Abstract

1
Regional Case Study I Seismic Hazards in the
Seattle Area
Brad Adams, ES473 Environmental Geology, Spring
Term 2009

• Seattle Fault Details and History
• Consist, of segmented east trending, north
  verging thrust faults (Fig. 2)
• Its being active since 40 MA to the present
• Mostly shallow crustal fault earthquakes 24 km
  or less
• Two large deep earthquakes in 1949 and 1965
• There is an uplifted area to the south caused by
  a M 7 around 1,100 yrs ago
• Basin is filled with unconsolidated sediment

• Seismic Hazards in Seattle
• Co-seismic Mass-Wasting
• Mostly landslides
• Very common with Seattle Fault Zone
• Largest occurred in the 1949 Olympia earthquake
  liquefaction
• Unconsolidated soil partially liquefies during
  seismic events amplifying the
• ground shaking
• Increases the damage to infrastructure
• Different sediments have varying liquefaction
  potential

Abstract Detection of active faults and seismic
hazards in the Seattle area is problematic, owing
to thick surficial deposits and abundant
vegetative cover. Aeromagnetic mapping techniques
have given insight into this once enigmatic area.
The Seattle fault zone is segmented and comprised
of several east-trending, north-verging splays
along a reverse-thrust system. Three distinct
magnetic anomalies are associated with the fault
zone, and in combination with regional
stratigraphic records, help elucidate seismic
history in the area. The fault has been active
from 40 Ma to the present, with a major M 7
earthquake occurring 1,100 years ago. The Seattle
area is prone to co-seismic ground failure such
as that associated with the Olympia 1949 and
Seattle-Tacoma 1983 earthquakes. In 2001, the
Nisqually event yielded a M 6.5 tremor that was
felt as far south as Salem. The Nisqually
earthquake had a similar hypocenter to that of
the 1949 Puget Sound event. The area has been
associated with widespread liquefaction processes
during past earthquakes. The expansion of urban
development in the Seattle area over the past two
decades has significantly increased the risk of
widespread, catastrophic damage when the next
seismic event occurs. This paper presents a
summary of past seismic activity in the Seattle
area and risk potential for catastrophic
earthquakes in the future.
Introduction The Seattle area overlies a
distinct fault zone with a history of past and
recent seismic activity. The fault locations
remain a mystery due to abundant surficial cover
obstructing geologists view of the ground
surface. Seattle has the potential for moderate
to severe damage to infrastructure and loss of
lives if a large seismic event were to occur.
Figure 2. Map of the Seattle Fault Zone.
Depicting sediment type and age, also location
and type of known faults.
Table 1. Geologic deposits and the risk of
liquefaction in Seattle area.

• 1949 Olympia Earthquake
• M 7.1
• Focal depth 0f 40 mi
• Landslides occurred as far away as 110 mi
• Total of 23 landslides but possibly more
• The largest landslide occurred at the Tacoma
  narrows with a
• Volume around 650,000 yd 3
• 1965 Seattle-Tacoma Earthquake
• M 6.5
• 24 individual landslides within 60 miles
• Assumed there was and under reporting of
  landslides

• Potential Damage from a Seismic Event
• In the 49 and 65 earthquakes liquefaction
  caused 25 million and 12 million respectively
  in damage
• Co-seismic landslides can block roads, and move
  buildings off foundations
• Seattle population and infrastructure has grown
  since the 49 and 65 earthquakes
• More people and more building means more damage
  or bigger risk

• Aeromagnetic Survey of Seattle Fault
• Aerial survey using a special instrument that
  detects magnetic fields
• Not all rocks give of the same magnetic fields
• Differences in the magnetic field could indicate
  faulting
• Seattle fault zone has three east trending
  distinct magnetic anomalies
• (Fig .1)

• Conclusions
• Not all of the crustal faults are known.
• The Seattle basin is a very active seismic zone.
• The Seattle Fault Zone is segmented.
• Liquefaction and co-seismic landslides are
  common and devastating.
• There were two major recent seismic events one in
  1949 the other in 1965.
• Urban population growth has lead to an increase
  in the potential damage during the next seismic
  event

Figure 3. A map of Washington and Oregon showing
the intensity of the 1949 and 1965 Earthquakes.
References Cited Schuster, R. L., 2004,
Earthquake-Induced Ground Failure in Western
Washington U.S. Geological Survey. Blakley, R.
J., 2002, Location, structure, and seismicity of
the Seattle fault zone, Washington Geological
Society of America Grant, W.P., 1996, Evaluation
of Liquefaction Potential in Seattle,
Washington Chleborad, A. F., 1996, Ground
Failure Hazards in Puget Sound Assessing
Earthquake Hazards and Reducing Risk in the
Pacific Northwest
Figure 1. Aeromagnetic photograph of three
magnetic anomalies in Seattle area. Anomaly A is
a elongated, narrow magnetic high B is broad
magnetic low C is a complex magnetic low.