Cycle of earthquake-induced aggradation and related tidal channel shifting, upper Turnagain Arm, Alaska, USA



    1. US Geological Survey, 4200 University Drive, Anchorage, AK, 99508–4667, USA
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      US Geological Survey, Federal Center, Mail Stop 905, Denver, CO, 80225, U.S.A.


Aggradation of intertidal sediment in upper Turnagain Arm, a macrotidal estuary located in southern Alaska near Anchorage, began because of subsidence caused by sediment consolidation and tectonic lowering of the land during the earthquake of March 27, 1964. At the head of the Arm, decrease in percent sand of surface sediments with time and return of rodents, pulmonate gastropods, trees, bushes, and grasses to the intertidal region, indicate that the rate of sediment deposition has slowed and the Upper Turnagain Arm area has returned to a state of preearthquake equilibrium only 17 years after the event. Comparison of channel locations before and after the earthquake indicates the possibility of regional postseismic tilting of the estuary.