## Geology

###### *S.D. Cannon, S.P. Reidel,^(^*[^1]*^)^ A.C. Rohay* {#s.d.-cannon-s.p.-reidel-a.c.-rohay .unnumbered}

The Hanford Site contains all the main geologic characteristics of the
Columbia Basin (DOE 1988). The Columbia Basin is the area bounded by the
Cascade Range to the west, the Rocky Mountains to the northeast, and the
Blue Mountains to the southeast (Figure 4.3-1). Four major geologic
processes occurring over millions of years formed the soil, rocks, and
geologic features (ridges and valleys) in the Columbia Basin and
therefore the Hanford Site. The area was flooded with numerous basaltic
lava flows between 17 and 6 million years ago, as tectonic forces folded
the basalt. In this landscape, the ancestral Columbia River flowed
across the area leaving behind layers of sediment called the Ringold
Formation. Between 1-2 million and 12,000 years ago the area was
inundated by a series of Ice-Age floods (including the Missoula floods),
which deposited more sediment in what is referred to informally as the
Hanford formation.

**Lava Flows.** Lava flows erupted over a period of time from 17 to 6
million years ago. Under the Hanford Site, basaltic lava deposits
(Columbia River Basalt Group) are over 4 km (13,000 ft) thick (Reidel
and Hooper 1989), spreading over portions of Idaho, Oregon, and
Washington. The Columbia Basin encloses the Columbia River Basalt Group.
A depression in the south-central part of the Columbia Basin is referred
to as the Pasco Basin. The Pasco Basin is bounded by the Saddle
Mountains to the north, Naneum Ridge to the west, Rattlesnake Hills to
the south, and the Palouse Slope to the east---generally the area north
and east of where the Snake River flows into the Columbia River.
Geographically, the ridges surrounding the Hanford Site and vicinity
define the Pasco Basin, which contains Ringold Formation sediment from
the ancestral Columbia River and sediment deposited by the Ice-Age
floods.

**Crustal Folding.** During and after the eruption of the lava flows,
the Earth's tectonic forces buckled and folded the basalt in the western
Columbia Basin into generally east-west trending, long, narrow ridges
(anticlines) and intervening valleys (synclines). Collectively, this is
identified as the Yakima Fold Belt.

**Ancestral Columbia River Deposits.** The Columbia River within the
Pasco Basin changed its course over the past 15 million years,
depositing gravel, sand, silt, and clay (Tallman et al. 1981, Fecht *et
al*. 1987, DOE 1988, Reidel et al. 1994, Lindsey 1996). Uplifting basalt
ridges diverted the course of the Columbia River from a southerly
direction (toward Goldendale) to the east (toward Wallula Gap) and left
behind the sediment called the Ringold Formation (Fecht et al. 1987).
Later regional uplift associated with the Cascade Mountains caused the
river to cut through its own earlier deposits (the Ringold Formation)
exposing the White Bluffs (Figure 4.3-2). The Columbia River continues
to erode the White Bluffs. Groundwater seepage from irrigation along the
bluffs also makes them unstable. Consequently, the White Bluffs are
landsliding and sloughing into the Columbia River along the shoreline
(Hays and Schuster 1987, USBR 2002).

**Ice-Age Floods.** The Ice-Age floods began as early as 2.5 million
years ago (Bjornstad et al. 2001) with the most recent occurring 18,000
to 12,000 years ago. During the freezes and thaws that occurred in the
Ice-Age, an ice dam across the Clark Fork River along the Montana-Idaho
border formed and failed many times, each time releasing a wall of water
that surged southwest through the Columbia Basin, inundating the area
that is now the Hanford Site. As the water moved across eastern
Washington, it eroded the basalt, forming channels of barren rocky land
referred to as the Channeled Scabland. At other localities the water
deposited bars of gravel and sand. The waste management facilities in
the 200 Areas

![](media/image1.png){width="6.49375in" height="8.403472222222222in"}

> **Figure 4.3-1**. Geologic Elements of the Pasco Basin Portion of the
> Columbia Basin, Washington

![](media/image2.jpeg){width="6.331944444444445in" height="2.9375in"}

> **Figure 4.3-2**. White Bluffs at the Hanford Site, Washington,
> Display River and Lake Deposits Left from the Ancestral Columbia River

of the Hanford Site are located on one prominent flood bar of sand and
gravel, the Cold Creek bar (Bretz et al. 1956, DOE 1988). Where the
waters back-flooded up tributary valleys, they left behind deposits of
sand and silt known as the Touchet Beds. Touchet Bed silt deposits lie
beneath the southern/central plateau of the Hanford Site.

