Rapid resetting of an estuarine recorder of the 1964 Alaska earthquake

Tides and plants have already restored much of a landscape that the 1964 Al aska earthquake destroyed. At the head of a macrotidal estuary near Anchora ge, in the vicinity of Portage, subsidence during the earthquake changed me adows, 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 th ickets 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 t he postearthquake deposits at Portage date from the first decade after the 1964 earthquake. Their thickness of 23 sites in a 0.5 km(2) area was 1.4 +/ - 0.2 m in 1973, 1.6 +/- 0.2 m in 1991, and 1.6 +/- 0.3 rn in 1998. (2) Man y of the deposits probably date from the first months after the earthquake. The deposits contain sedimentary couplets in which coarse silt or very fin e sand is capped by fine or medium silt. About 100 such couplets make up th e lowest 0.5 m or more of the postearthquake deposits in two outcrops. Thes e couplets thicken and thin rhythmically, both as groups of 5-20 couplets a nd 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 multip lied. The vegetation now resembles the fossil assemblage rooted in the buri ed landscape from 1964. Had the 1964 Alaska earthquake been repeated a decade later, the two earthq uakes would now be recorded by two superposed, buried landscapes near Porta ge. Much more than a decade is probably needed to reset similar recorders a t mesotidal estuaries of the Cascadia subduction zone.