3.0 Affected Environment - Knik Arm Bridge and Toll Authority

Knik Arm Crossing DraftFinal EIS
Affected Environment
Earthquake ground shaking hazards depend on earthquake magnitude, probability of
occurrence (return period), and soil type. Several site-specific probabilistic seismic hazard
analyses have been conducted for this Draft EIS to assess ground shaking hazards under local
soil conditions and the relative potential contributions from different faults. The results of
these analyses indicate that both deep subduction zone earthquakes and more nearby shallow
crustal earthquakes would be the most significant sources of seismic hazard to the proposed
project (Crouse 2005; Martirosyan and Biswas 2004, 2005). Detailed results of these
analyses, including ground accelerations predicted by the different earthquake models for
various return periods and soil types, are provided in the Knik Arm Crossing Seismic Studies
Technical Report (KABATA 2006j).
3.5.3.2.2 Surface faults
No active surface faults are known to cross the proposed KAC alternatives. The closest
potentially active surface fault to the Study Area is the Little Susitna River Scarp fault,
located about 7 miles west of the Mat-Su alignments (Figure 3.30). This fault is suspected of
offsetting late Pleistocene deposits (Plafker et al. 1993).
Near-surface folding and reverse faulting of Tertiary and Quaternary strata are suspected to
be actively occurring throughout the Cook Inlet basin (Haeussler et al. 2000). Several of the
fault-cored folds along the west side of Upper Cook Inlet exhibit evidence of active
deformation of the seafloor or ground surface in this area. A similar fold was mapped along
the west side of Knik Arm and crosses the Mat-Su alternatives, referred to as the Lorraine-
Alaska Gulf fold (Haeussler et al. 2000; Magoon et al. 1976). Based on its similarity to other
well-studied features in Cook Inlet basin, it is possible that it could represent the site of nearsurface
active folding or reverse faulting.
3.5.3.2.3 Ground failure and liquefaction
Earthquakes cause ground shaking that can result in ground failure and structural damage or
loss. Ground failure in the event of a major earthquake can take several forms, such as
landsliding, surface cracking, land spreading, liquefaction, and subsidence. Winterhalder et
al. (1979) and the Municipality of Anchorage (2001a) mapped the relative potential for
seismically induced ground failure in the Anchorage area based on a combination of the
above causes and localized ground conditions (Figure 3.31). The Below-the-Bluff Roadway
portion of the proposed KAC project and the Anchorage approach alternatives each cross
areas of high-to-very high predicted ground failure (predictions based largely on the potential
for translational landsliding).
Liquefaction generally occurs during earthquakes in loose, saturated sands and silty sands,
because of the rapid buildup of pore water pressure and resulting loss of strength and bearing
capacity. Although most clay deposits typically have little-to-no liquefaction potential,
sensitive clays (also known as quick clays) such as those within the Bootlegger Cove
Formation (Section 3.5.3.1.2), can also produce an effect similar to liquefaction during an
earthquake (Hungr et al. 2001; Noson et al. 1988). Failure of the Bootlegger Cove sensitive
clays and resulting translational landsliding during the 1964 earthquake are
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