During the largest Ice-Age floods, most of the southern Pasco Basin was
under water (Figure 4.3-3). Ice-Age floods became hydraulically dammed
behind Wallula Gap, forming Lake Lewis, which probably lasted a week or
less. The largest and most frequent floods came from glacial Lake
Missoula in northwestern Montana. Other floods probably escaped
down-valley from glacial lakes Clark and Columbia along the northern
margin of the Columbia Plateau (Waitt 1980, Baker and Bunker 1985) or
down the Snake River from glacial Lake Bonneville (Malde 1968, O'Connor
1993) or from subglacial outbursts (Shaw et al. 1999).

Since the end of the Ice-Age floods, winds have reworked the deposits of
sand and silt, shifting them into dune sands in the lower elevations and
loess (windblown fine sand and silt) around the margins of the Pasco
Basin. Anchoring vegetation has stabilized many sand dunes. Where human
activity has disturbed this vegetation, dunes have been reactivated.
Most recently, many dunes were reactivated by the removal of vegetation
resulting from the June-July 2000 fire at the Hanford Site.

The Hanford Site today is a composite of what the lava flows, earth's
tectonic forces, river changes, and Ice-Age floods of long ago left
behind and the winds since then have reshaped. The resulting landmass
elements that affect and are affected by activities at the Hanford Site
are its physical and structural characteristics, and the strata and
structure of its rocks.

### Physical and Structural Characteristics

The physical characteristics of the Hanford Site include the mountains,
valleys, and riverbeds. These are the landmarks that usually have
recognizable names on topographical maps. The structural geology of the
Hanford Site demonstrates pressure effects on the landmass, which have
produced many of the landmarks (Figure 4.3-4). The Earth's internal
forces continually push and press the landmass into a

![](media/image3.png){width="6.499305555555556in"
height="2.817361111111111in"}

> **Figure 4.3-3**. Conceptualization of an Ice-Age Flood in the
> Vicinity of the Hanford Site, Washington, which Occurred
> Intermittently between 18,000-13,000 years ago

variety of shapes: ridges (anticlines) and valleys (synclines) and
associated faults. Unlike the physical characteristics which define the
landscape, structural geology defines the fabric of a landmass.

The fabric of the Hanford Site is defined by the Palouse Slope and
Yakima Fold Belt (DOE 1988). The underlying basalt of the Palouse Slope
dips toward the central Columbia Basin and exhibits some structural
deformation. A wedge of Columbia River basalt underlies the Palouse
Slope thinning gradually toward the east and north and lapping onto the
adjacent highlands. The Yakima Fold Belt is the name collectively given
to all the ridges formed from the folded basaltic lava flows that are
found within the central and western parts of the Columbia Basin (DOE
1988).

The main ridges and valleys on the Hanford Site area are as follows:

-   Saddle Mountains form the ridge that is the northern boundary of the
    Pasco Basin and Hanford Site. The Saddle Mountains are an east-west
    ridge about 110 km (68 mi) long and 5 km (3 mi) wide.

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-   Wahluke Slope is on the north side of the Columbia River. The
    Wahluke Slope is a portion of the synclinal valley of basalt between
    the Saddle Mountains and Umtanum Ridge.

-   Umtanum Ridge makes up the southern boundary of the Wahluke
    Syncline. Gable Butte and Gable Mountain are the parts of this ridge
    that lie between the 100 and 200 Areas. Gable Butte and Gable
    Mountain are folded layers of rock that are the high points along
    the Umtanum anticlinal ridge, which is buried in places by the
    Ringold Formation and Hanford formation. The valley north of the
    Umtanum Ridge is the Wahluke Syncline and the valley south is Cold
    Creek.

-   Rattlesnake Hills, southwest of the Hanford Site, are also the
    result of folded rock layers. Rattlesnake Mountain, the highest of
    the Rattlesnake Hills, reaches an elevation of 1110 m (3660 ft)
    above mean sea level, the highest elevation in the area.

