Preliminary Atlas of Active Shallow Tectonic 

Deformation in the Puget Lowland, Washington 

Compiled by 

Elizabeth A. Barnett, Ralph A. Haugerud, Brian L. Sherrod, Craig S. Weaver, Thomas L. Pratt, and 

Richard J. Blakely 

Open-File Report 2010-1149

2010

U.S. Department of the Interior 

U.S. Geological Survey 
ii

U.S. Department of the Interior 

KEN SALAZAR, Secretary 

U.S. Geological Survey 

Marcia K. McNutt, Director 

U.S. Geological Survey, Reston, Virginia 2010 

For product and ordering information: 

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Suggested citation: Barnett, Elizabeth A., Haugerud, Ralph A., Sherrod, Brian L., Weaver, Craig S., 

Pratt, Thomas L., and Blakely, Richard J., compilers, 2010, Preliminary atlas of active shallow tectonic 

deformation in the Puget Lowland, Washington: U.S. Geological Survey Open-File Report 2010-1149, 

32 p., 14 maps [http://pubs.usgs.gov/of/2010/1149/]. 

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endorsement by the U.S. Government. 

Although this report is in the public domain, permission must be secured from the individual 

copyright owners to reproduce any copyrighted material contained within this report. 
iii

Contents

Abstract ........................................................................................................................................................................1 

Introduction...................................................................................................................................................................1 

The Puget Lowland Geology and Geography and the Atlas Map Area ...................................................................2 

Tectonic Framework of Puget Lowland Shallow Faults................................................................................................2 

Active Faults Identified by Recent Geophysical and Geological Field Projects ..........................................................3 

The Maps .....................................................................................................................................................................4 

Paleoseismic Studies: Map 1 and Inset Map 3.........................................................................................................5 

Seismic Reflection and Refraction Surveys: Map 2 and Inset Map 4.......................................................................5 

Thickness of Unconsolidated Quaternary Deposits: Map 5 and Inset Map 6...........................................................5 

Aeromagnetic Anomalies and Residual Aeromagnetic Anomalies: Maps 7 and 8 and 

Inset Maps 9 and 11 .................................................................................................................................................6 

Isostatic Residual Gravity Anomalies: Map 11 and Inset Map 12 ............................................................................6 

Active Shallow Fault Locations: Confluence of Geological and Geophysical Data .....................................................7 

Summary of Shallow Crust Deformation Data and Quaternary Fault Traces: Map 13 and Inset Map 14 ................7 

Recently Identified Active Shallow Faults in Puget Lowland: Paleoseismic and Geophysical Evidence..................7 

Southeastern Olympic Peninsula: Saddle Mountain Fault Zone, Canyon River Fault and Frigid 

Creek Fault ..............................................................................................................................................7 

Southern and Central Puget Lowland: Seattle, Tacoma, and Olympia Faults......................................................8 

South Whidbey Island Fault Zone (SWIF) ............................................................................................................9 

North Puget Lowland: Darrington-Devils Mountain, Utsalady Point, Strawberry Point and Boulder 

Creek Faults ............................................................................................................................................9 

Acknowledgments ......................................................................................................................................................10 

References Cited........................................................................................................................................................10 

Figures 

1. Map showing Puget Lowland region described in atlas. ...........................................................................21 

2. Tectonic-block model of Cascadia fore-arc terranes ................................................................................22 

3. Map showing GPS velocities derived from continuous and campaign survey sites..................................23 

4. Map showing inferred Quaternary fault structures in Puget Lowland mapped before 1985 .....................24 

5. Map showing Quaternary fault traces, including active Fault Zones, mapped before 2009 in the 

Puget Lowland .................................................................................................................................25 

6. Map showing LiDAR-modeled coseismic shoreline uplift from A.D. 900-930 Seattle Fault 

earthquake .......................................................................................................................................26 

7. Schematic depiction of how data layers from atlas have been compiled in summary maps ....................27 

8. Diagram of known ground-surface deformation in Puget Lowland caused by earthquakes on 

shallow faults since end of last glaciation ........................................................................................28 
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Tables 

1. References for faults and paleoseismic studies featured in atlas. ............................................................29 

2. Trench excavations, fault scarps, and references.....................................................................................32 

Maps 

Map 1: Paleoseismic Studies in Puget Lowland 

Map 2: Seismic Survey Locations in Puget Lowland 

Inset Map 3: Paleoseismic Studies in Seattle/Tacoma Region 

Inset Map 4: Seismic Survey Locations in Seattle/Tacoma Region 

Map 5: Thickness of Unconsolidated Quaternary Deposits in Puget Lowland 

Inset Map 6: Thickness of Unconsolidated Quaternary Deposits in Seattle/Tacoma Area 

Map 7: Aeromagnetic Anomalies in Puget Lowland 

Map 8: Residual Aeromagnetic Anomalies in Puget Lowland 

Inset Map 9: Aeromagnetic Anomalies in Seattle/Tacoma Region 

Inset Map 10: Residual Aeromagnetic Anomalies in Seattle/Tacoma Region 

 Map 11: Isostatic Residual Gravity Anomalies in Puget Lowland 

Inset Map 12: Isostatic Residual Gravity Anomalies in Seattle/Tacoma Region 

Map 13: Summary of Shallow Fault Deformation Data and Quaternary Fault Traces, Draped over 

