HOLOCENE FAULT SCARPS AND SHALLOW 

MAGNETIC ANOMALIES ALONG THE SOUTHERN 

WHIDBEY ISLAND FAULT ZONE NEAR 

WOODINVILLE, WASHINGTON 

By Brian L. Sherrod, Richard J. Blakely, Craig Weaver, Harvey Kelsey, Elizabeth Barnett, and Ray Wells 

Open-File Report 2005-1136 

March 2005 

U.S. Department of the Interior 

U.S. Geological Survey
HOLOCENE FAULT SCARPS AND SHALLOW 

MAGNETIC ANOMALIES ALONG THE SOUTHERN 

WHIDBEY ISLAND FAULT ZONE NEAR 

WOODINVILLE, WASHINGTON 

By Brian L. Sherrod1, Richard J. Blakely2, Craig Weaver1, Harvey Kelsey3, Elizabeth Barnett1, and Ray Wells4

1

U.S. Geological Survey, Dept. of Earth and Space Sciences, Box 351310, Univ. Washington, Seattle, WA 98195 

2

U.S. Geological Survey, 345 Middlefield Road, M/S 989, Menlo Park, CA 94025 

3

Dept. of Geology, Humboldt State University, Arcata, CA 95521 

4

U.S. Geological Survey, 345 Middlefield Road, M/S 973, Menlo Park, CA 94025 

Open-File Report 2005-1136 

March 2005 

This report is preliminary and has not been reviewed for conformity with U.S Geological Survey editorial standards nor 

with the North American Stratigraphic Code. Any use of trade, product, or firm names in this publication is for descriptive 

purposes only and does not imply endorsement by the U.S. Government. 

U.S. Department of the Interior 

U.S. Geological Survey 
ABSTRACT 

The southern Whidbey Island fault zone (SWIFZ), as previously mapped using borehole 

data, potential-field anomalies, and marine seismic-reflection surveys, consists of four subparallel, northwest-trending fault strands, extending ~100 km from near Vancouver Island to the 

Washington mainland. The vertical component of displacement changes both across and along 

the fault, and strike slip motion may be important as well. Abrupt uplift at a coastal marsh on 

south-central Whidbey Island suggests that the SWIFZ experienced a MW 6.5 - 7.0 earthquake 

between 3200 and 2800 years B.P. The SWIFZ has been hypothesized to extend southeastward 

beneath the mainland, making landfall between the cities of Seattle and Everett. Linear, lowamplitude aeromagnetic anomalies in this mainland region are on strike with the SWIFZ and 

may facilitate mapping the SWIFZ onshore. The most prominent of the residual aeromagnetic 

anomalies, the Cottage Lake aeromagnetic lineament, extends at least 16 km, lies approximately 

on strike with the SWIFZ on Whidbey Island, and passes within about 27 km of downtown 

Everett. Glacial deposits are slightly magnetic in this region, as indicated by magnetic 

susceptibility measurements and a ground-magnetic survey, and, in places, the Cottage Lake 

aeromagnetic lineament is associated with topographic lineaments. Spectral analysis and 

modeling experiments indicate that the source of the Cottage Lake aeromagnetic lineament 

extends to depths greater than 2 km and into Eocene sedimentary strata. 

Subtle scarps on Pleistocene surfaces are visible on high-resolution lidar topography at a 

number of locations in the mainland region, often closely associated with aeromagnetic 

lineaments. In the field, scarps exhibit northeast-side-up vertical relief of 1 to 5 m. One group of 

scarps extends a total distance of 18 km along the Cottage Lake aeromagnetic lineament, where 

they defines two strands: One strand lies east of Crystal Lake and is referred to here as the 

Crystal Lake lidar lineament, another strand follows the Little Bear Creek drainage and is called 

here the Little Bear Creek lidar lineament. Four excavations across these two lidar lineaments 

show evidence for multiple folding and faulting events since deglaciation, most likely above 

buried reverse/oblique faults. One trench exposed a normal fault, although it was not possible to 

determine whether slip was caused by glacial or tectonic processes. In this same trench, evidence 

for younger folding suggests that surface deformation during an earthquake formed the scarp 

seen on lidar maps near Crystal Lake. Subsequent scarp degradation buried a forest soil, from 

which charcoal samples indicated that the folding event occurred after 11,670 years B.P. Two 

trenches were also excavated along the Little Bear Creak lineament near Grace, Washington. 

One of these trenches showed evidence for multiple events, including both folding and faulting, 

with the youngest event disturbing late Holocene wetland soils. In total, paleoseismological 

evidence suggests that the SWIFZ produced at four events since deglaciation about 16,400 years 

ago, the most recent after 2.7 ka. Liquefaction features near Everett indicate that strong shaking 

has occurred there at least three times in the last 1100 years, although evidence that those 

earthquakes occurred on the SWIFZ has not yet been found. 

INTRODUCTION 

The southern Whidbey Island fault zone is imaged by a variety of geophysical methods as 

a broad, complex, northwest-striking structure extending from southern Whidbey Island to 

Vancouver Island (Figures 1 and 2). The SWIFZ appears in gravity and seismic tomography 

inversions as a structural contact between low-velocity, low-density sediments of the Everett 

basin and high-velocity, high-density Crescent Formation that underlies the Kingston arch 

2
(Brocher and others, 2001; Gower and others, 1985; Johnson and others, 1996; Ramachandran 

and others, 2005). The SWIFZ was originally mapped on the basis of regional gravity and 

magnetic anomalies as a single strand extending northwest from about Murphys Corner on the 

Washington mainland to Admiralty Inlet west of Whidbey Island (Gower and others, 1985; 

Yount and Gower, 1991). Johnson and others (1996) showed evidence in seismic-reflection 

data, exposures in sea cliffs, and nearby boreholes for Quaternary displacement on three separate 

strands of the SWIFZ and extended the zone northwestward nearly to Vancouver Island. Kelsey 

and others (2004) interpreted differences in relative sea-level histories at two coastal marshes on 

opposite sides of one strand of the SWIFZ as evidence for 1-2 m of vertical, north-side-up 

displacement during a MW 6.5 to 7.0 earthquake about 3000 years ago. 

While other shallow faults in the Puget Lowland show distinct aeromagnetic patterns 

associated with hanging-wall geology (Blakely and others, 2002), the aeromagnetic expression 

of the SWIFZ is broad, complex, and diffuse, possibly because strongly magnetic lithologies, 

like the Crescent Formation, lie at significant depths. The SWIFZ is expressed in aeromagnetic 

anomalies across Admiralty Inlet, but the southeastward extension of the SWIFZ from southern 

Whidbey Island onto the Washington mainland is subdued in unprocessed aeromagnetic data. 

Aeromagnetic anomalies do display a subtle northwest-striking pattern of magnetic lineaments in 

the Maltby-Woodinville area, particularly evident after application of data enhancement 

methodologies (Blakely and others, 2004). This pattern is on strike with the SWIFZ offshore 

and is spatially associated with a narrow, 18-km-long zone of topographic scarps and lineaments 

identified in a GIS analysis of lidar data. The scarps are typically 1 to 5 m high, have northeastside up, and lie on Pleistocene and younger surfaces. On the basis of their spatial association 

with topographic scarps and lineaments, we earlier interpreted some of the northwest-striking 

aeromagnetic lineaments as onshore splays of the SWIFZ, thereby extending the zone of past 

deformation approximately 30 km beyond the mainland coast (Blakely and others, 2004). 

