Tidal_PAD_V1_Sec4.pdf - Snohomish County PUD

Snohomish County PUD –Pre-Application Document
4 – Description of Existing Environment
lahars, originating from the now-4,300-meter-high mountain within the past approximately 0.85
million years (Swanson et al. 1989). A number of more-recent volcanic events have occurred
during the Holocene era, within the past 10,000 years, since the retreat of the Cordilleran ice
sheet.
Seismic activity occurs in the region in three different zones: subduction zone earthquakes
where the North American plate and the Juan de Fuca plate make contact; deep earthquakes,
where the subducted portions of oceanic plate slip into the upper mantle; and shallow quakes,
which occur on faults within the North American continental area (USGS 2006b). The largest of
these quakes are subduction quakes; magnitudes of earthquakes in the Cascadia subuction zone
can exceed 9.0 on the Richter scale (USGS 2006b). Deep zone or Benioff zone earthquakes may
be as large as magnitude 7.5, but are generally more dispersed, and are not accompanied by
aftershocks (USGS 2006). Shallow quakes generally have somewhat smaller magnitudes, but
their proximity to the surface and to urban areas as well as the potential to trigger liquefaction
events or mass-wasting can make shallow earthquakes just as devastating as deeper earthquakes
(USGS 2006b). Figure 4-1 shows the major surface faults of the Puget Sound area, which
cause shallow, crustal earthquakes. Table 4-1 summarizes which potential project sites contain
known faults.
The Seattle fault is the only crustal fault in the area that has been verified to be recently active,
and has been responsible for a number of crustal earthquakes within the past 14,000 years
(Johnson 1999). It has not, however, been the source of the 20 th -century earthquakes in the
Seattle area; the 1949 and 1965 earthquakes originated in the Benioff zone (USGS 2006). The
North Whidbey Island and South Whidbey Island faults are currently being studied, but no recent
activity has proven them to be currently or recently active. Geologic investigations indicate that
seismic events associated with the Seattle fault have caused liquefaction and ground subsidence
in the past and that the potential for liquefaction still exists (Johnson et al. 2006).
4-6 January 31, 2008