Residual Aeromagnetic Anomalies 

Inset Map 14: Summary of Shallow Fault Deformation Data and Quaternary Fault Traces, Draped 

over Residual Aeromagnetic Anomalies 
Abstract 

This atlas presents an up-to-date map compilation of the geological and geophysical 

observations that underpin interpretations of active, surface-deforming faults in the Puget 

Lowland, Washington. Shallow lowland faults are mapped where observations of deformation 

from paleoseismic, seismic-reflection, and potential-field investigations converge. Together, 

results from these studies strengthen the identification and characterization of regional faults and 

show that as many as a dozen shallow faults have been active during the Holocene. The suite of 

maps presented in our atlas identifies sites that have evidence of deformation attributed to these 

shallow faults. For example, the paleoseismic-investigations map shows where coseismic surface 

rupture and deformation produced geomorphic scarps and deformed shorelines. Other maps 

compile results of seismic-reflection and potential-field studies that demonstrate evidence of 

deformation along suspected fault structures in the subsurface. Summary maps show the fault 

traces derived from, and draped over, the datasets presented in the preceding maps. Overall, the 

atlas provides map users with a visual overview of the observations and interpretations that 

support the existence of active, shallow faults beneath the densely populated Puget Lowland. 

Introduction 

Research during the last quarter century accounts for nearly all of what is known today 

about the locations, paleoseismic histories, and damage potential of shallow faults that rupture 

the ground surface in the Puget Lowland, northwest Washington. In 1985, when Gower and 

others summarized geological and geophysical evidence for the region’s shallow faults, only one 

of these structures, the Saddle Mountain Fault Zone, had demonstrated earthquake activity 

during the Holocene, or past 10,000 years (Carson, 1973; Wilson and others, 1975, 1979). 

Today, recent investigations show that there might be as many as a dozen shallow faults that 

have been active during the Holocene, which together heighten the seismic hazard of the Puget 

Lowland. These faults, as well as those considered to have been active during at least the 

Quaternary (the past 1.6 million years), are compiled in the U.S. Geological Survey Quaternary 

Fault and Fold Database (http://earthquake.usgs.gov/regional/qfaults/), an online database and 

map resource for fault traces and their source studies. 

The primary motive behind producing this atlas is to present in map view both the active, 

surface-deforming shallow faults in the Puget Lowland and a compilation of the geological field 

and geophysical observations that underpin their interpretations and locations. By doing so, the 

atlas updates the Gower and others (1985) map and complements the U.S. Geological Survey 

Quaternary Fault and Fold Database. Although we include Quaternary faults in this atlas, here 

we emphasize currently known or suspected faults that have Holocene displacements and present 

their primary supporting geophysical and geological data evidence. These active and potentially 

active faults critically impact earthquake hazards in the densely populated metropolitan areas of 

the Puget Lowland. 

The suite of maps is thematically organized, beginning with field evidence for coseismic 

fault-generated surface deformation and followed by geophysical subsurface observations of 

potentially related fault structures. In the final summary maps, deformation observations from 

the preceding maps are compiled and stacked; fault traces are then draped over the observations 

that support their interpretations. The maps listed below are more thoroughly described in the 

sections that follow: 
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· Map 1 and Inset Map 3: Paleoseismic Studies 

· Map 2 and Inset Map 4: Seismic Surveys and Locations 

· Map 5 and Inset Map 6: Thickness of Unconsolidated Quaternary Deposits 

· Map 7 and Inset Map 9: Aeromagnetic Anomalies 

· Map 8 and Inset Map 10: Residual Aeromagnetic Anomalies 

· Map 11 and Inset Map 12: Isostatic Residual Gravity Anomalies 

· Map 13 and Inset Map 14: Summary of Shallow Fault Deformation Data and Quaternary 

Faults Traces Draped Over Residual Aeromagnetic Anomalies

The Puget Lowland Geology and Geography and the Atlas Map Area 

The Puget Lowland is a north-south-trending structural basin that is flanked by Mesozoic 

and Tertiary rocks of the Cascade Range on the east and by Eocene rocks of the Olympic 

Mountains on the west. Quaternary glaciations and subsequent fluvial systems have shaped the 

basin by leaving behind a thick blanket of sediments (fig. 1) (Tabor and others, 1978, 1993, 

2000; Shuster and others, 2005). The Puget Lowland encompasses this formerly glaciated plain, 

which fringes the surrounding mountains and contains Puget Sound and the eastern part of the 

Strait of Juan de Fuca. In order to include recent shallow fault studies located outside the 

lowland (fig. 1), we include in the map area of the atlas the east half of the Olympic Mountains, 

the west part of the Cascade Range, and the south part of the Georgia Strait. Thus, in the atlas, 

we define the Puget Lowland as the map area shown in figure 1.

Tectonic Framework of Puget Lowland Shallow Faults 

Active shallow faults accommodate regional north-south shortening in the Puget 

Lowland. Situated near the north end of the Cascadia fore arc, the lowland is subject to tectonic 

compression as mobile fore-arc blocks migrate northward between the Cascade volcanic arc to 

the east and the convergent margin of the Cascadia Subduction Zone (CSZ) to the west (fig. 2) 

(Wells and others, 1998). Oblique subduction of the Juan de Fuca oceanic plate beneath the 

North America continental plate drives the Oregon Coast fore-arc block northward, compressing 

western Washington against the relatively stationary backstop of the British Columbia Coast 

Mountains (fig. 2) (Wells and others, 1998; Wells and Simpson, 2001). 

