Lots of folks know that Seattle is prone to earthquakes, but
not a lot of people know where the fault lines are in the
area...
Seattle
Quaternary Fault and Fold Map
552 - Hood Canal Fault
Zone
Counties: Mason, Jefferson, Kitsap
Slip Rate: <.2 mm per year
The northerly striking, Hood Canal fault
zone was originally inferred from gravity anomalies and
some aeromagnetic data for the Puget Lowlands and the
Olympic Mountains directly to the west. Much of the
length Hood Canal marks the eastern edge of a prominent
gravity high. Interpretation of seismic-reflection data
suggests that Quaternary sediments are deformed along a
north-northeast-striking zone of faults beneath the
northern part of the canal.
The presence of an active fault zone,
beneath the southern part of the canal and onland north
of the canal, are more speculative and based principally
on geophysical anomalies. Interpretation of seismically
imaged stratigraphic relations implies considerable
deformation of Tertiary bedrock and complex depositional
patterns in Quaternary deposits that have been affected
by high-angle faulting. In seismic-reflection profiles,
these high-angle faults appear to be principally normal
faults associated with some reverse faults.
Possible strike-slip offsets along these
faults, however, might not be apparent in these
profiles, and several regional studies infer from
regional relations that this fault zone is principally
characterized by right-lateral strike-slip movement.
Some interpretations of regional tectonic relations may
also suggest that the most recent movement along faults
of this zone is as young as late Holocene in age.
Inferences of right-lateral strike-slip movement and the
possibility of late Holocene movement along this fault
zone are based mostly on the apparent westward
termination of the Seattle fault zone [570] near Hood
Canal. Late Holocene activity along the Seattle fault
zone is well documented.
The age, character, and origin of faults
that appear to deform Quaternary sediments beneath Hood
Canal, however, are not tightly constrained. For
example, some apparent faults and faulted relations
imaged from seismic data, might instead reflect
inherited Tertiary topography enhanced and modified by
multiple episodes of Quaternary glacial erosion and
glacial and marine deposition. Possible slip rates and
recurrence intervals for this fault zone have not been
reported. Consequently, at this time this largely
inferred fault zone is classified herein as a Class B
structure until more detailed studies and
characterization of this zone is reported.
570 - Seattle Fault
Zone
Counties: King, Kitsap
Slip type: Thrust (dip is South)
Slip Rate: Between .2 and 1 mm/yr
This 4- to 7-km-wide east-trending fault
zone extends from the Cascade Range foothills on the
east across the Puget Lowland to Hood Canal, crossing
Lake Sammamish, Lake Washington, Puget Sound, Bainbridge
Island, and the Kitsap Peninsula. Various strands of the
fault zone lie largely concealed beneath the major
population centers of Seattle, Bellevue, and Bremerton.
It forms the northern boundary of a belt of bedrock
exposures that cross much of the Puget Lowland.
The depth to bedrock north of the fault
zone is as much as 1 km (Yount and others, 1985 #4746;
Johnson and others, 1999 #4729). The fault zone has been
imaged on seismic-reflection profiles collected in Puget
Sound and adjacent waterways (Yount and Gower, 1991
#4744; Johnson and others, 1994 #4730; Pratt and others,
1997 #4737; Johnson and others, 1999 #4729), correlates
with large gravity and magnetic anomalies (Danes and
nine others, 1965 #4723; Blakely and others, 2002
#4716), and is represented by a prominent velocity
anomaly on tomographic models (Brocher and others, 2001
#4718; Calvert and others, 2001 #4722).
These data indicate the zone consists of
three or more south-dipping thrust faults that form the
structural boundary between the Seattle uplift on the
south and the Seattle basin on the north. Blakely and
others (2002 #4716) have named three of these structures
the "frontal fault," the "Blakely Harbor fault," and the
"Orchard Point fault." Nelson and others (2003 #5868)
termed the "frontal fault" the "Seattle fault." The
Seattle fault zone also includes north-dipping reverse
or thrust faults, such as the Toe Jam Hill fault (Nelson
and others, 2000 #4733; 2002 #4736; 2003 #5868), which
forms a complex scarp in densely forested terrain on
Bainbridge Island.
