Arctic GIS Workshop Poster Abstract22-24
January 2001 |
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Geologic Map and Cross Sections of Anchorage, Alaska, Using GIS
The Alaska Division of Geological & Geophysical Surveys has developed a geologic map and cross sections of Anchorage, Alaska, based on previous mapping, photointerpretation, and available subsurface data. Using PC-based Geographic Information System (GIS) software, the existing geologic map was updated and simplified 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. Borehole-data software operating within GIS allows projecting the lithologic logs into cross-section layouts along selected lines. To create geologic cross sections, stratigraphic units were manually correlated using the log sections as guides. Identification and correlation of subsurface units is hampered by complex glacial geology, sparseness of deep boreholes, and significant variation in lithologic descriptions among dozens of 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. Cross sections through downtown Anchorage show a sedimentary basin filled with soft wet clay (fine-grained facies of Bootlegger Cove formation), which locally extends more than 200 ft below sea level. Data from many earthquakes worldwide indicate that thick deposits of soft clay significantly amplify ground motion, particularly at low frequencies of shaking (<1 Hz). This was the observed behavior of ground motion in and near downtown Anchorage during the 1964 great Alaska earthquake (Mw 9.2). Limited subsurface data also reveal a thick deposit of similar clay in parts of southern Anchorage. These areas of thick, soft clay were the locations of major ground failures in 1964 and, in recent seismic experiments, are the areas showing highest measured amplification of low-frequency ground motions. The new geologic map and cross sections provide critical geologic control for earthquake-hazard maps now in preparation. These include maps of site-amplification factors, NEHRP seismic soil types, and liquefaction susceptibility.
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