GPS studies show that, between northern Oregon and Canada, the fore-arc blocks are 

rotating clockwise and translating northward at about 6 to 8 mm/yr (fig. 3) (Wells and others, 

1998; Wells and Simpson, 2001; Mazzotti and others, 2002). Within the Puget Lowland, GPS 

data record north-south shortening rates of from 3.0 to 4.0 mm/yr (red arrows in fig. 3) (Mazzotti 

and others, 2002, 2003) to 4.4 mm/yr (McCaffrey and others, 2007) after the interseismic loading 

signal (green and black arrows in fig. 3) of the locked CSZ is removed. The accumulated strain 

of this shortening is released by large earthquakes on the shallow east-west- to northwestsoutheast-trending faults and also accommodated in folds that traverse the Puget Lowland (figs. 

4, 5). 

Gower and others (1985) delineated some of the Puget Lowland shallow fault zones (fig. 

4) on the basis of geophysical and geological field observations that were available in the early 

1980’s. Gower and others (1985) noted that, although the majority of recorded earthquakes 

originate within the North America continental crust at depths less than 35 km, earthquake 

hypocenters do not correlate well with inferred fault locations (fig. 4). Furthermore, researchers 
3

at that time had not yet studied the suspected shallow faults in enough detail to determine 

whether or not they were currently active. 

More recent paleoseismic studies, however, have demonstrated that shallow faults in the 

Puget Lowland have produced large, surface-deforming Holocene earthquakes (maps 1, 3; table 

1). In addition, geophysical investigations have better contrained the locations of these faults in 

the subsurface (maps 7-12; table 1). Earthquake histories for several of these Puget Lowland 

faults have emerged from paleoseismic investigations of deformed shorelines, marsh subsidence 

or uplift, tsunami deposits, liquefaction features, and fault scarps (maps 1, 3; tables 1, 2). Faults 

for which paleoseismic studies have documented Holocene activity are shown with solid black 

lines on figure 5 and maps 13, 14 (see fig. 5 and table 1 for references); dashed black lines 

indicate faults that either have documented earlier Pleistocene deformation or, at most, suspected 

Holocene activity. 

Active Faults Identified by Recent Geophysical and Geological Field 

Projects 

 Recent advances documenting Puget Lowland shallow fault activity have resulted 

from both large, collaborative research initiatives and smaller, independent studies. The USGS 

Urban Hazards Studies program provides support for aeromagnetic, gravity, and seismic 

reflection and refraction experiments to map the Puget Lowland subsurface structure and better 

resolve the location and geometry of underlying faults. Large-scale studies include a highresolution aeromagnetic survey that extends from Olympia north to the Canadian border and 

from the Cascade Range foothills west to the east edge of the Olympic Mountains (maps 7-10) 

(Blakely and others, 1999), as well as a detailed set of seismic reflection and refraction surveys 

acquired between 1997 and 2002. These seismic experiments, called Seismic Hazards 

Investigations of Puget Sound (SHIPS), consist of marine surveys within Puget Sound, the Strait 

of Georgia, and the Strait of Juan de Fuca (map 2) (Brocher and others, 1999; Fisher and others, 

1999); an onshore-offshore, 112-km-long two-dimensional seismic refraction/reflection line 

across the Seattle Basin (Brocher and others, 2000b); a dense passive seismic array that took 

advantage of the Seattle Kingdome implosion as a seismic source (Brocher, 2002); and two 

deep-borehole seismic-site-characterization studies in Seattle (Odum and others, 2004). 

 The large geophysical experiments conducted in the urban area coincided with the 

acquisition of over 10,000 square kilometers of high-resolution digital topographic data, known 

as LiDAR (Light Detection And Ranging) (map 1; see summary in Haugerud, 2003; see also, 

Puget Sound Lidar Consortium, http://pugetsoundlidar.ess.washington.edu/). LiDAR data 

processing removes the dense forest cover characteristic of the Pacific Northwest, allowing 

geologists to “see through the trees” and revealing a high-resolution image of the bare-earth 

ground surface. Subtle surface features such as fault scarps become visible at the centimeter 

scale of LiDAR imagery. 

Pre-LiDAR field investigations documented active crustal faults such as the Saddle 

Mountain Fault Zone, in the southeastern Olympic Mountains (Wilson and others, 1975, 1979), 

and the Seattle Fault Zone (fig. 5) (Atwater and Moore, 1992; Bucknam and others, 1992; Jacoby 

and others, 1992; Karlin and Abella, 1992; Schuster and others, 1992; Sherrod and others, 2001). 

The arrival of LiDAR imaging, however, greatly facilitated the pinpointing of fault-related 

surface features such as individual fault scarps. These initial pre-LiDAR findings, in addition to 
4

the availability of LiDAR data, prompted several new fault investigations throughout the Puget 

Lowland (Haugerud, 2003). 

