Dames & Moore, 1999 - USDA Forest Service

make up the earth's surface. The stresses accumulate because of friction between the plates as they
attempt to move past one another. The movement can be between plates (such as when one plate moves
over another, as in subduction zones) or within the plates themselves.
The recorded earthquakes (magnitude 4.0 or greater) in Washington State are presented on Figure 4.2-2.
With this information, predictions have been made about the severity of hture earthquakes. Intraplate
seismic events appear to be more widespread geographically, and result from various structural sources in
the shallow crust. These events often occur along mapped or postulated faults in the earth's surface. The
largest instrumented earthquake in greater site vicinity was a magnitude 6.0 (Richter scale) event in 1990
approximately 44 miles from the Site (NOAA, 1998). The event resulted in an estimated horizontal
acceleration of 0.03 g at the Site. A more in-depth discussion of the seismic potential is presented in Section
4.2.4 of this report.
The seismicity of the area is noted to be moderate. The Uniform Building Code (UBC, 1994) maps the
seismicity as a Zone 2B on a scale of 0 to 4, with 0 having no seismic activity, and 4 having high seismicity.
Three translational faults were mapped in the Site underground mine: steep, eastward-dipping normal
faults, and westward-dipping thrust faults. No surface expressions of recent or historic faults are noted in
the area. This observation would lead one to suspect that the faults are not active.
Based on the geology of the area, the potential hazards associated with seismicity are likely limited to
ground shaking and liquefaction. Ground shaking is rapid movement of the surface which can result in
damage to structures. Liquefaction is a condition in which saturated, loose sand soil behaves as a fluid
during seismic events, therefore, resulting in a reduction in soil strength at the time of the event, much like
quicksand.
4.2.1.3 Volcanic Activity
Referring to Figure 4.2-la, Washington has several major composite volcanoes and one area of extensive
basaltic shield volcanoes which have been active within the last 2 million years and, in some cases, active
within the last 20 years. The nearest volcano is Glacier Peak, approximately 20 miles west of the Site,
which experienced documented activity approximately 12,000 years ago (Tabor and Crowder, 1969).
4.2.1.4 Geology and Mineral Resources
A discussion of the North Lake Chelan geology is presented herein to support the selection of the aquatic
reference reaches. Refening to Figure 4.2-3, the geology of the upper Lake Chelan area consists of bedrock
overlain by unconsolidated to semi-consolidated glacial, fluvial, lacustrine and colluvial deposits of silt,
sand and gravel (Cater and Crowder, 1967). The geologic basement rocks (bedrock) found within the North
Lake Chelan watershed consist primarily of massive granitics and metamorphosed sedimentary rocks
including hornblende, gneiss, and schist. The bedrock outcrops at the surface in the high mountain ranges
and is encountered at depths ranging from 20 to greater than 100 feet deep in the valleys. The bedrock is, in
places, fractured, folded and faulted.
As noted on Figure 4.2-3, the North Lake Chelan basin has been mapped as containing a number of
economic mineral deposits with associated historic mineral claims andlor mine workings. In terms of the
drainages sampled during the aquatic reference reach sampling program (Section 4.6), the mineralization
G:\WPDATA\OONPORTSWOLDEN-2W.DOC
17693-005-019Uuly 19. 1999;4:5 1 PM;DRAFT FINAL Rl REPORT