A detailed description of the aeromagnetic survey and evidence for onshore extension of 

the SWIFZ were provided by Blakely and others (2004). In their report, Blakely and others 

(2004) used a numbering scheme to designate lidar features. In particular, they referred to 

lineament 4 as a set of topographic features that follows the Little Bear Creek drainage and 

crosses the site of a proposed sewage treatment facility in southern Snohomish County, and they 

referred to features 10 and 11 as two scarps east of Crystal Lake. In this report, we designate 

lineament 4 as the Little Bear Creek lineament and the scarps east of Crystal Lake as the Crystal 

Lake lineament. Both of these topographic features lie along the larger Cottage Lake 

aeromagnetic lineament. In the following section, we summarize the results of Blakely and 

others (2004) and their implications for more recent paleoseismic field investigations. 

AEROMAGNETIC ANOMALIES 

In 1997 the U.S. Geological Survey conducted a high-resolution aeromagnetic survey of 

the Puget Sound and adjacent areas (Blakely and others, 1999). Flight altitude was targeted at 

250 m above ground or as low as permitted by safety considerations; over the MaltbyWoodinville area, flight altitude was nominally 262 m above terrain. North-directed flight lines 

were spaced 400 m apart over the Puget Lowland; east-directed tie lines were spaced at 8 km 

intervals. Two base magnetometers operated continuously during the survey in order to 

subsequently correct for time-varying fields. Subtraction of the International Geomagnetic 

Reference Field provided total-field aeromagnetic anomalies. Taking all sources of error into 

account, we estimate the overall precision of individual measurements to be 0.5 nT or better 

3
(Blakely and others, 2004). [nT is an abbreviation for nanotesla, a unit of measure for magnetic 

flux density, or the “strength” of the magnetic field.. 1 nT = 10-9 Tesla. See Blakely (1995) for 

additional information.] 

Figure 3A shows unprocessed aeromagnetic anomalies over the study area. Most 

anomalies in Figure 3A, especially those of regional extent, are caused by variations in 

magnetization of subsurface lithologies, and the evidence for and implications of these anomalies 

are discussed below. A “dimpled” pattern is also evident in some parts of Figure 3A, especially 

along the mainland coast. This short-wavelength pattern originates from large buildings and 

other manmade structures. For example, the anomaly southeast of Mukilteo (label C, Figure 3A) 

is situated over a Boeing facility and probably is caused by associated buildings and other 

infrastructure. 

Magnetic anomalies and possible source lithologies 

The onshore portion of the study area is dominated by two broad, high-amplitude 

anomalies (anomalies A and B, Figure 3A). Anomaly A south of Snohomish extends over Bald 

Hill and Devils Butte, which are underlain by Tertiary volcanic rocks of Mount Persis (Johnson 

and others, 1996; Minard, 1985; Tabor and others, 1993; Yount and Gower, 1991). These 

locally exposed Tertiary volcanic rocks probably extend in the subsurface throughout the region 

beneath anomaly A. 

A second broad, high-amplitude magnetic anomaly (anomaly B, Figure 3A) is situated 

over the Maltby-Woodinville area. The high amplitude of anomaly B requires highly magnetic 

rocks, and its broad gradients indicate that the source lies at depths of several kilometers. 

Potential sources include Tertiary volcanic rocks of Mount Persis and basaltic rocks of the 

Eocene Crescent Formation. 

A subtle pattern of aeromagnetic anomalies is superimposed on anomaly B and adjacent 

areas. The pattern is characterized by northwest-striking lineaments, each extending more or less 

continuously for distances of 2 to 16 km. The narrow gradient across each lineament indicates 

that they originate from the shallow subsurface, and we can highlight their expression by 

computationally emphasizing anomalies caused by shallow sources in Figure 3A. Figure 3B 

shows one such method, known as “residual magnetic anomalies”. Residual anomalies are 

calculated by transforming observed anomalies into those anomalies that would be observed at a 

slightly higher elevation (50 m higher in this case), a process known as upward continuation, 

then subtracting that result from the observed anomalies (Blakely and others, 2004). 

The northwest-striking pattern of residual magnetic anomalies appears throughout the 

onshore portion of the study area but is best represented in the Maltby-Woodinville area. The 

pattern is bounded to the northeast by a pronounced northwest-striking lineament (lineament E, 

Figure 3B) that also forms the northeastern margin of anomaly B. South of magnetic lineament 

E, the lineaments each separate positive regions to the northeast from less positive regions to the 

southwest. The lineaments are not associated with manmade structures and must be caused by 

subsurface lithologies. The longest and most pronounced of these lineaments (lineament D, 

Figure 3B), referred to as the Cottage Lake aeromagnetic lineament in Blakely and others (2004), 

is spatially associated with scarps identified in lidar terrain mapping. Without exception, the 

sense of offset on these topographic scarps when observed in the field is northeast-side-up. 

Similarly, the sense of each magnetic gradient (northeast-side more positive than the southwest 

side) is consistent with northeast-side-up offset on a normally magnetized body. Blakely and 

others (2004) concluded that lineament D (the Cottage Lake aeromagnetic lineament) and other 

linear aeromagnetic anomalies in the Maltby-Woodinville area are caused by strands of the 

4
SWIFZ. 

The lithologic source of the magnetic lineaments is of considerable interest. The sharp 

gradients of the lineaments indicate that they originate in part from relatively shallow depths, 

within the upper 1 to 2 km, and thus linear magnetic sources extend structurally higher than the 

source of anomaly B. Two nearby exploratory wells provide insights into possible magnetic 

lithologies at these depths. The Standard Oil Company of California Alderwood #1 well (label 

SA, Figure 3A) encountered unconsolidated Pleistocene glacial and interglacial deposits to a 

depth of 110 m; penetrated mudstone, siltstone, and sandstone of the upper Eocene to lower 

Oligocene Blakeley Formation to a depth of 1591 m; and bottomed in sedimentary and volcanic 

rocks interpreted by Rau and Johnson (1999) as Eocene sandstone of Scow Bay. The Standard 

Oil Company of California Socal-Schroeder #1 well (label SS, Figure 3A) passed through 207 m 

of unconsolidated Pleistocene glacial and interglacial deposits; penetrated Blakeley Formation 

and Renton Formation to a depth of 2438 m; then encountered Tertiary volcanic rocks 

interpreted by Rau and Johnson (1999) as volcanic rocks of Mount Persis. The volcanic-derived 

lithologies encountered near the bottom of each of these wells are clearly potential sources for 

the aeromagnetic lineaments, if those lithologies lie at shallow enough depths. Magnetic 

susceptibility measurements in the Maltby-Snohomish area (Blakely and others, 2004) found that 

even Pleistocene glacial deposits and coarse-grained units within the Blakeley Formation are 

sometimes weakly magnetic. 

GROUND-MAGNETIC SURVEY 

An open field traversed by the Little Bear Creek lidar lineament (lineament 4 on Fig. 4) 

afforded an excellent opportunity to conduct a ground-magnetic survey in a region of linear 

aeromagnetic anomalies. The motivation for this investigation was to help determine the 

lithologic source of aeromagnetic lineaments throughout the Maltby-Woodinville area. The open 

field was also the site of the “Beef Barley” trench excavation (Fig. 4), discussed subsequently, 

and the ground-magnetic survey provided an opportunity to characterize surrounding and 

underlying lithologies. The open field was ideal for this type of investigation: First, it contained 

no significant metallic objects (buildings, metal fences, pipes, culverts, etc.) to contaminate the 

magnetic field. A small amount of wire was uncovered during the trench excavation (see 

following), but its volume was too small to significantly affect the magnetic measurements. 

Second, the lack of tree cover facilitated accurate GPS determinations. 