Subsequent trench excavations of geomorphic scarps along inferred Holocene faults, 

some of which were already noted by Gower and others (1985) (fig. 4), and some of which were 

more recently discovered (fig. 5), have demonstrated that many of these faults have been active 

in the Holocene (table 2). Results from these Puget Lowland paleoseismic studies, including 

studies of coastal deformation and paleoecology, extended the currently known active faults 

from the Canadian border south to Olympia. Puget Lowland faults, summarized on maps 13 and 

14 and also table 1, are, from north to south: the Boulder Creek Fault, the Utsalady Point Fault, 

the Lake Creek-Boundary Creek Fault (also known as the Little River Fault), the DarringtonDevils Mountain Fault, the Southern Whidbey Island Fault Zone (SWIF), the Seattle Fault Zone 

(SFZ), the Tacoma Fault Zone, the Saddle Mountain Fault Zone, the Frigid Creek Fault, and the 

Canyon River Fault (Wilson and others, 1979; Atwater and Moore, 1992; Bucknam and others, 

1992; Johnson and others, 1996; Sherrod, 1998, 2001; Sherrod and others, 2001, 2002; 

Haugerud, 2003; Nelson and others, 2003b; Johnson and others, 2004b; Kelsey and others, 2004; 

Sherrod and others, 2004a; Hughes, 2005; Walsh and others, 2007; Barnett, 2007; Nelson and 

others, 2007; Sherrod and others, 2008; Blakely and others, 2009; Personius, 2009; Witter and 

others, 2009; see also, tables 1 and 2 for additional references). 

The integration of geophysical and geological field observations of deformation provides 

a rich dataset that serves to better characterize the locations, geometry, and earthquake histories 

for shallow faults in the Puget Lowland (table 1). For example, recent studies of the SFZ, the 

SWIF, the Tacoma Fault Zone, and the Saddle Mountain Fault Zone link subsurface-fault 

structures mapped from seismic and potential-field surveys to surface-deformation features 

identified in LiDAR and geological field data (Blakely and others, 2002; Nelson and others, 

2003b; Johnson and others, 2004c; Sherrod and others, 2004a, 2008; Blakely and others, 2009). 

The Maps 

The atlas contains seven 1:600,000-scale maps of the Puget Lowland that depict various 

types of investigations and fault-generated deformation evidence. In, addition, 1:400,000-scale 

inset map follows each map. The larger scale maps provide greater detail of the most densely 

populated region of the Puget Lowland, which also has the greatest seismic risk, the SeattleTacoma region. The resolution of all PDF map files is sufficient for onscreen display and 

printing of finer scale details at larger scales. In order to provide visual and spatial continuity 

throughout the maps, some features such as fault scarps (orange lines) and trench locations 

(stars) appear on all maps, as do geographic boundaries and place names. Earthquakes, which 

have been recorded regionally since 1970, are continual reminders that the Puget Lowland region 

is seismically active (Pacific Northwest Seismic Network, 2008; www.pnsn.org). Shallow 

(depths less than 35 km) earthquakes larger than magnitude 4 are featured on all maps. (In 

addition, figure 4 shows epicenters of earthquakes recorded since 1970 as small as, as well as 

magnitude 2 and focal mechanisms for larger events.) Summary maps 13 and 14 show currently 

known shallow fault traces draped over residual aeromagnetic anomalies. 

It is important to note that, except where faults have ruptured or deformed the ground 

surface, fault traces drawn on the maps are not pinpoint locations. Instead, many fault traces are 

based on ongoing projects and only represent inferred and, in some cases, approximate locations 

of where fault planes either intersect or are thought to project to the ground surface. Thus, the 

fault traces on these maps should be used only as guides to the general locations of currently 
5

identified active fault zones and should not be used for site-specific evaluations of potential 

earthquake hazards. 

Paleoseismic Studies: Map 1 and Inset Map 3 

Map 1 and inset map 3 depict fault-related surface deformation features along with the 

locations of trenches excavated across LiDAR-identified fault scarps. (See table 2 for the trench 

study names, the sense of fault motion, and the references that describe trench excavations.) 

Current sources of LiDAR survey data are shown in the maps, as are locations of shoreline 

studies and directions of observed shoreline subsidence or uplift was observed (Bucknam and 

others, 1992; Bucknam, 1999; Sherrod, 2001, 2004a). Also shown on inset map 3 are data from a 

LiDAR-based model of regional marine-terrace uplift during the A.D. 900 SFZ earthquake, 

which indicates that the region experienced as much as 8.5 m of coseismic uplift near the 

southern tip of Bainbridge Island (see also, fig. 6) (Bucknam and others, 1992; Muller and 

Harding, 2005). 

Seismic Reflection and Refraction Surveys: Map 2 and Inset Map 4 

 By imaging the subsurface, seismic reflection profiles can reveal the deeper structure of 

the Puget Lowland. Although fault planes are not clearly distinguishable in the seismic reflection 

profiles, imaging of deformed strata can be used to infer the subsurface locations of shallow 

faults that deform the ground surface (table 1). 

Map 2 and inset map 4 depict the locations of both marine seismic surveys and land 

profiles. Also shown are the projected surface traces of faults that are based on deformed strata 

observed in the marine seismic reflection profiles (Johnson and others, 1996; Pratt and others, 

1997; Johnson and others, 1999, 2001b, 2004c). 

Until recently, marine seismic reflection profiles from Puget Sound and the Strait of Juan 

de Fuca provided the majority of subsurface information on fault structures that cross the Puget 

Lowland (Pratt and others, 1997; Brocher and others, 1999; Fisher and others, 1999; Johnson and 

others, 1999; Brocher and others, 2001; Johnson and others, 2004c; Dash and others, 2007; see 

also, table 1 for additional references). Recent land-based seismic reflection studies of the SFZ 

and SWIF have extended the marine-survey imaging of the fault zones onto land, as indicated on 

map 2 and inset map 4 (Brocher and others, 2000b, 2002; Stephenson and others, 2006; Liberty 

and others, 2008; Sherrod and others, 2008). 

