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Reconnaissance engineering geology of the Yakutat area, Alaska, with emphasis on evaluation of earthquake and other geologic hazards

1979; United States Government Publishing Office; Linguagem: Inglês

10.3133/pp1074

2330-7102

Lynn A. Yehle,

Geotechnical and Geomechanical Engineering

Yakutat, situated about 360 km northwest of Juneau, Alaska, near the shore of the Gulf of Alaska, has a setting that calls for superlatives.Within the Yakutat region are some of the tallest mountains, some of the heaviest snowfalls, and the largest glacier (Malaspina) in North America.Between the abrupt mountain front and the Gulf of Alaska lies a very gently sloping plain of outwash derived from repeated cycles of advance and retreat of glaciers during the Quaternary Pe riod.The latest melting probably took place 500 to 600 years ago.Yakutat is built upon the moderately steep moraine that is the product of melting of one of these glaciers.Near Yakutat, surficial deposits may be more than 213 m thick and they may overlie siltstone, sand stone, and mudstone.The eight general categories of mapped surficial deposits are artificial fill, organic, eolian, beach, delta-estuarine, allu vial, and outwash deposits, and end and ground moraine deposits of the area.The Yakutat region is part of an active tectonic belt that rims most of the North Pacific Ocean.The latest episode of activity probably began in early Miocene time.Many faults are active, as indicated by numerous earthquakes in the area.One fault is known to have broken the ground surface in historic time, the nearly vertical Fairweather fault, whose closest segment is about 53 km northeast of Yakutat.Movement along this fault caused the major earthquake of July 10, 1958 (magnitude 7.9).The great earthquake of September 10, 1899 (magnitude 8.6), probably was caused by tectonic uplift within a broad area centered about 47 km north of Yakutat.Uplift averaged 2-3 m; one small area shows uplift of about 14.4 m, the greatest onshore uplift ever measured for an earthquake sequence.A sequence of earthquakes in July 1973, centered offshore approximately southeast of Yakutat, has been related to an inferred fault, the Transition fault.Many earthquakes have been felt at Yakutat since written historic records were first kept.Five very large earthquakes (magnitude 7.0 to 8.6) occurred within a radius of 130 km from Yakutat in the period from 1893 through 1975.Of these earthquakes, those of September 10, 1899, and July 10, 1958, were the strongest, causing some damage to buildings at Yakutat.Earthquakes of equally large size undoubtedly will shake the Yakutat area in the future.Several geologic effects that have characterized large earthquakes in the past may be expected to accompany large earthquakes in the future.These effects include (1) tectonic uplift; (2) severe ground shaking; (3) liquefaction of some delta-estuarine, alluvial, and fine grained outwash deposits; (4) ejection of water and sand as fountains from individual craters; (5) compaction and differential subsidence of some materials, especially young and intermediate delta-estuarine de posits and some artificial fills; and (6) landslides, where terrain is steep or where beach spits and delta-estuarine deposits are newly deposited and poorly consolidated.Large water waves commonly are formed during large coastal earth quakes.Tsunami (seismic sea) waves arriving from large earthquakes at great distances can be forecast, but advance warnings of locally formed waves cannot be provided.To date, the highest recorded earthquake-related waves at Yakutat were about 5 m high and oc curred during the largest earthquake of the sequence during Septem ber 10, 1899.Geologic hazards in the Yakutat region not necessarily associated with earthquakes include (1) subaerial and submarine landsliding, (2) stream flooding and erosion of surficial deposits, (3) high waves, and (4) glacier advances and breakout of glacier-dammed lakes.Recommended future geologic and geophysical studies in the Yak utat area and surrounding region could provide additional informa tion needed for land-use planning.Expansion of general and detailed geologic mapping and collection of data on geologic materials, joints, faults, and stability of slopes are strongly recommended to help de lineate areas of economic mineral deposits, to identify hazardous slopes, and to locate suitable areas for construction.There should be expansion of the studies of earthquakes in the region.Such work might lead to prediction of the location of future large earthquakes.Installation of additional seismological instruments could provide in formation on the location of any unknown active faults and an index of the overall tectonic activity of the region.Additional offshore geo physical studies are needed to determine the nature and position of submarine faults and their relationship to the stability of sediments on the sea floor.Determination of the natural periods of oscillation of Yak utat Bay and adjoining fiords as well as the nearby Continental Shelf would assist in the prediction of heights of tsunami and other water waves that might be associated with seismic shaking.Other studies might concentrate on analysis of slopes that appear unstable and possibly are subject to landsliding.Finally, glaciers that (1) are advancing rapidly, (2) appear likely to form glacier-dammed lakes, or (3) appear susceptible to sudden breakage should be studied.