Survey Specifications 

The survey was conducted prior to the Beef Barley trench excavation. Profiles were 

conducted on foot while carrying a Geometrics 858 magnetometer system. The 858 system 

includes a cesium-vapor magnetic sensor, real-time GPS navigational equipment, and a small 

breast-pack computer. Seventeen profiles were directed northeast and spaced nominally 9 m 

apart; a single profile was directed northwestward through the future trench site. Measurements 

were made at 1-second intervals while walking at a normal pace. According to the manufacturer, 

the Geometrics 858 magnetometer has a sensitivity of 0.01 nT when operated at 1.0-second 

intervals. A stationary base station was established within the open field and operated 

simultaneously with the ground-magnetic survey in order to subsequently correct for diurnal 

variations of the earth’s magnetic field and external field fluctuations. The base-station sensor 

was a Geometrics 856 proton-precession magnetometer. According to the manufacturer, this 

magnetometer has a resolution of 0.1 nT and an accuracy of 0.5 nT. 

5
Ground Magnetic Anomalies 

Figure 4 shows the ground-magnetic survey. Magnetic field values measured along each 

profile were corrected for time-varying magnetic fields as recorded at the base station, and then 

interpolated to a projected rectangular grid using a standard minimum-curvature algorithm. No 

other processing was applied to the data shown in Figure 4. Individual anomalies typically range 

between -40 and +20 nT and have gradient widths on the order of a few tens of meters. 

Individual anomalies measured at ground level with these characteristics would not appear at 

aeromagnetic altitudes (262 m). 

The ground-magnetic anomalies are small in amplitude, considering that they were 

measured 2 m above ground, but we are struck by the linear nature of the anomalies and their 

dominantly northwest-strike parallel to the Little Bear Creek topographic lineament (lidar 

lineament 4 of Blakely and others (2004)) and on strike with the SWIFZ. Several pronounced 

northwest-striking lineaments are evident, shown by the hand-drawn dotted lines on Figure 4; 

one passes through the Beef Barley trench site, three lie near lidar lineament 4 (Blakely and 

others, 2004), and a fifth (ground-magnetic lineament GA, Figure 4) lies southwest of the trench. 

The trench could not be extended to this southwestern-most lineament because of environmental 

restrictions. 

Depth and lithology of likely sources 

The sharp gradients of these linear anomalies indicate that their sources lie at shallow 

depth. To confirm that assertion, we applied a graphical approach to estimate the depth to the 

magnetic sources of ground-magnetic lineament GA (Figure 4) beneath profiles 6 and 8. The 

technique, known as “Peters’ method” (Blakely, 1995; Peters, 1949), is based on an important 

property of magnetic anomalies: The horizontal gradient at the margins of a magnetic anomaly 

depends on the depth to source; i.e., a shallow magnetic body produces a sharper magnetic 

anomaly relative to an anomaly caused by the same body at deeper depth. Peters’ method is 

graphical in nature: Draw two parallel lines (dotted lines in Figure 5) with slope equal to one 

half of the maximum gradient of the anomaly (dashed lines in Figure 5), one line tangent to the 

peak of the anomaly and the other line tangent to the minimum part of the anomaly. The 

horizontal separation of the two lines is proportional to the depth to the top of the body. The 

proportionality constant is 1.2 for very thin bodies and 2.0 for very thick bodies. We chose a 

value of 1.2, assuming that glacial strata are relatively thin in this region. Peters’ method 

assumes that the magnetic anomaly is caused by a two-dimensional body with vertical sides and 

uniform and nearly vertical magnetization. Although the magnetic sources under investigation 

here are two-dimensional in nature, the other underlying assumptions are less likely to be 

realized. Therefore, our estimates should be treated as only approximate. 

Based on Peters’ method, we estimate that the top of the magnetic source of lineament 

GA lies in the upper 8 to 20 m (Figure 5). Geo-technical boreholes indicate that Pleistocene 

glacial deposits extend to depths of about 100 m in this area (AspectConsulting, 2005). Thus, 

Pleistocene glacial deposits are apparently the lithologic source of the linear anomalies seen in 

ground-magnetic profiles. In the area of the Beef Barley trench specifically, boreholes 

encountered glacial till, overlain by outwash, at depths increasing west-southwestward across the 

trench site (Aspect Consulting, 2005). Extrapolating from these boreholes, we estimate the 

glacial till to lie approximately 10 m below ground level at ground-magnetic anomaly GA, 

roughly equivalent to the magnetic source depth (8.6 m) estimated from Peter’s method. 

Several explanations for the ground-magnetic anomalies are possible. (1) The anomalies may 

originate from lateral variations in magnetic properties caused by contrasting depositional 

6
processes within the Pleistocene section, such as contacts between outwash and channel deposits. 

(2) The anomalies may be caused by concentrations of secondary magnetic minerals, possibly 

related to persistent elevated ground-water levels. (3) The anomalies may be caused by faulted 

offsets of Pleistocene strata. We favor the third interpretation because of the linear nature of the 

anomalies and their orientation relative to the Little Bear Creek lidar lineament, linear 

aeromagnetic anomalies, and the onshore extension of the SWIFZ. 

Magnetic susceptibility measurements of deposits exposed in the trench wall ranged from 

0.62x10-3 to 10.30x10-3 SIU, with an average of 2.25x10-3 SIU. [SIU is an abbreviation for 

Système Internationale unit. Magnetic susceptibility is proportional to induced magnetization; it 

is a dimensionless quantity, but its numerical value depends on the units of measure. See 

Blakely (1995) for additional information.] This average susceptibility would produce an 

induced magnetization of about 0.10 A/m. [A/m, or ampere per meter, is the unit of 

magnetization. See Blakely (1995) for additional information.] Susceptibility measurements of 

glacial till recovered from geo-technical boreholes in the area had similar magnetic 

susceptibilities (average 2.75x10-3 SIU). 

The range of magnetic susceptibilities observed in the Beef Barley trench is marginally 

sufficient to explain the ground-magnetic anomalies. We verified this assertion with a simple 

forward model (Blakely, 1995): a horizontal layer with magnetic susceptibility of 7x10-3 SIU 

relative to overlying strata, a value near the high end of the measured range, and with a 

northwest-striking, 5-m vertical offset of its upper surface located directly beneath groundmagnetic anomaly GA. If the depth to the layer is 13 m on the southwest side of the vertical 

offset and 8 m on the northeast side, the calculated anomaly has approximately the same 

amplitude and wavelength as observed in ground-magnetic anomaly GA. 

Relationship between ground-magnetic and aeromagnetic anomalies 

We suggest that the linear anomalies in Figure 4 are caused by linear offsets in magnetic 

glacial deposits in the upper 8 to 20 m. It is important to recognize, however, that glacial 

deposits alone cannot also account for the aeromagnetic lineaments. Individual ground-level 

anomalies with characteristics described here would not be detectable at aeromagnetic altitudes 

(approximately 262 m), yet the ground-magnetic anomalies do lie near and parallel to the Little 

Bear Creak aeromagnetic lineament (Figure 3B). As suggested by Blakely and others (2004), the 

aeromagnetic anomalies may be caused by faults that intersect both the Pleistocene section and 

deeper, more magnetic lithologies. 

PALEOSEISMOLOGY OF THE SWIFZ 

Geophysical studies of the SWIFZ suggest that several offshore splays project onto the 

mainland between Edmonds and Mukilteo (Figure 6). Lidar images reveal several post-glacial 

scarps that sit astride magnetic anomalies and topographic lineaments. Although scarps may be 

formed by a variety of surficial processes, the correlation of scarps with deeper geophysical 

structure suggests that the scarps may be related to faulting on strands of the SWIFZ. To test this 

hypothesis, we excavated trenches across scarps at four sites to look for earthquake-related 

deformation of surficial deposits. The potential for recent earthquakes in this area is also 

suggested by evidence for a recent earthquake along the SWIFZ on Whidbey Island (Kelsey and 

others, 2004). 