Thickness of Unconsolidated Quaternary Deposits: Map 5 and Inset Map 6 

The Puget Lowland contains thick Quaternary-age deposits, most of which are derived 

from the advance and retreat of repeated glacial episodes that covered the region, overprinted by 

modern fluvial and marine processes. Jones (1996) compiled a point dataset of measured and 

inferred thicknesses of unconsolidated deposits throughout the Puget Lowland from nearly 4,000 

water, oil, coal, and gas well logs and 700 seismic reflection shot sites. Where wells do not reach 

bedrock, the deepest wells in the area provide a minimum thickness; in addition, Jones (1996) 

interpolated a regional maximum thickness on the basis of nearby wells that do penetrate 

bedrock. Deposit thicknesses vary from a thin veneer, on the Puget Lowland margins along the 

Olympic and Cascade Mountains, to more than 1-km thick, in the Seattle and Everett Basins. 

Map 5 and inset map 6 depict contours of deposit thickness and well locations from Jones 

(1996). Also shown is a generalized bedrock surface derived from the Jones (1996) point data 

that depicts a simplified image of the Puget Lowland stripped of its thick layer of unconsolidated 
6

deposits. Jones’ (1996) interpolation accounts for any discrepancies between the point data and 

the contours and color shading-relief. 

 Locations where deposits are thin along the northern and southern margins of the Seattle 

and Everett Basins coincide with the locations of inferred active fault structures (see also, fig. 5 

and maps 13 and 14). Subsequent seismic reflection studies indicate that these basins are 

structurally bounded by faults (Pratt and others, 1997; Brocher and others, 2001; Johnson and 

others, 2004c). The thick deposits of the Seattle Basin are located at the northern boundary of the 

SFZ, and the Everett Basin is bounded by the SWIF and the Utsalady Point and Strawberry Point 

faults (maps 13 and14). 

Aeromagnetic Anomalies and Residual Aeromagnetic Anomalies: Maps 7 and 8 and Inset 

Maps 9 and 10 

Potential-field observations provide another means to constrain subsurface fault locations 

and geometries. Faults that juxtapose rocks that have contrasting magnetic properties produce 

small magnetic fields near the topographic surface that can be detected with airborne magnetic 

surveys. Magnetic anomaly maps derived from such surveys provide a way to map faults 

covered by vegetation, water, or young geologic materials. Maps 7 through 10 show a highresolution aeromagnetic survey of the Puget Sound region conducted in 1997 (Blakely and 

others, 1999, 2002). Map 7 and inset map 9 show the original data; map 8 and inset map 10 show 

the original data reprocessed to emphasize magnetic anomalies originating from shallow 

magnetic sources by filtering out deeper source anomalies. 

Maps 7 and inset map 9 show the location of abrupt magnetization contrasts, informally 

called “maxspots,” as estimated from magnetic anomalies using potential-field geophysical 

software (Blakely and Simpson, 1986). These maxspots form lineaments, some of which mark 

the location of shallow faults, as indicated by the spatial association between magnetic 

lineaments and fault scarps shown on maps 7 and 9. 

Isostatic Residual Gravity Anomalies: Map 11 and Inset Map 12 

Along with magnetic observations, some gravity anomalies observed in the Puget 

Lowland indicate possible faults and fault-bounded structural basins. Map 11 and inset map 12 

show isostatic residual gravity anomalies that are based on the compilation of Finn and others 

(1991). Isostatic residual gravity anomalies (Simpson and others, 1986) reflect density of the 

middle and upper crust. Gravity gradients are present where rocks of contrasting densities are 

juxtaposed, such as along fault zones; therefore, they are particularly useful for defining 

sedimentary basins and their bounding faults. The Seattle, Everett, and Tacoma Basins appear in 

map 11 as broad gravity lows that reflect the relatively low density of basin-filling sedimentary 

rocks. Also depicted on map 11 and inset map 12 are the locations of sharp density contrasts, 

also called maxspots, estimated from gravity anomalies (Blakely and Simpson, 1986). For 

example, a sharp gravity gradient located along the southern margin of the Seattle Basin marks 

the Seattle Fault. 

 The spatial density of gravity measurement stations varies throughout the Puget Lowland; 

accordingly, the data resolution varies. To show this visually, the locations of the gravity stations 

are depicted on map 11 and inset map 12, and areas of greater station spatial density reflect 

higher resolution data (Finn and others, 1991). For example, the highest resolution gravity data 

on these maps cover the region within the Puget Sound between Victoria, B.C., and Tacoma. 
7

Active Shallow Fault Locations: Confluence of Geological and 

Geophysical Data 

Summary of Shallow Crust Deformation Data and Quaternary Fault Traces: Map 13 and Inset 

Map 14 

Shallow faults in the Puget Lowland are mapped where multiple geophysical and 

geological observations of crustal deformation converge. Together, these observations bolster the 

identification and characterization of potentially damaging faults and provide a rich database of 

surface- and subsurface-deformation evidence from geological field surveys (map 1 and inset 

map 3), seismic reflection profiles (map 2 and inset map 4), structural basin boundaries (map 5 

and inset map 6), and potential-field anomaly gradients (maps 7 through 12). In summary maps 

13 and 14, deformation observations from maps 1 through 12 have been compiled and stacked; 

fault traces are then draped over the observations that underpin their characterizations. 