![](media/image4.jpeg){width="6.0in" height="7.540277777777778in"}

> **Figure 4-3.4**. Physical and Structural Geology of the Hanford Site,
> Washington (Hartshorn *et al*. 2002)

-   Cold Creek is a valley of rock and sediment that lies between
    Umtanum Ridge and Yakima Ridge. Yakima Ridge is one of the
    anticlinal ridges of the Yakima Fold Belt. The Cold Creek bar, a
    deposit of Ice-Age flood sediment, lies in this area.

### Strata and Structure of Sediment and Rock

The strata and structure of the sediment and rocks (Figure 4.3-5) are
important characteristics of the Hanford Site (DOE 1988).

#### Columbia River Basalt Group

The bedrock of the Hanford Site is basalt. Beneath the Hanford Site lay
a minimum of 100 basalt flows with a maximum combined thickness of more
than 4 km (13,000 ft) (DOE 1988), all part of the Columbia River Basalt
Group.

To organize the many basalt deposits into a consistent nomenclature,
geologists have named and grouped them based on their physical and
chemical properties. The basalt deposit closest to the surface at the
Hanford Site, and therefore most often referred to, is Saddle Mountains
Basalt (Figure 4.3-5). Saddle Mountains Basalt consists of ten distinct
basaltic lava deposits (members). The most recent basalt flow underlying
most of the Hanford Site is the Elephant Mountain Member of the Saddle
Mountains Basalt. A younger basalt flow, the Ice Harbor Member, is found
in the southern portion of the site near the 300 Area (DOE 1988).

In addition to basalt, the Hanford Site has sedimentary formations. Some
of the sediment at the Hanford Site is found between the basaltic lavas
and is called the Ellensburg Formation. The majority of the sediment is
above the basalt with the Ringold Formation overlain by the Cold Creek
unit, and topped with the Hanford formation (Figure 4.3-5).
Understanding the formations, along with clastic dikes and the soil of
the Hanford Site, contributes to our understanding of how, for example,
contaminants might travel in the 100, 200, and 300 Areas.

#### Ellensburg Formation

The Ellensburg Formation is the sediment found interbedded with the
Columbia River Basalt Group. The Ellensburg Formation formed as early as
17 million years ago, although the youngest portion on the Hanford Site
may have formed as recently as 8 million years ago (DOE 1988). The
Ellensburg Formation was created when volcanic rock and sediment from
uplands surrounding the Columbia Plateau interfingered with the basalt
of the Columbia River Basalt Group (Swanson et al. 1979a, b). The
thickest accumulations of the Ellensburg Formation lie along the western
margin of the Columbia Basin. While deposition along the western margin
was primarily from volcanic debris flows and related stream and sheet
floods, no volcanic debris flows have been identified at the Hanford
Site (Reidel *et al*. 1994). Volcanic ash known as tuff is the dominant
material in the Hanford Site portion of the Ellensburg Formation. The
Ellensburg Formation is commonly exposed between basalt flows along the
ridges of the Yakima Fold Belt.

#### Hanford Formation, Cold Creek Unit, and Ringold Formation

Sediments overlying basalt in the Pasco Basin and Hanford Site include
the Ringold Formation, Cold Creek unit, and the Hanford formation. These
formations are primarily exposed in the lower elevation areas around the
Hanford Site, including White Bluffs.

![](media/image5.png){width="6.5881944444444445in"
height="6.998611111111111in"}

**Figure 4.3-5**. Strata of Rocks and Sediment at the Hanford Site,
Washington

**Ringold Formation.** The Ringold Formation formed between 8.5 and
about 3.0 million years ago and consists of gravel, sand, silt, and clay
deposited across portions of the Columbia Basin by the ancestral
Columbia River (Tallman et al. 1981, DOE 1988, Lindsey 1996). For
detailed descriptions of the Ringold Formation see DOE (1988) and
Lindsey (1995, 1996). Although exposures of the Ringold Formation are
limited to White Bluffs within the central Pasco Basin and to Smyrna and
Taunton Benches along the Saddle Mountains north of the Pasco Basin,
extensive data on the Ringold Formation are available from boreholes.
The Ringold Formation at the Hanford Site is up to 185 m (600 ft) thick.
Erosion removed the upper 300 feet of the Ringold Formation over most of
the Hanford Site. Along White Bluffs, the Ringold Formation is up to
285 m (900 ft) thick (DOE 1988).