Summary of Whidbey Island paleoseismology 

Relative sea level (RSL) histories at two coastal marshes straddling the SWIFZ on 

Whidbey Island show evidence for abrupt uplift between 3200 and 2800 cal yr B.P. (Figures 2 

7
and 7). Sea level curves for Crockett and Hancock marshes, developed using radiocarbon ages 

from sand/peat contacts beneath each marsh, should be nearly identical if the sites experienced 

no past deformation. However, combined RSL curves show that the Crockett marsh curve is 

displaced about 2 m relative to the Hancock marsh curve (Figure 7). Kelsey et al. (2004) 

inferred that an earthquake caused the changes they observed in the RSL histories from the two 

marshes. The earthquake, caused by reverse offset on a fault tip concealed beneath a fold on 

central Whidbey Island, occurred between 3200 and 2800 cal yr B.P. (Kelsey and others, 2004). 

The fold and fault, imaged on high-resolution seismic reflection profiles in Admiralty Inlet and 

Holmes Harbor, lies hidden beneath a mantle of deformed late Quaternary glacial deposits. The 

seismic data show an asymmetrical syncline, with a steeper limb on the north side, developed in 

Quaternary glacial deposits. Kelsey et al. (2004) inferred that the syncline is cut by a steep 

reverse fault, possibly dipping 60°. 

Prehistoric liquefaction at the Snohomish River delta 

Bourgeois and Johnson (2001) found evidence for several events at the Snohomish River 

delta near Everett, Washington. Outcrops along tidal channels, exposed at low tide, revealed 

evidence for several episodes of strong shaking. This evidence consists mainly of upward 

terminating sand dikes and sand-filled cracks, some up to 1 meter wide. The dikes and cracks 

cut layers containing organic materials usable for radiocarbon dating. The radiocarbon ages and 

stratigraphic relationships show that strong shaking events occurred 1205-1025 years B.P., 1095-

1015 years B.P., and 575-365 years B.P. Because the evidence for these events consists of 

liquefaction features, Bourgeois and Johnson (2001) could not attribute the strong shaking to a 

specific source. Shaking may have been caused by a Cascadia subduction zone earthquake, 

intraslab earthquake, or shallow earthquake in the Puget Lowland. 

Evidence for past surface rupture from lidar and historical aerial photographs 

Lidar mapping and historical aerial photographs of northern King County and southern 

Snohomish County reveal several features along the mainland trace of the SWIFZ (Figures 6, 8 

and 9). The extent of lidar coverage from King County is shown on Figure 6 and only covers 

part of the area traversed by the mainland projection of the SWIFZ. Poor quality and artifacts in 

the King County lidar data limited the resolution of our lidar analysis (Blakely and others, 2004). 

We identified several lidar scarps in the area near Crystal Lake and Cottage Lake (Figure 9). 

These features consist mainly of short (<1 km), north-side-up scarps and longer topographic 

lineaments (~0.5 - ~4 km long). No lidar data exists for most of Snohomish County. Therefore, 

we relied on historical aerial photographs to search for features possibly related to past surface 

rupture in southern Snohomish County. Features mapped from historical aerial photographs near 

Devil’s Butte in Snohomish County (Figure 8) appear on both the 1933 and 1938 Army Corps of 

Engineers photographs (Collins and others, 2005). These features consist of short scarps and 

longer topographic lineaments paralleling the Snohomish River near Devil’s Butte. 

The lidar and aerial photograph features fall close to, or directly on, previously identified 

low-amplitude magnetic anomalies (Figure 6). Some of these anomalies are thought to represent 

strands of the SWIFZ in the shallow subsurface. The largest and most continuous aeromagnetic 

anomaly is the Cottage Lake aeromagnetic lineament. Two topographic lineaments that lie on 

top of and southwest of the larger Cottage Lake aeromagnetic lineament – the ‘Crystal Lake 

lineament’ and the ‘Little Bear Creek lineament’ – were selected for field study in the summer of 

2004. To test the hypothesis that the topographic lineaments coincided with active faults, we 

mapped excavations placed across scarps at four sites. 

8
SELECTION OF STUDY SITES AND FIELD METHODS 

Between June and October, 2004, the USGS opened and mapped four excavations across 

scarps lying along the onshore extension of the SWIFZ. The excavation sites lay along discrete 

topographic lineaments, found using lidar maps, within the larger Cottage Lake aeromagnetic 

lineament. Specifically, two of the excavations cross scarps along a 3-km-long topographic 

feature known as the Little Bear Creek lineament, and two lie along a 3-km-long scarp near 

Crystal Lake (Figures 6 and 9). The excavations were placed perpendicular to the scarps to 

depths between 1.2 and 2.4 m. Excavation walls higher than 1.2 m were sloped at about 1:1 for 

safety. After establishing a 1-m by 1-m grid system in each excavation, field teams flagged 

stratigraphic contacts and structural features using colored surveyor’s tape and nails, and mapped 

selected excavation walls on rectified digital photomosaics. 

EXCAVATIONS ACROSS THE CRYSTAL LAKE LINEAMENT 

A north-side-up scarp lying along the Cottage Lake aeromagnetic lineament is the most 

prominent scarp observed in the lidar data (Figure 9). The scarp, previously identified as Feature 

10 and Lineament 11 in Blakely et al. (2004) and herein called the Crystal Lake lineament, is 

about 3 km long, and is sub-parallel to the eastern shoreline of Crystal Lake. Profiles across the 

scarp using the lidar data show a broad, north-side-up scarp between 2.5 and 4 m in height. Field 

observations confirm the scarp location and approximate height. The scarp is not clear on the 

lidar map at its southeast end due to poor lidar returns in dense vegetation. 

Two sites along the Crystal Lake lineament were selected for excavation on the basis of 

lidar data and field reconnaissance (Figure 9). The two excavation sites, named Mountain 

Beaver and Flying Squirrel, lie 100 to 200 m east of Crystal Lake in a mature Douglas-fir forest. 

Mountain Beaver site 

The Mountain Beaver site consists of a 16-m long excavation with sloping walls (about a 

1:1 slope). This excavation exposed deposits from the last glacial advance into the Puget 

Lowland (Figure 10). This glacial advance, regionally called Vashon, culminated around 16,400 

cal yr B.P. when the glacier disintegrated, leaving behind outwash deposits and glacial drift 

(Porter and Swanson, 1998). Glacial drift exposed in the excavation consisted of dense gray 

sandy silt with faceted pebbles and cobbles (Qvt). Overlying the drift is a thin layer of dense silt 

that is laminated in places (Qste). The silt is in turn overlain by a pebbly drift consisting of 

dense sandy silt with abundant faceted pebbles and cobbles, and discontinuous stratified sand 

layers (Qd). 

A fault separates the pebbly drift (Qd) from overlying sand and pebbly silty sands (Qrs) 

that contain large intact blocks of Qd (Figure 10 and 11). The fault truncates and deforms sand 

and silt layers in the pebbly drift (Qd) such that a normal sense of movement on the fault is 

clearly observed. Photographs of the fault show truncated and deformed beds, and show a shear 

fabric parallel to the fault (Figure 11). Overlying the fault is a sequence of massive to 

interbedded sand and pebbly silty sands that pinch out towards the middle of the trench wall 

(Qrs). A clast of charcoal from a silty fine sand layer (sample MB001 from Qrs) yielded an 

AMS radiocarbon age of 31,330±340 C14 yr B.P. Overlying Qrs is a buried A-horizon (Hab). 