For example, figure 7 schematically shows the construction of the summary maps and the 

array of deformation evidence that constrain the mapping of surface fault traces such as the SFZ. 

As depicted by data layers in figure 7 and throughout maps 1 through 12, multiple fault-scarp 

excavations and shoreline-deformation studies quantify surficial Holocene deformation located 

above sharp magnetic- and gravity-anomaly gradients and above deformed subsurface seismic 

strata. The SFZ also bounds the edge of the deepest structural basin in the Puget Lowland, the 

Seattle Basin. North of the Seattle Fault Zone, trench excavations of LiDAR-identified fault 

scarps, subsurface stratigraphic deformation recognized in seismic reflection profiles, and 

magnetic-anomaly gradients constrain the location of the SWIF (Sherrod and others, 2008). 

These faults and most other Puget Lowland fault traces are differentiated between those 

that have demonstrated Holocene activity and those for which evidence for Holocene activity is 

inconclusive but Quaternary activity is suspected (fig. 5; maps 13 and 14). Most Puget Lowland 

fault traces are from the USGS Quaternary Fault and Fold Database 

(http://earthquake.usgs.gov/regional/qfaults/), but we have updated several fault traces using 

work completed since its publication: the South Whidbey Island Fault (SWIF) strands have been 

mapped farther east across the Puget Sound toward the Cascade Range (Blakely and others, 

2004; Sherrod and others, 2005a, 2005b, 2008); the Tacoma Fault also has been extended farther 

east (Johnson and others, 2004c; Brocher and others, 2001; Sherrod and others, 2004a, 2004b); 

and the Boulder Creek Fault is a newly recognized fault near the Canadian border (Haugerud, 

2005; Barnett, 2007; Barnett and others, 2007; Siedlecki, 2007) (fig. 5; maps 13 and 14; table 1). 

In addition, the USGS Quaternary Fault and Fold Database does not list the Olympia Fault, but 

we have included it here as a subject of ongoing research (fig. 5; maps 13 and 14; table 1) 

(Blakely and others, 1999; Sherrod, 2001). 

Recently Identified Active Shallow Faults in Puget Lowland: Paleoseismic and Geophysical 

Evidence 

Southeastern Olympic Peninsula: Saddle Mountain Fault Zone, Canyon River Fault and Frigid 

Creek Fault 

The Saddle Mountain Fault scarps provide the earliest clear indications of fault-related 

surface deformation in the Puget Lowland (see Tables 1 and 2 for information of specific faults) 

(Carson, 1973; Wilson and others, 1975, 1979). After identifying fault scarps on aerial 
8

photographs of clear cut timberland, Wilson and others (1979) excavated trenches across these 

scarps and sampled drowned trees from neighboring wetlands for radiocarbon-dating material. 

Their early work and more recent trenching and wetland studies demonstrate that Holocene 

reverse faulting created the scarps and submerged an adjacent forest as recently as 1,000 years 

ago (Wilson and others, 1979; Hughes, 2005; Witter and others, 2009). A ground-magnetic 

survey supported the presence of east-side-up fault structures beneath the Saddle Mountain Fault 

Zone that might relate to the observed deformation features (Blakely and others, 2005, 2009). 

Other fault scarps that flank the east side of the Olympic Mountains corroborate recent 

tectonic deformation. Trench excavations across the Canyon River (Walsh and others, 2007), 

Frigid Creek (Blakely and others, 2009), and Lake Creek-Boundary Creek/Little River Fault 

scarps (Nelson and others, 2007) confirm Holocene activity along these faults. Blakely and 

others (2009) suggested that, on the basis of paleoseismic work and aeromagnetic-data analysis, 

these fault zones form an en echelon chain of active fault segments along the east edge of the 

Olympic Mountains. Moreover, they might compose the western fault boundary of the SFZ and, 

thus, be components of the deformation zone that accomodates shortening in the Puget Lowland 

(Blakely and others, 2009). 

Southern and Central Puget Lowland: Seattle, Tacoma, and Olympia Faults

Underlying the urban areas of Seattle and Tacoma, the Seattle Uplift (fig. 4; maps 13, 14) 

is a wedge of uplifted crust situated between two active fault zones (Gower structures I and K on 

figs. 4 and 5; see also SFZ and Tacoma Fault Zone on maps 13, 14): the SFZ to the north, 

interpreted as a south-dipping reverse fault that separates the Seattle Basin from the Seattle 

Uplift (Yount, 1992; Bucknam and others, 1992; Pratt and others, 1997; Johnson and others, 

1999; Blakely and others, 2002; Nelson and others, 2003b; Brocher and others, 2004a); and the 

north-dipping, reverse Tacoma Fault Zone to the south (Sherrod and others, 2004). Observations, 

measurements, and radiocarbon dates from earthquake-triggered features demonstrate Holocene 

deformation along the Seattle and Tacoma Fault Zones include the following: fault scarps on 

Bainbridge Island (Nelson and others, 2003b), Waterman Point (Nelson and others, 2003a), Vasa 

Park (Sherrod, 2002), Catfish Lake (Sherrod and others, 2004a, 2004b), Sunset Beach (Nelson 

and others, 2007), and Saddle Mountain (Hughes and others, 2005; Witter and others, 2009); 

coastal uplift (maps 1, 3) (Bucknam, 1992; Sherrod, 2001; Sherrod and others, 2002, Sherrod 

and others, 2004a; Muller and others, 2006; Ota and others, 2006); landslides (Jacoby and 

others, 1992); and turbidites in Lake Washington (Karlin and Abella, 1992, 1996), and tsunami 

deposits (Atwater and Moore, 1992). 

