ABSTRACT

Erosion and Deposition on a Beach Raised by the 1964 Earthquake Montague Island, Alaska   [open pdf - 13MB]

"On March 27, 1964, an earthquake shook parts of Alaska and caused great devastation in inhabited areas. However, maximum uplift took place, and large bedrock faults were active, on Montague Island, an uninhabited island off the southern coast of Alaska. The southern end of Montague Island was uplifted as much as 33 feet or more during the earthquake (fig. 1), and two active faults several miles long caused many landslides and broke or displaced many hundreds of trees (Plafker, 1967). Around the coast, marine deposits and a marine-abraded bedrock platform were lifted above sea level and exposed to subaerial processes. The uplift associated with the earthquake has provided physio-graphic conditions on a scale much larger than could be simulated in a laboratory and with the added advantage that the features and changes were natural. The bay-head deposits of MacLeod Harbor (figs. 1, 2) on the northwest coast of the island provided an undissected surface, about 1 square mile in area and with a slope of only 20', which had been suddenly uplifted 33 feet. On this surface, consequent drainage channels were init.iated and were rapidly eroded vertically and laterally; thus, a unique opportunity was provided to study the effects of rapid uplift on the mode and rate of fluvial processes and the resultant valley forms and long profiles. The earthquake also brought to view a bedrock platform and associated deposits, which allowed observation and measurement of features that are usually under water and therefore difficult to study.On both sides of the island the amount of subaerial modification of the 1964 raised beach could be measured. Especially important was the opportunity to study the area only 15 months after the earthquake. In the MacLeod Harbor area, rivers in soft sediments have adjusted so swiftly that evidence of the post-earthquake history of the area will soon be removed by erosion. Subaerial degradation of the raised beaches and abandoned seadiffs is so rapid that within a few years little of the original marine form will be left unchanged, and the growth of vegetation, which has already begun on the beach, will hasten its obliteration."

Report Number:
Geological Survey Professional Paper No. 543-H
Author:
Publisher:
Date:
1969
Copyright:
Public Domain
Retrieved From:
U.S. Geological Survey: http://www.usgs.gov/
Format:
pdf
Media Type:
application/pdf
URL:
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