A charcoal clast collected from the stratigraphic middle of Hab in the west wall (not shown in 

Figure 10) of the excavation yielded an AMS radiocarbon age of 10,190±40 C14 yr B.P. (12,090 

– 11,670 cal yr B.P.). Gravel lenses (not shown in Figure 10) and a silty colluvial deposit (Hc 

and Hcs) bury the former A-horizon and underlying deposits. The loose character of unit Hc 

9
makes it an ideal habitat for mountain beaver (Aplodontia ruppia). Several filled and open 

Aplodontia burrows were noted during field work, and several live Aplodontia were observed in 

the excavations. The tops of the excavation walls are mantled with surface accumulations of 

modern forest litter (Ha), with thicker accumulations near the base of the scarp. 

Flying Squirrel site 

The Flying Squirrel site is a 20-m long excavation across a 3.5–m high, north-side-up 

scarp (Figure 12). Eight lithologic units mapped in the excavation provide a stratigraphic section 

spanning from the Vashon advance to the present. Vashon till is exposed at the base of the 

stratigraphic section and consists of a gray sandy loam with faceted pebbles and cobbles of 

mainly felsic and intermediate rocks (unit Qvt on Figure 12). An erosional unconformity 

separates the till from overlying Vashon recessional outwash deposits. 

The Vashon recessional outwash deposits consist of four mappable units. The oldest unit 

is a gray, gravelly, fine to coarse sand to sandy gravel (Qrg). Interbedded with unit Qrg are two 

similar units - a gray, pebbly, fine to coarse sand with crossbedding (Qrgs), and a gray, fine to 

coarse, crossbedded sand (Qrs). A thin silt layer with laminations (Qste) caps the recessional 

outwash deposits. Unit Qste is assumed to be a good indicator of paleohorizontal; however, dips 

of Qste range from an average of about 10o to almost 40o below the foot of the scarp (between 6 

and 10 m). 

Unit Qste is overlain by a gray, pebbly sandy loam (unit Qd) with planar and cross 

stratification observed in several places. A thick, reddish-brown sandy loam to loam, with weak 

sub-blocky pedogenic structures and scattered pebbles and cobbles (Hc) overlies the entire 

section. Tree roots and burrows of Aplodontia ruppia extend downward from ground surface 

into Hc. Former burrows and filled cavities from rotted tree roots occur in several unmapped 

sections of the excavation. A thin layer of organic detritus (unit Ha) overlies unit Hc and 

mantles the ground surface. 

EXCAVATIONS ACROSS THE LITTLE BEAR CREEK LINEAMENT 

The Little Bear Creek lineament follows the Little Bear Creek drainage and lies parallel 

to and along an aeromagnetic anomaly about 1.5 km southwest of the Cottage Lake 

aeromagnetic lineament (Figures 6 and 9). The southeastern part of the lineament is aligned 

along a deep ravine and several scarps. To investigate whether the lineament is a strand of an 

active fault, we selected two sites with degraded scarps for further study based on field 

reconnaissance. Detailed logs of the sites, named French Onion and Beef Barley after the 

pervasive aroma of soups produced in a nearby facility, are shown in Sherrod and others (2005). 

French Onion site 

The French Onion site is an excavation across a small scarp on the south side of a narrow 

ridge (Figure 13). The ridge is oriented northeast-southwest and bordered on the southeast and 

northwest by deep ravines eroded into late Quaternary glacial deposits. A small scarp lying 

along the Little Bear Creek lineament crosses the southwest end of the ridge. A shallow 

excavation across the scarp revealed eight stratigraphic units composed of glacial lake deposits, 

till, and Holocene soils. 

The oldest units found in the French Onion excavation are glacial lake deposits, 

presumably of Late Pleistocene age (Figure 13). A brown to gray, laminated claystone (Qscl) 

and a brown mudstone (Qmud) lie at the base of the exposed section. Overlying Qscl is a 

massive to laminated, gray claystone (Qmcl). A small lens of glacial drift, probably a till (Qt), is 

located between 12 and 15 m and is nearly surrounded by Qmud. All of these units were slightly 

10
indurated, possibly as a result of overconsolidation below the bed of a thick glacier. No 

materials for radiometric dating were observed in the glacial deposits, but the overconsolidation 

is consistent with these units being deposits from an earlier glacial advance that were 

subsequently overridden by glacial ice during the Vashon advance. 

Overlying the glacial deposits in the French Onion trench is a series of Holocene soils 

and colluvial deposits. A thick layer of buried organic-rich sandy loam to loam lies directly on 

the Qmud unit (unit Hab between 6 and 9 m on Figure 13). Immediately overlying Hab are two 

colluvial units – Hcol and Hpcol on Figure 11. Hcol is a brown sandy loam to loam and contains 

scattered charcoal clasts. Hpcol is similar to Hcol but has abundant pebble-sized clasts of 

mudstone. The mudstone pebbles were likely derived upslope from weathering of units Qscl, 

Qmud, and Qmcl. Hpcol overlies Hcol between 4 and 6 meters on Figure 11. A dark brown 

organic detritus layer (Ha) caps the stratigraphic section. 

Beef Barley site 

The Beef Barley site consists of an excavation ~30 meters long and ~2.4 meters deep, 

oriented N55oE (Figures 14 and 15). Approximately the upper meter of exposed soil consisted of 

fill containing shards of colored glass, a small amount of fence wire, and machine-cut lumber. 

The soils below the fill appeared undisturbed and likely span the time interval from Vashon 

recessional outwash to development of the Little Bear Creek drainage and historic settlement. 

The oldest stratigraphic units found in the Beef Barley excavation were two discrete 

sequences of recessional outwash deposits (Figure 14). Neither sequence is overconsolidated, 

indicating that neither was overridden by glacial ice. The lack of overconsolidation suggests that 

the sequences are Vashon recessional outwash deposits. The first sequence (sequence Q1; units 

Q1a to Q1g on Figure 14) consists of yellowish brown to olive brown pebbly sand, loamy sand, 

and sandy loam. Locally, gravel beds within sequence Q1 dip ~36o to the SSW while thin silts 

and clay layers in sequence Q1 dip as much as 22o to the SSW. An angular unconformity (Q1-

Q2 unconformity; yellow line on Figure 14) separates sequence Q1 from an overlying sequence 

of recessional outwash deposits (sequence Q2). Sequence Q2 consists of yellow brown to olive 

gray gravelly sandy loam, sandy loam, sandy clay, and clay (Figure 14). All of sequence Q2 is 

essentially flat-lying to very gently south-dipping. 

A second unconformity separates sequence Q2 from an overlying gravelly deposit (Q2-

H/Q3 unconformity; green line on Figure 14). This unconformity is flat-lying between 8 and 16 

meters (see Figure 14) and rises slightly between 2 and 6 m. This slight rise could result either 

from a tectonic origin for the unconformity or from normal erosional processes that created the 

unconformity. Three conformable units lie above the Q2-H/Q3 unconformity. 

Unit H/Q3 is an olive brown sandy loam that lies directly over the Q2-H/Q3 

unconformity. This unit extends the length of the excavation but thins considerably in the 

northeastern part of the excavation. Dark brown organic-rich sandy loams (unit H4 and H4.1) 

overlie unit H/Q3 and most likely represent prehistoric and historic surface soils developed at the 

site. Charcoal samples collected at the base of unit H4 yielded AMS ages of 2450±40 C14 yr 

B.P. (BETA-200523; 2730 to 2350 cal yr B.P.) and 2660±50 C14 yr B.P. (BETA- 200524; 2850 

to 2740 cal yr B.P.). Unit H4.1 is slightly disturbed in places, most likely by equipment used to 

place fill over the site in the early part of the 20th century. 