The most recent uplift event that occurred around 1,100 years ago (A.D. 900-930) 

probably produced most of the Seattle Uplift deformation features (Atwater and Moore, 1992; 

Bucknam and others, 1992; Atwater, 1999). A large earthquake, possibly magnitude 7 or greater 

(Bucknam and others, 1992; Pratt and others, 1997; Muller and Harding, 2005; ten Brink, 2006) 

uplifted shorelines within the SFZ (Bucknam and others, 1992), caused subsidence of the Seattle 

Basin north of the fault, and triggered a tsunami in Puget Sound (Atwater and Moore, 1992) (fig. 

5; maps 1, 3). On the south edge of the Seattle Uplift, warped shorelines and intertidal marshes 

that straddle the Tacoma Fault also recorded regional uplift north of the Tacoma Fault and 

subsidence south of the fault around 1,100 years ago (Bucknam and others, 1992; Sherrod and 

others, 2002, 2004a, 2004b). 

Recent seismic-reflection and aeromagnetic studies helped to constrain subsurface 

geometry of the Seattle and Tacoma Fault Zones. Seismic reflection profiles image subsurface 
9

deformation attributed to the SFZ (maps 2, 4, 13, 14) (Pratt and others, 1997; Johnson and 

others, 1999; Liberty and Pratt, 2008) and the Tacoma Fault Zone (Pratt and others, 1997; 

Brocher and others, 2001; Johnson and others, 2004c). Because of large offsets, the fault zones 

also appear as linear anomalies in gravity (map 11) and aeromagnetic data (map 7) (Gower and 

others, 1985; Blakely and others, 2002) and as velocity anomalies in seismic-tomography results 

(Brocher and others, 2001). 

Still farther south, a prominent northwest-southeast-trending aeromagnetic anomaly south 

of Olympia (map 7) and evidence for abrupt submergence of a southern Puget Sound marshland 

and forest about 1,100 years ago (maps 2, 13) suggest the approximate location of the inferred 

Olympia Fault (structure labeled “L” in fig. 4) (Blakely and others, 1999; Sherrod, 2001). No 

fault scarps have been found above this structure, and, other than a broad zone of inferred 

deformation, the location of the fault trace remains poorly known. 

Paleoseismic evidence collected throughout the Puget Lowland indicates that several 

faults described above may have ruptured during or around the time of the A.D. 900-930 event. 

Figure 8 shows age ranges of earthquakes on shallow faults throughout the Puget Lowland and 

the apparent cluster around this event. 

South Whidbey Island Fault Zone (SWIF) 

North of the SFZ, the SWIF cuts across central and northern Puget Lowland (structure 

labeled “G’ in fig. 4, 5; map 13) as a possible transpressional fault (Johnson and others, 1996). 

The SWIF, located by marine seismic-reflection and aeromagnetic data (Johnson and others, 

1996), extends from offshore Vancouver Island southeastward nearly 100 km and projects onto 

land. Blakely and others (2004) and Sherrod and others (2005a, 2005b, 2008; maps 7, 9, 13) 

mapped the fault strands about another 40 km southeastward toward the Cascade Mountains by 

modeling aeromagnetic-anomaly data and excavating LiDAR-identified fault scarps. A coastaluplift study on Whidbey Island (Kelsey and others, 2004) and surface-rupture studies conducted 

northeast of Seattle (Sherrod and others, 2008) documented at least four earthquakes along the 

eastern extension of SWIF within the past 16,000 years. Two of the youngest occurred between 

3,200 and 2,800 years ago and less than 2,700 years ago (Kelsey and others, 2004; Sherrod and 

others, 2008). 

North Puget Lowland: Darrington-Devils Mountain, Utsalady Point, Strawberry Point, and Boulder 

Creek Faults 

The Darrington-Devils Mountain, Utsalady Point, and Strawberry Point Faults might 

compose elements of an active oblique-slip transpressional deformation zone between 

southeastern Vancouver Island and the Cascade Range foothills (structure labeled “A” in figs. 4, 

5; Map 13) (Johnson and others, 2001b, 2004b). Trench studies of scarps located along the 

Darrington-Devils Mountain (Personius and others, 2009) and Utsalady Point Faults indicate that 

Holocene faulting generated these scarps (map 1). Trench excavations across the Utsalady Point 

Fault scarps indicate that the fault ruptured at least once, and possibly twice, during the last 2000 

years (Johnson and others, 2004b). 

 Recent paleoseismic work northwest of Mount Baker has shifted the previously mapped, 

northernmost limit of active shallow faults over 50 km north, from the Darrington-Devils 

Mountain Fault Zone to just south of the Canadian border. Excavations of LiDAR-identified 

fault scarps along the mapped trace of the Boulder Creek Fault (map 13) (Misch, 1966), as well 

as coring of adjacent wetlands, has yielded evidence of three or more Holocene oblique-thrusting 
10

events (Haugerud and others, 2005; Barnett, 2007; Barnett and others, 2007). Subsequent 

trenching (for example, the Smuggler trench) along nearby en echelon fault scarps also has 

demonstrated faulting (map 1; table 2). Just south of the Boulder Creek fault, Dragovich and 

others (1997a, 1997b) attribute the 1990 Deming earthquake swarm (Qamar and Zollweg, 1990) 

(fig. 4) to activity along the Macaulay Creek Thrust Fault (map 13) on the basis of field evidence 

and earthquake data collected during the swarm. No surface rupture, however, was identified, 

and so Holocene activity along the thrust fault remains unresolved. 