**Cold Creek unit**. The Cold Creek unit (DOE 2002b) includes all
material underlying the Hanford formation, overlying the Ringold
Formation in the vicinity of 200 West, and may extend over most of the
central Pasco Basin. The Cold Creek unit distinguishes itself from the
Hanford and Ringold formations because it was formed when the Ringold
Formation was eroding and relatively little was being deposited at the
Hanford Site. This subunit is found locally in the subsurface of the
Cold Creek synclinal valley. Distribution of the Cold Creek unit depends
in part on erosion and weathering of the underlying Ringold Formation
and post-depositional erosion by the Ice-Age floods (Slate 1996). The
thickness of the Cold Creek deposit ranges from 0-20 m (0-66 ft).
Locally, the Cold Creek unit contains very hard rock that formed during
soil development as precipitation evaporated and left behind minerals
forming what geologists call caliche or hardpan. This layer can
influence contaminant migration by slowing its rate of downward movement
and potentially diverting contaminants laterally (Slate 1996). The thin,
fine-grained layers in the Hanford formation also cause lateral
migration (Serne *et al*. 2002).

**Hanford formation.** The Hanford formation is the informal name for
the strata that lie on top of Cold Creek unit above the Ringold
Formation. The Ice-Age floods inundated the Hanford Site a number of
times beginning as early as 1-2 million years ago (Bjornstad *et al*.
2001). The last major flood sequence occurred about 12,000 years ago.
When the Ice-Age floodwaters entered the Pasco Basin, they quickly
became impounded behind Wallula Gap, which was too restrictive for the
volume of water involved. Floodwaters formed a temporary lake with
shorelines up to 381 m (1250 ft) in elevation. The lake only lasted a
few days (O'Connor and Baker 1992). After the floodwater receded, the
Hanford formation deposits were left in the low-lying areas over most of
the Hanford Site.

On the Hanford Site the Hanford formation is thickest in the vicinity of
the 200 Areas where it is up to 100 m (300 ft) thick (DOE 2002b).
Gravel, sand, and silt (Touchet Beds) dominate the Hanford formation
(Reidel et al. 1992). The different sediment types of the Hanford
formation commonly interfinger laterally. The relative proportion of
each sediment type at any given location is related to its distance from
main high-energy flows at the time of deposition (DOE 2002b). Generally,
gravel was deposited in the center of the Pasco Basin, while finer
grained sand and silt were deposited along the margins of the basin.

#### Clastic Dikes

Clastic dikes are fissures filled with sand, silt, clay, and coarser
debris. They are commonly associated with, but not restricted to,
Ice-Age flood deposits in the Columbia Basin. Many dikes occur as
sharp-walled, near-vertical tabular bodies filled with multiple layers
of unconsolidated sediment. Thin clay/silt linings separate the margins
of dikes and internal layers (Fecht *et al*. 1999). Dikes vary in width
from less than 1 mm (0.039 in.) to greater than 2 m (6.5 ft). Vertical
extents range from less than 1 m (3 ft) to greater than 50 m (164 ft)
with a large number greater than 20 m (65 ft) (Fecht et al. 1999).

Clastic dikes are complex natural structures that may facilitate or
retard the migration of contaminants. Where large leaks have occurred,
clastic dikes may also act as barriers, limiting the extensive lateral
spreading of contaminants caused by horizontal layers of Hanford
sediment (Murray *et al*. 2002, 2003; Ward *et al*. 2002a, b).

#### 100 Areas Strata and Structure

The 100 Areas are located along the Columbia River in the northern
portion of the Hanford Site (Figure 4.0-1). With the exception of the
100-B/C Area, the 100 Areas lie on the north limb of the Wahluke
synclinal valley. The 100-B/C Area lies over the axis of the synclinal
valley. The top of the basalt in the 100 Areas ranges in elevation from
46 m (150 ft) near the 100-H Area to -64 m (-210 ft) below sea level
near the 100-B/C Area. The Ringold Formation and Hanford formation occur
throughout this area. The Cold Creek unit deposits have not been
identified in the 100 Areas.

