of Earth and Space Sciences, University of Washington, Seattle, Washington 98195-1310, USA
² 5630 200th Street S.W., #202B, Lynnwood, Washington 98036-6260, USA
³ Department of Geology, University of Tromsø, N-9073 Tromsø, Norway
⁴ U.S. Geological Survey, M.S. 999, 345 Middlefield Road, Menlo Park, California 94025, USA
⁵ Department of Earth Sciences, Dartmouth College, Hanover, New Hampshire 03755-3571, USA
⁶ GeoEngineers, Inc., 8410 154th Avenue N.E., Redmond, Washington 98052, USA
⁷ Department of Geology, University of Tromsø, N-9073 Tromsø, Norway
⁸ 928 11th Avenue S.W., Puyallup, Washington 98371, USA
⁹ Active Fault Research Center, National Institute for Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8567, Japan

Tides and plants have already restored much of a landscape that the 1964 Alaska earthquake destroyed. At the head of a macrotidal estuary near Anchorage, in the vicinity of Portage, subsidence during the earthquake changed meadows, thickets, and spruce groves into barren tidal flats. Tidal-flat silt and sand soon buried the pre- earthquake landscape while filling intertidal space that the subsidence had made. The flats supported new meadows and thickets by 1973 and new spruce by 1980. Three new findings confirm that the flats aggraded rapidly and that their vegetation is maturing. (1) Most of the postearthquake deposits at Portage date from the first decade after the 1964 earthquake. Their thickness of 23 sites in a 0.5 km² area was 1.4 ± 0.2 m in 1973, 1.6 ± 0.2 m in 1991, and 1.6 ± 0.3 m in 1998. (2) Many of the deposits probably date from the first months after the earthquake. The deposits contain sedimentary couplets in which coarse silt or very fine sand is capped by fine or medium silt. About 100 such couplets make up the lowest 0.5 m or more of the postearthquake deposits in two outcrops. These couplets thicken and thin rhythmically, both as groups of 5–20 couplets and as pairs of successive couplets. Probably, the groups of thick couplets represent the highest tides, the groups of thin couplets represent some of the lesser high tides, and the pairs record inequality between twice-daily high tides. (3) In the 1980s and 1990s, thickets expanded and spruce multiplied. The vegetation now resembles the fossil assemblage rooted in the buried landscape from 1964.

Had the 1964 Alaska earthquake been repeated a decade later, the two earthquakes would now be recorded by two superposed, buried landscapes near Portage. Much more than a decade is probably needed to reset similar recorders at mesotidal estuaries of the Cascadia subduction zone.

Key Words: Alaska • earthquakes • estuaries • Holocene • stratigraphy

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