Acknowledgments 

We would like to thank the following people for providing both data and advice: Tom 

Brocher, USGS (seismic data); Wes Thelan, PNSN (mapGMT help); Todd Williams, Pacific 

Plate Boundary Observatory (PBO data); Ray Wells, U.S. Geological Survey (GPS data and 

figures); Jordan Muller (shoreline data); Renate Hartog and Jon Connolly, PNSN (data); Diana 

Martinez, Puget Sound Regional Council (LiDAR information); Stephane Mazzotti, Pacific 

Geoscience Centre (GPS data); and very helpful and informative content reviews from Joan 

Gomberg, Katie Keranen, and Brian Atwater of U.S. Geological Survey

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21

Figure 1. Map showing Puget Lowland region described in atlas, which includes Puget Lowland (green 

outline), eastern part of Olympic Mountains, western part of Cascade Range, and southern part of 

Georgia Strait. For simplicity and to incorporate most datasets relevant to shallow faults, in the text, 

Puget Lowland refers to the entire map extent shown in figure 1 and in maps. 
22

Figure 2. Tectonic-block model of Cascadia fore-arc terranes, modified from Wells and Simpson (2001) 

and Wells and others (1998). Gray arrows indicate relative plate motion. Block motion (circled 

numbers) is in mm/yr. Rotation of Sierra Nevada block and of Oregon Coast Range block about their 

respective Euler poles induce north-south shortening of western Washington against backstop of the 

Canadian Coast Mountains. Red arrows indicate north-south shortening of Puget Lowland region. 

Orange triangles denote Cascade Arc volcanoes. 
23

Figure 3. Map showing GPS velocities derived from continuous and campaign survey sites, modified from 

Mazzotti (written commun., 2009). Black and green arrows predominantly reflect interseismic loading of 

the Cascadia subduction zone (CSZ) (McCaffrey and others, 2007; Plate Boundary Observatory, 

2008). Red arrows are residual GPS velocities of crust after interseismic loading signal of CSZ is 

removed (Mazzotti and others, 2003; written commun., 2009). Error ellipses are circles at tips of 

velocity vectors. 
24

Figure 4. Map showing inferred Quaternary fault structures in Puget Lowland mapped before 1985 

(orange dashed lines) from Gower and others (1985); structures are labeled A-L. Shallow earthquakes 

(<35 km) denoted by red circles; deep earthquakes (>35 km) by green circles. Pre-instrumental (earlier 

than 1970) earthquakes denoted by purple circles. Focal mechanism (“beach ball”) color and symbol 

size corresponds to same depth and magnitude as earthquakes (PNSN, 2008). 
25

Figure 5. Map showing Quaternary fault traces, including active fault zones, mapped before 2009 in the 

Puget Lowland, from USGS Quaternary Fault and Fold Database (USGS, 2006) and subsequent 

studies. Faults that have documented Holocene earthquakes are shown in black; faults that have 

unknown Holocene activity, but probable activity during Quaternary, in gray. Newly mapped fault traces 

not included in USGS Quaternary Fault and Fold Database also are shown. Inferred Quaternary fault 

structures from Gower and others (1985) are denoted by orange dashed lines and labeled in gray. 

Abbreviations: BCF, Boulder Creek Fault; CRF, Canyon River Fault; DDMFZ, Darrington-Devils 

Mountain Fault Zone; HC, Hood Canal Fault; LCF, Lake Creek-Boundary Creek Fault; MCF, Macaulay 

Creek Fault; OF, Olympia Fault; SFZ, Seattle Fault Zone; SMFZ, Saddle Mountain Fault Zone; SPF, 

Strawberry Point fault; SWIF, South Whidbey Island Fault; TFZ, Tacoma Fault Zone; UPF, Utsalady 

Point Fault. 
26

Figure 6. Map showing LiDAR-modeled coseismic shoreline uplift from A.D. 900-930 Seattle Fault 

earthquake (Muller and Harding, 2005). Colored triangles denote uplift in meters. Open brown triangles 

mark observed uplift or subsidence from coastal study sites (Sherrod, 2001, 2004b). Seattle Fault Zone 

traces shown in gray. 
27

Figure 7. Schematic depiction of how data layers from maps (Maps 1 through 12) have been compiled in 

summary maps 13 and 14. Box color links data layers to corresponding maps. Wide array of geological 

and geophysical data underpins location and characterization of fault zones; for example, location of 

Seattle Fault Zone (dashed, orange box) is based on evidence from trench excavations, seismic 

profiles, basin geometry, shoreline deformation, and potential-field anomaly maps. Data layer maps, 

from top to bottom: fault traces, scarp and trench locations, coastal deformation studies, deformation 

observed in seismic data, Quaternary deposit thickness, gravity anomalies, aeromagnetic anomalies, 

and summary map of compiled data layers. 
28

Figure 8. Diagram of known ground-surface deformation in Puget Lowland caused by earthquakes on 

shallow faults since end of last glaciation. Grey bars denote maximum age range for earthquakes on 

faults. Data derived from trench excavations, other paleoseismic studies, and dating of faulting-related 

organic material when possible (Sherrod, written commun., 2008). 
29
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