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Table 8.2: Conversion Table for Various Data
DATUM
M (ft)
NGVD 1
Mean Sea
Level City of Portland
National
Weather Service NAVD88 2
CONVERSION 0.0 0.0 0.43 (1.4) 0.55 (-1.8) 1.07 (3.5)
1 NGVD – National Geodetic Vertical Datum
2 NAVD88 – North American Vertical Datum of 1988
8.2.2 Seismic Hazard
The overall seismic hazard in the Portland region reflects the contributions of three seismic
sources: (1) interplate earthquakes along the Cascadia Subduction Zone located near the Pacific
coast; (2) relatively deep intraplate subduction zone earthquakes that may be located as far inland
as the Portland metropolitan region; and (3) relatively shallow crustal earthquakes of located in
the Portland region. During the last 150 years the Portland metropolitan area has been subjected
to six earthquakes of Richter magnitude (also called the Local magnitude - M L ) 5 or greater,
including the 1962 M L 5.5 Portland and the 1993 M L 5.6 Scotts Mills earthquakes (Wong et al.
2000). Recent studies have been performed that indicate the presence of three crustal faults
beneath the Portland area that could generate earthquakes of M L 6.5 or greater (Blakely et al.
1995; Pratt et al. 1999 in Wong et al. 2000). The Cascadia Subduction Zone has been estimated
to be capable of generating a moment magnitude (M w ) 8 to 9 earthquake (Geomatrix 1995).
8.2.2.1 Interplate Earthquakes
An interplate earthquake occurs due to movement at the interface of tectonic plates.
These earthquakes are usually relatively shallow thrust events, occurring in the upper 50
km of the earth’s crust. The Cascadia Subduction Zone, consisting mainly of the interface
of the Juan De Fuca and the North America Plates, off the coast of Oregon and
Washington, provide the potential for subduction zone events in the Pacific Northwest.
Figure 8.2 shows a schematic of the Cascadia Subduction Zone near Oregon and
Washington. For the purpose of this study, the eastern edge of the seismogenic portion of
the interface was assumed to be about 80 km west of Portland, and approximately 25 km
below mean sea level.
8.2.2.2 Intraslab Earthquake
An intraslab earthquake originates within a subducting tectonic plate and occurs at
distances from the edges of the plate. It is caused by the release of built up stresses within
a tectonic plate as it dives below an overriding plate. For Oregon, the Juan De Fuca Plate
provides the potential for such an event, as it dips below the North America Plate. The M s
7.1 Olympia Earthquake of 1949, the M s 6.5 Puget Sound Earthquake of 1965, and the
M w 6.8 Nisqually Earthquake of 2001 are examples of intraslab earthquakes occurring in
the Juan De Fuca Plate below Washington.
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