3.0 Affected Environment - Knik Arm Bridge and Toll Authority

Knik Arm Crossing DraftFinal EIS
Affected Environment
and depressions left behind by the Naptowne Glaciation. Present use of these areas is
primarily for wildlife habitat, with minor homesite use in the more well-drained soil zones. In
addition, Kichatna-Delyndia soils are used as a sand and gravel source (NRCS 1998).
NRCS soils data related to soil suitability for building (1998) incorporate the effects of
wetness, ponding, low strength, frost action, and slopes into categories ranging from slightly
to severely limiting for this purpose (Figure 3.29). Soils on the west side of Burma and Upper
Point MacKenzie Roads, including those associated with the Point MacKenzie Agricultural
Area, exhibit some of the best conditions for construction of homesites and small commercial
buildings. Soils along the east side of Knik-Goose Bay Road have similar building suitability
ratings. Soils in the remaining areas are rated as having moderate-to-severely limiting
building suitability characteristics, largely because of the presence of wet hydric soils and
low-strength peats and mucks.
Anchorage approach alternatives
Soils mapped along the steep slopes of Knik Arm and Ship Creek are composed of well
drained Smithfa, Deception-Estelle-Kichatna, and Kashwitna-Kichatna complex sandy silt
loams derived from gravelly till, outwash alluvium, and eolian deposits. In addition to soils
on urban lands, the soils of Government Hill and the Ship Creek terraces are classified as
Cryorthents. Cryorthents typically consist of very well-drained gravelly sandy loam derived
from glacial outwash. Soils in Ship Creek bottom consist of poorly-to-moderately welldrained
Moose River-Niklason complex derived from alluvium parent material
(NRCS 2001).
3.5.3.2 Seismic hazards
3.5.3.2.1 Seismicity and ground shaking
The Study Area is located in one of the most tectonically active regions of the world. The
Upper Cook Inlet basin is characterized by numerous potentially active fault-cored folds
located in between two major linear faults (Figure 3.30) and underlain by the subduction
zone between the North American and Pacific plates. Historical seismicity in the region has
come from several sources:
• Megathrust and deeper earthquakes associated with the subduction zone, such as the
1964 Alaska earthquake of magnitude 9.2 on the Richter scale.
• Strike-slip earthquakes associated with the surficial transform boundary between plates,
such as the Denali and Castle Mountain faults.
• Shallow crustal earthquakes within the upper North American Plate, such as those which
may be associated with the smaller structures on Figure 3.30. These are considered to be
capable of earthquakes in the range of magnitude 6 to 7 (Crouse 2005; Haeussler et
al. 2000; Martirosyan and Biswas 2004; KABATA 2006j).
12/18/07 3-119