Alaska Statute 41, "to determine the potential of Alaskan laud for production of metals, minerals, fuels, and geothermal resources; the location and supplies of groundwater and construction materials; the potential geologic hazards to buildings, roads, bridges, and other installations and structures; and shall conduct such other surveys and investigations as will advance knowledge of the geology of Alaska." Cover: Portage area, Seward Highway, and upper Turnugain Arm in 1984, looking northwest. The great Alaska earthquuke of 1964 caused this area to subside about 7. 9 9 (2.4 m). Extensive tidalflooding resulted.
Microfossil diatom and pollen data from a number of cores at coastal marshes at Girdwood Flats and Kenai Flats in Cook Inlet, Alaska, suggest that there was a phase of gradual land subsidence prior to the main shock of the ad 1964 earthquake. This phase followed a long period of gradual land uplift since the last large earthquake, which occurred about 730–900 years ago. Caesium (137Cs) records indicate that subsidence started approximately 15 years before the 1964 earthquake. This pre-seismic subsidence is evident in the coastal marsh sediment sequences in changes in diatom and pollen assemblages that indicate changes in tidal-marsh environments or a change from raised-bog to marsh conditions. The microfossil evidence indicates|0.15 m pre-seismic land subsidence at both Girdwood Flats and Kenai Flats and co-seismic subsidence of|1.8 m and|0.2 m respectively.
This paper deals with the site response (SR) in the Anchorage basin in south-central Alaska. The investigation is based on the analysis of seismograms of 114 earthquakes recorded by 22 weak-motion stations and 46 earthquakes recorded by 19 strong-motion stations in the study area. We have computed SR for 41 sites, using standard spectral ratio and horizontal-to-vertical spectral ratio methods in the frequency range from 0.5 to 11 Hz. Based on these results, we have calculated band-average site response values in two frequency ranges: low frequency (from 0.5 to 2.5 Hz) and high frequency (from 3 to 7 Hz). There is a good correlation between SR values and surficial geology of the Anchorage area in the low-frequency range. SR values increase by a factor of three from the foothills of the Chugach Mountains in the east to the west towards the deeper part of the basin. The highest site response values (SR>2.5) in the same frequency range are observed in the west-central part of the city, which is underlain by cohesive facies of the Bootlegger Cove formation. The SR has a good correlation with the uppermost 30-m time-average shear-wave velocity with a correlation coefficient of 0.82. Moreover, the low-frequency SR values are close to the NEHRP site coefficients for 1 sec. However, high-frequency SR values lack correlation with 30-m average shear-wave velocity and short-period NEHRP site coefficients.
This geologic map and preliminary cross sections of central and east Anchorage, Alaska, are based on previous mapping, limited new photointerpretation, and available subsurface data. Using PC-based Geographic Information System (GIS) software, I updated and simplified the existing geologic map by adding recent fill deposits and combining units of similar genesis, composition, and age that are also recognizable in the subsurface. The GIS database consists of a USGS geologic map and over 4,000 geotechnical boreholes and water-well logs provided by numerous public and private sources. Geologic cross sections were developed by using GIS to project graphic lithologic logs into scaled vertical layouts along selected lines. I manually correlated stratigraphic units using the log sections as guides. Identification and correlation of subsurface units are somewhat hampered by complex glacial geology, sparseness of deep boreholes, and significant variation in lithologic descriptions among many drillers. Although these limitations result in some generalized, undifferentiated geologic units, the differences among interpreted units are of the level desired by the geotechnical user community for highlighting engineering and seismic behavior.
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