EVIDENCE FOR PREHISTORIC DEFORMATION 

Stratigraphic and structural observations collected from the four excavations yield 

insights into past earthquakes along the SWIFZ (Figure 16). We mapped each excavation in 

11
detail, looking for evidence of surface deformation from folding and faulting and establishing 

criteria to distinguish tectonic from glacial deformation. Evidence for strong shaking at each 

site, in the form of liquefaction features, was also mapped. At sites along the Crystal Lake 

lineament, we observed evidence for past folding and faulting, and at sites along the Little Bear 

Creek lineament we observed evidence for past folding, faulting, and liquefaction. These past 

folding, faulting, and liquefaction events were each assigned a unique identifier and are 

discussed below. 

Crystal Lake lineament 

The clearest indication of prehistoric deformation along the Crystal Lake lineament is 

post-glacial folding (Event C) and a normal fault that offsets glacial deposits (Event B). Event B 

is interpreted from deformed drift and outwash cut by a low-angle normal fault (Figure 11). A 

low-angle fault observed in the Mountain Beaver excavation (dip is ~20o on Figures 9 and 11) 

separates Vashon drift/ablation till (Qd) from Vashon recessional outwash deposits (Qrs channel 

fill). The fault truncates interbeds within Qd. Drag folds in Qd and Qrs suggest a normal sense 

of movement on the fault (Figure 11). Stratigraphic observations suggest that the last episode of 

movement on the fault could be as old as late Pleistocene, and was possibly caused by syn- or 

post-depositional slumping or tectonic movement. The sediments cut by the normal fault are 

strongly deformed and are associated with features of soft-sediment deformation. Therefore, it is 

likely that Event B and the normal fault are the result of slumping or landsliding in a saturated, 

recessional-outwash environment rather than tectonic deformation. However, a tectonic origin 

for the normal fault cannot be ruled out completely. 

After Event B, the entire section, including the fault, was warped by several meters of 

north-side-up folding. If the normal fault was created by landsliding during Event B, then the 

original dip of the fault plane may have been quite low or even flat, accounting for the ~20o postfolding dip of the fault plane. Post-glacial folding (Event C) along the Crystal lake lineament is 

indicated by dipping laminated silt layers, deformed recessional outwash deposits, and a buried 

soil. Laminated silts observed in the Flying Squirrel and Mountain Beaver excavations had dips 

averaging about 10o in both excavations, but higher dips in parts of both excavations suggest 

post-depositional warping. Below the scarps at both excavations, dips of the silt layer increase, 

reaching a maximum of almost 40o in the Flying Squirrel excavation and ~30o in the Mountain 

Beaver excavation (Figures 10 and 11). While 10o dips may be common in glacial deposits, dips 

of laminated silts exceeding 30o are not common; suggesting that the higher dips resulted from 

post-depositional deformation. Folding in the Mountain Beaver excavation is mirrored by dips 

of units Qvt, Qste, Qd, and Qrs, and the normal fault described above – suggesting that the entire 

section was warped during a single event. Geometry of stratigraphic sections in the Mountain 

Beaver excavation indicates that warping was greater than 2 m during Event 2. Thus, the 

original scarp at the Mountain Beaver site likely was at least 2 m high. It is possible that the 

Mountains Beaver excavation records only Event C. Lidar profiles across the scarp show that 

the present-day scarp height is between 2.5 to 4 m, suggesting that folding during Event C could 

have been 4 m high. Over time, the original scarp at the Mountain Beaver site degraded and 

colluvium buried the former soil at the base of the scarp (unit Hab on Figure 10). 

Unit Hab at the Mountain Beaver site is a buried A-horizon. Filled root cavities extend 

from the bottom of the former soil horizon into the underlying units. The surface of the buried 

soil rises slightly towards the scarp, suggesting that parts of the former soil were eroded over 

time. Colluvium from the scarp (units Hc and Hcs) buried the remaining soil after the event. 

Thus, radiocarbon ages from charcoal in the buried soil horizon provide a maximum age for the 

12
timing of the folding event described above. A single radiocarbon age from a sample collected 

near the middle of unit Hab yielded an age of 12,090 – 11,670 cal yr B.P. This single age 

indicates that Event C occurred after 11,670 cal yr B.P. 

Little Bear Creek lineament 

Observations of fractures, faults, and shear zones in the French Onion excavation suggest 

a complex deformation history. We observed at least three structural fabrics in the excavation, 

one dipping to the south, one to the north, and a subhorizontal fabric. When viewed in outcrop, 

these fabrics give the older glacial units (Qscl, Qmud, Qmcl) a shattered appearance. We 

observed each fabric in the older glacial units and traced only a few faults into the overlying 

Holocene units. However, it was not possible to determine if the faults actually offset the 

Holocene units or if the Holocene units were simply weathering into one of the older faulted 

units. With few exceptions, we could not determine the sense of motion of any of the faults. We 

infer that the south and north-dipping fabrics are older than the subhorizontal fabric (discussed 

below). We did not assign any events to these fabrics because it is likely that none of the faults 

observed in the French Onion excavation moved after Vashon deglaciation. It is possible that 

fault-line erosion created the scarp at the French Onion site rather than movement on a fault. 

The south-dipping fabric in the French Onion excavation is concentrated in the southern 

part of the excavation (Figure 13). We observed reverse motion on at least three of the southdipping faults. One of the south-dipping reverse faults cuts a late Quaternary till (Qt between 13 

and 14.5 m on Figure 13). Several south-dipping faults appear to push Qscl against Qmud 

(between 7 and 10 m on Figure 13), suggesting a reverse sense of motion on this set of faults. 

These south-dipping reverse faults are difficult to explain with movement of glacial ice across 

the region. Glacial flutes indicate that ice flowed approximately north to south across the region 

in the last glacial advance, and we assume that previous glaciers flowed in similar directions. If 

these faults are glaciotectonic, then the hanging wall was thrust upwards into the bed of the 

glacier – an unlikely scenario. More likely, the south-dipping faults are tectonic and were later 

cut by north-dipping and subhorizontal faults produced by movement of glacial ice. 

The north-dipping fabric is concentrated in the northern part of the excavation. The 

largest of the faults observed in this set of features appear to merge between grid points 9 and 10 

m on Figure 13. Bedding truncations and displacements on the largest of the north-dipping faults 

were not observed so it is not possible to indicate a sense of slip for this set of features. 

A subhorizontal shear fabric cuts across the north-dipping and south-dipping faults in the 

French Onion excavation. Microfaults observed in the subhorizontal fabric had both reverse and 

normal displacements, but were dominated by normal displacement. In general, the sense of net 

tectonic transport was to the south. A south-directed tectonic transport is consistent with the 

subhorizontal fabric resulting from simple shear at the base of a glacier. Nelson at al. (2003) 

observed similar shear fabrics developed in weathered bedrock immediately below Vashon till 

and attributed the shears to movement of glacial ice across the weathered bedrock surface. 

The Beef Barley excavation has a rich history of deformation and strong ground shaking. 

One or possibly two angular unconformities and reverse faulting suggest multiple episodes of 

surface deformation. Liquefaction features observed throughout the excavation disturb recent 

soils and indicate strong ground shaking in the late Holocene. 