Slip on both south- and north-dipping
faults within the zone probably is associated with
offset on a south-dipping "master fault" (e.g., Pratt
and others, 1997 #4737) at depth. Surface-deforming
earthquakes have occurred on the Seattle fault in the
latest Holocene, most recently about 1050-1020 cal yr BP
(Bucknam and others, 1992 #602; Atwater, 1999 #4715;
Nelson and others, 2000 #4733; 2002 #4736; 2003 #5868;
2003 #6250).
572 - Southern Whidbey
Island Fault Zone
Counties: Clallam, Island, Jefferson,
Snohomish
Slip Type: Thrust (Dip is SW and NE)
Slip Rate: Between .2 and 1mm per year
This northwest-trending fault zone
extends more than 65 km across Possession Sound,
southern Whidbey Island, Admiralty Inlet into the
eastern Strait of Juan de Fuca. The fault zone is as
wide as 5-7 km, correlates with gravity and magnetic
anomalies (Finn and others, 1991 #4753; Blakely and
others, 1999 #4747), and has been interpreted as a
complex zone of transpressional deformation (Johnson and
others, 1996 #4751).
575 - Saddle Mountain
Faults
County: Mason
Slip Type: Reverse (Dip SE)
Slip Rate: Between .2 and 1mm per year
The Saddle Mountain faults (east and
west) are two of four faults described and discussed by
Wilson (1975 #5721) and Wilson and others (1979 #5663).
In a later report, Wilson (1983 #5822) also discussed
possible relationships of these faults and earthquakes
in the Puget Sound region. These two northeast-striking
faults are present along the southeast flank of the
Olympic Mountains and they show evidence for two or more
faulting events during the late Quaternary (Wilson, 1975
#5721; Wilson and others, 1979 #5663).
Detailed study of the Saddle Mountain
faults, which included trenching and isotopic dating,
indicates that latest movement along the Saddle Mountain
faults is Holocene in age (Wilson, 1975 #5721; Wilson
and others, 1979 #5663). Recently acquired lidar
topography suggests that scarps related to the Saddle
Mountains faults cut across Holocene alluvium and
continue about 10 km southwest of the mapped traces of
these faults (Haugerud and others, 2003 #6211).
The northwest-striking Dow Mountain
fault intersects the eastern Saddle Mountain fault and
shows evidence for late Quaternary offset (Wilson, 1975
#5721; Wilson and others, 1979 #5663). The Dow Mountain
fault, however, has a short mapped trace, less than 1 km
in length, and is not shown herein for scale reasons.
The nearby, inferred Crescent Valley fault of Wilson
(1975 #5721) is not shown on the geologic map by
Dragovich and others (2002 #5715) and is also not shown
or discussed herein.
581 - Tacoma Fault Zone
Counties - Pierce, Mason
Slip Type - Thrust (Dip N)
Slip Rate - Between .2 and 1mm per year
Prominent geophysical anomalies extend
west-northwest across the south-central Puget Lowland
from the Tacoma region to Hood Canal (Danes and nine
others, 1965 #4723; Gower and others, 1985 #4725;
Brocher and others, 2001 #4718). Local and regional
experts are in agreement that the western, east-trending
part of the geophysical anomalies are caused by the
Tacoma fault, but no consensus exists for the eastern
part.
We anticipate that ongoing
investigations will clarify present uncertainties in the
location and geometry of the eastern portion and eastern
end of the fault. Brocher and others (2001 #4718) and
Johnson and others (2004 #6235) suggest that the Tacoma
fault is a backthrust to the south-dipping Seattle fault
[570], and suggest that structural relief increases
westward along the Tacoma fault.
Coastal marsh uplift north of the Tacoma
fault at Lynch Cove and Burley (Bucknam and others, 1992
#602), and coastal marsh subsidence south of the Tacoma
fault at Wollochet Bay (Sherrod and others, 2002 #6240),
are consistent with a minimum of 2-3 m of
up-to-the-north slip along the western part of the
Tacoma fault about 1,000 yr ago.
Source: USGS -
http://earthquake.usgs.gov/regional/qfaults/wa/