The oldest period of deformation (Event D) in the Beef Barley excavation is marked by 

the angular unconformity between sequences Q1 and Q2 (Figures 14 and 15). Although no 

material was found to provide ages for either sequence, lithologic comparisons to other 

exposures and the lack of overconsolidation suggest that the unconformity separates two 

13
sequences of Vashon recessional outwash. We speculate that the unconformity was created 

when an earthquake on the Little Bear Creek lineament caused folding of sequence Q1 (Event 

D). Following Event D, the folded units were beveled off by flowing water and sequence Q2 

was deposited. Thus, Event D likely occurred between deglaciation and establishment of nonglacial environments, or roughly between 16,400 and ~12,000 cal yr B.P. A limiting minimum 

age of 12,000 years B.P. was selected because non-glacial environments were established in 

many areas of the Puget Lowland by ~12,000 years B.P. (Nelson and others, 2003; Sherrod, 

2002). Non-glacial environments inferred at the Mountain Beaver site at the time of Event C 

suggest that Event C postdates Event D by hundreds to thousands of years (Figure 16). 

A possible earthquake (Event E) occurred in either the latest Pleistocene or early 

Holocene, marked in the Beef Barley excavation by the unconformity between sequence Q2 and 

unit H/Q3. This unconformity could be the result of folding on the Little Bear Creek lineament, 

or, alternatively, the unconformity could be the result of normal fluvial processes acting at the 

site at the time unit H/Q3 was deposited. Neither alternative can be dismissed based on the data 

collected in the excavation. If the unconformity is the result of folding on the Little Bear Creek 

lineament, no age data exist to constrain the age of the event beyond being younger than ~12,000 

yr B.P and older than ~2850 cal yr B.P., the maximum age of unit H4. 

The youngest event (Event F) recorded in the Beef Barley excavation included folding, 

reverse faulting, and liquefaction. Evidence for folding is best seen between 14 and 20 m on the 

excavation logs (Sherrod et al. (2005), and Figure 15). Reverse faulting is best seen between 

13.5 and 15 m on the excavation logs. Good piercing points to measure displacement during this 

youngest event are lacking but the folding probably did not exceed 1 meter and the faulting is 

likely limited to only a few decimeters (less than 30 cm). However, none of these estimates 

account for out-of-plane movement (lateral displacement), and actual slip could have been quite 

a bit larger. 

Liquefaction accompanied the folding and faulting of the youngest event, evidenced by 

numerous liquefaction features observed throughout the excavation (Figures 14 and 15). These 

features include sand-filled dikes injected upwards through fine-grained facies of sequences Q1 

and Q2. Pieces of Q1 and Q2 were entrained by the flow of sand, including blocks of silt in a 

large liquefaction feature between 13.5 and 16 meters on the log (Figure 15). The liquefaction is 

associated with and may have taken advantage of the reverse faults observed between 14 and 15 

meters, suggesting that the faulting and liquefaction occurred during the same event. 

Alternatively, it is possible that the faults created during an older event were places of weakness 

followed by younger liquefaction features. That the liquefaction is young is also evidenced by 

the degree to which unit H4 is disturbed by the liquefaction. Units H/Q3 and H4 were deformed 

into hummocks and swales above the liquefaction features, and one sand dike was observed 

cutting through units H4 and H4.1 to the former land surface. Since the liquefaction disturbs unit 

H4, the events that caused the shaking had to postdate the youngest charcoal collected from H4, 

or about 2730 cal yr B.P. It is possible that multiple events in the late Holocene caused strong 

ground shaking at the Beef Barley site. Bourgeois and Johnson (2001) documented at least three 

episodes of liquefaction in the late Holocene at the Snohomish River delta, about 26 km N/NW 

of the site. Any of these events could also have shaken the ground hard enough at the Beef 

Barley site to generate the liquefaction features observed in the excavation. 

In this study, liquefaction is unique to the Beef Barley site. Site conditions explain why 

the Beef Barley site liquefied while the other sites did not. Saturated soils and groundwater are 

located near the ground surface at the Beef Barley site. Near surface soils at the Beef Barley site 

14
include fine-grained sediments likely to act as a barrier to upwards flow of liquefied materials. 

None of the other excavation sites contained the unique set of conditions conducive to 

liquefaction that are found at the Beef Barley site. 

STRUCTURE AND PREHISTORIC EARTHQUAKES ON THE SWIFZ 

Offshore marine seismic surveys indicate that several strands of the SWIFZ traverse 

Admiralty Inlet and Whidbey Island before making landfall between the cities of Edmonds and 

Mukilteo (Figure 17). The magnetic survey data presented in Blakely et al. (2004) and in this 

report show that after making landfall, strands of the SWIFZ continue to the southeast and cross 

through southern Snohomish County and northern King County near Maltby and Woodinville. 

Lidar mapping shows that scarps on the ground surface lie on or near several magnetic 

anomalies, suggesting that the shallow subsurface anomalies are strands of an active fault zone. 

Excavations across two scarps confirm that the Crystal Lake and Little Bear Creek lineaments 

are active strands of the SWIFZ. 

We envision one model of the SWIFZ as a set of anastomosing faults, two of which are 

the Crystal Lake and the Little Bear Creek lineaments. These strands have lidar scarps and 

paleoseismological evidence that suggests earthquakes have ruptured the ground surface at least 

once in the Holocene. Other strands may be active as well, including Lineament “X” (Figure 9), 

the “GA anomaly” (Figure 4), and possible strands associated with scarp-like features identified 

on historical aerial photographs. However, poor quality of existing lidar coverage in King 

County and lack of lidar in southern Snohomish County preclude making any statements as to 

surface ruptures associated with these features. Based on the aeromagnetic mapping in southern 

Snohomish and King Counties, the SWIFZ could exceed 12 km in width, but at present, active 

strands are only known between Woodinville and Maltby (Figures 6 and 17). 

Prehistoric earthquake activity on at least two strands of the SWIFZ is based on 

excavations across scarps identified on lidar maps. These excavations, combined with published 

data from Whidbey Island (Kelsey and others, 2004), indicate that at least four earthquakes 

struck the SWIFZ in the last 16,400 years (labeled as events C, D, W, and F on Figure 16). An 

additional earthquake and at least three ground shaking events have occurred along the SWIFZ 

when additional excavation information and events from the Snohomish River delta are 

considered (Bourgeois and Johnson, 2001). The liquefaction events could have sources other 

than the SWIFZ, such as a large Seattle fault event about 1100 years ago and intraslab events 

(Bucknam and others, 1992). However, if we included the liquefaction events and other 

evidence as indicators of possible SWIFZ events, then the number of events in the last 16,400 

years increases to nine (Figure 16). 

FUTURE RESAERCH DIRECTIONS 

We now have strong evidence that two fault strands in the Maltby-Woodinville area, 

presumably part of the SWIFZ, slipped multiple times in the Holocene, yet questions remain 

regarding the overall hazards posed by the SWIFZ. The focus of our present study and that of a 

previous report (Blakely and others, 2004) was controlled partly by available lidar coverage. As 

shown in Figure 6, that lidar survey did not include mainland regions along the Puget Sound, nor 

areas northeast of Maltby. Thus, we do not have a clear view of the length and width of the 

active deformation zone. This uncertainty could be addressed with new, high-quality lidar 

surveys and follow-on trench excavations. The region between Maltby and Snohomish is of 

particular interest because of lineaments noted in airborne photography, many of which 

15
correspond to aeromagnetic anomalies. 

The region between the Little Bear Creek lineament and Lake Washington is also an 

important concern. Numerous northwest-striking aeromagnetic lineaments have been interpreted 

in this region, but corresponding scarps have not been identified. Several explanations are 

possible: (1) The aeromagnetic lineaments may reflect sub-surface faults that have no 

topographic expression, either because slip surfaces did not intersect the ground or because the 

ground surface has been altered by human development or by geologic processes occurring 

subsequent to latest slip. (2) The lidar data may have been of insufficient quality to detect small 

scarps. (3) The aeromagnetic lineaments may be caused by geologic processes other than 

faulting and thus pose no hazard. An experimental trench across a well-defined aeromagnetic 

lineament, one without topographic expression, might determine which explanation is most 

plausible. 

Questions also remain regarding the vertical extent and three-dimensional aspects of 

individual fault strands. High-resolution seismic-reflection profiles conducted across selected 

lineaments might define the configuration of deformed strata and the amounts of vertical offset 

on faults at depth. Additional ground-magnetic surveys, analyzed jointly with existing 

aeromagnetic anomalies, might help determine the vertical extent of identified faults. Ground 

magnetic surveys are impractical throughout much of the study region, however, because of the 

proximity of manmade structures. A helicopter-borne magnetic survey, flown as low as safely 

possible and along closely spaced flight lines, would serve as an excellent alternative. 

ACKNOWLEDGEMENTS 

 The U.S. Geological Survey National Earthquake Hazards Reduuction Program and an 

Inter-Agency Agreement between the U.S. Geological Survey and King County, Washington 

provided funding for this project. 

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Figure 1—(A) Kinematic model of Cascadia forearc, simplified from Wells and others (Wells and others, 1998) and 

Wells and Simpson (2001). Northward migration of the Oregon Coast Range squeezes western Washington against 

North America, producing faults and earthquakes in the Puget Lowland. (B) Generallized map of the Puget 

Lowland and surrounding regions. BB, Bellingham basin; EB, Everett basin; SB, Seattle basin; TB, Tacoma basin; 

DMF, Devils Mountain fault; UPF, Utsalady Point fault; SWIF, southern Whidbey Island fault; SF, Seattle fault; 

TF, Tacoma fault; OF, Olympia fault; V, Vancouver; B, Bellingham; E, Everett; S, Seattle; T, Tacoma; O, Olympia; 

WI, Whidbey Island; KA, Kingston arch; SU, Seattle uplift. Redrawn from Brocher and others (2001). 

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Figure 2—Evidence for the SWIFZ. Color-shaded relief map indicates intensity of the magnetic field observed in a 

high-resolution aeromagnetic survey (Blakely and others, 1999). Dashed lines show faults from published reports: 

black lines from Johnson and others (1996) and blue lines from Kelsey and others (2004). White diamonds show 

approximate location of faults identified using marine seismic-reflection surveys. Yellow triangles indicate relative 

sea-level study sites (Kelsey and others, 2004); northern triangle (Crockett marsh) is up relative to the southern 

triangle (Hancock marsh). CM, Crockett marsh; HM, Hancock marsh; WI, Whidbey Island; SP, Saratoga Passage; 

PS, Possession Sound; QP, Quimper Peninsula; KP, Kitsap Peninsula; NB, Nora Beach, UB – Useless Bay. 

Figure 3 (on following page)—(A) Aeromagnetic anomalies of the study area. Major roads indicated by brown 

lines. Labels A, B, C, D, and E refer to anomalies and lineaments discussed in text. GMS shows location of 

ground-magnetic survey. SA, Standard Oil Company of California Alderwood #1 well; SS, Standard Oil Company 

of California Socal-Schroeder #1 well. (B) Residual magnetic anomalies of study area. Upward-continuation 

distance 50 m. White dots show the location of magnetic contacts determined automatically with the method of 

Blakely and Simpson (1986). Black dashed lines indicate lineaments associated with specific magnetic contacts; 

black dotted lines are lineaments identified with lower confidence. Color interval for both maps determined by an 

equal histogram algorithm. 

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Figure 4—Ground-magnetic survey of the Beef Barley trench site. Thin black lines show location of profiles; profiles 6 and 8 (shown i

with white dashes. Bold blue line is location of Beef Barley trench, and black dashed line is lidar lineament 4 from Blakely and others 

indicate northwest-striking lineaments discussed in text; label GA is lineament referred to in text and identified in Figure 5.
Figure 5—Profiles 6 and 8 of ground-magnetic survey (shown in Figure 4). See Figure 2 for location. Dotted and 

dashed lines show gradient analyzed using Peters’ method (Peters, 1949; Blakely, 1995, p. 238), as discussed in text. 

Depth is relative to ground level. 

Figure 6 (on following page)—Summary diagram showing locations of aeromagnetic anomalies, faults from marine 

seismic reflections data, lidar scarps, and lidar lineaments. SA, Standard Oil Alderwood#1 well; SS; Standard Oil 

Socal-Schroeder #1 well. 

Figure 7—Relative sea-level histories from Crockett and Hancock marshes, Whidbey Island (Kelsey and others, 

2004). The upper plot shows the actual sea level data, composed or radiocarbon ages from samples collected from 

the interface between marine and freshwater deposits beneath each marsh. The lower plot shows how much vertical 

displacement on the SWIFZ is required to offset the curves. 
Figure 8—Historical aerial photographs along the Snohomish River near Devil’s Butte. (A) Uninterpreted aerial 

photograph. (B) Interpreted aerial photograph. These photographs illustrate the types of features observed on the 

aerial photographs. In general, the scarps and lineaments have the same approximate trend as the SWIFZ and the 

aeromagnetic anomalies located nearby. 

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Figure 9—(A) Uninterpreted lidar image of the Crystal Lake and Little Bear Creek lineaments. (B) Interpreted lidar 

image of the Crystal Lake and Little Bear Creek lineaments. Thick green lines are lidar lineaments and thin red 

lines are lidar scarps. The scarp east of Crystal Lake is marked as ‘scarp’. 
Figure 10—Simplified excavation log of the Mountain Beaver trench (east wall). (Inset) Lidar image showing the excavation site. Log 

actual mapped log to aid intercomparison of excavation logs in this report.
Figure 11—Enlarged excavation diagram showing the normal fault in the Mountain Beaver excavation. The photomosaic at the lower 

enlargement of the fault and truncated beds (photomosaic shows an area 2 meters wide). Log and photo are flipped relative to actual mintercomparison of excavation logs in this report.
Figure 12— Simplified excavation log of the Flying Squirrel trench (west wall). (Inset) Lidar image showing the excavation site. 
Figure 13— Simplified excavation log of the French Onion trench (east wall). (Inset) Lidar image showing the excavation site. Log is 

mapped log to aid intercomparison of excavation logs in this report.
Figure 14— Simplified excavation log of the Beef Barley trench (west wall). (Inset) Lidar image showing the excavation site. 
Figure 15— Enlarged excavation diagram showing the area where several small reverse faults, the unconformities, and a large liquefac

in the Beef Barley excavation. 
Figure 16—Summary diagram of paleoseismological evidence along the SWIFZ. The text below the plot lists each 

the approximate age and strand of each event. Event labels are the same as those mentioned in the text.
Figure 17—Map showing newly identified geologic features on the mainland with previously published evidence for 

the SWIFZ west of the mainland shore. Color-shaded relief map shows intensity of the magnetic field. Bold dashed 

lines west of the mainland shore are faults reported by Johnson and others (1996) and Kelsey and others (2004). 

White diamonds show locations of faulted Quaternary strata identified in marine seismic-reflection data (Johnson 

and others, 1996). Yellow triangles are sites of relative differences in Holocene sea-level changes (Kelsey and 

others, 2004). Red star is location of liquefaction evidence for multiple Holocene ground-shaking (Bourgeois and 

Johnson, 2001). Various dotted and dashed lines between Everett and Seattle are aeromagnetic, lidar, and airphoto 

lineaments. See Figures 2 and 6 for description of lines and labels. 

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