The Ringold Formation shows a marked west-to-east variation in the 100
Areas (Lindsey 1992). The main channel of the ancestral Columbia River
flowed along Umtanum Ridge and through the 100-B/C and 100-K Areas
before turning south to flow along Gable Mountain and/or through the
Gable Mountain-Gable Butte gap. This main channel deposited
coarse-grained sand and gravel of the Ringold Formation. Farther to the
north and east, however, the Ringold sediment is gradually dominated by
the fine-grained silt deposits and associated ancient soil with the
100-H Area showing almost no gravel.

#### 200 Areas Strata and Structure

The geology in the 200 West Area is notably different from that in the
200 East Area even though they are separated by a distance of only 6 km
(4 mi) (DOE 1988) (Figure 4.0-1). The 200 West Area has sections
containing all three formations including most of the Ringold Formation
as well as the Cold Creek unit and the Hanford formation (DOE 1988).

In the 200 East Area, some Ringold Formation is present in the southern
portion but has been completely eroded in the northern part. On the
north side of the 200 East Area, the Hanford formation rests directly on
the basalt, and no Ringold sediment is present. Erosion by the ancestral
Columbia River and Ice-Age flooding are believed to have removed the
Ringold Formation from this area. Material of questionable origin
overlies basalt within the B-BX-BY Waste Management Area, the location
of the B, BX, and BY tank farms where radioactive wastes are stored in
underground tanks (Wood et al. 2000). This material may be equivalent or
partially equivalent to the Cold Creek unit or it may represent the
earliest ice-age flood deposits overlain by a locally thick sequence of
fine-grained non-flood deposits. This unit is referred to informally as
Hanford-Cold Creek deposits.

#### 300 Area Strata and Structure

The 300 Area is located in the southeastern portion of the Hanford Site
(Figure 4.0-1). The 300 Area lies next to the Columbia River and sits on
the Hanford formation, which in turn overlies the Ringold Formation and
Columbia River Basalt Group. The Columbia River Basalt Group and Ringold
Formation beneath the 300 Area are folded into the Cold Creek synclinal
valley that lies near the intersection of the Yakima Fold Belt and the
un-deformed Palouse Slope (Figure 4.3-3) (DOE 1988). The uppermost
basalt flows belong to the Elephant Mountain Member over most of the
Hanford Site, but near the 300 Area younger flows belonging to the Ice
Harbor Member of the Saddle Mountains Basalt are present, causing the
overlying sediment layers to be relatively thin (Schalla et al. 1988).
Both Ringold Formation and Hanford formation sediment is found in the
300 Area.

### Surface Soil

Hajek (1966)^(^[^2]^)^ describes 15 different surface soil types on the
Hanford Site, varying from sand to silty and sandy loam (Figure 4.3-6).
Classifications including land use are also given in Hajek (1966).

Soil types found on the Hanford Site include the following:

**Ritzville Silt Loam**. Ritzville silt loam, a dark-colored silt loam
soil, is found midway up the slopes of the Rattlesnake Hills. It was
formed under bunch grass from silty wind-laid deposits mixed with small
amounts of volcanic ash. Characteristically greater than 150 cm (60 in.)
deep, Ritzville silt loam may be separated by bedrock that occurs
between 75 and 150 cm (30 and 60 in.).

**Rupert Sand**. Rupert sand, brown-to grayish-brown coarse sand grading
to dark grayish-brown at a depth of 90 cm (35 in.), is one of the most
extensive soil types on the Hanford Site. Rupert sand developed under
grass, sagebrush, and hopsage in coarse sandy alluvial deposits that
were mantled by wind-blown sand and formed hummocky terraces and
dune-like ridges.

**Hezel Sand**. Hezel sand, similar to Rupert sands, is laminated
grayish-brown strongly calcareous silt loam subsoil usually encountered
within 100 cm (39 in.) of the surface. When found as surface soil it is
very dark brown. Hezel sand was formed in wind-blown sands that mantled
lake-laid sediment.

**Koehler Sand**. Koehler sand is similar to other sandy soil found on
the Hanford Site, differing in that it mantles a lime-silica cemented
hardpan layer. It was developed in a wind-blown sand mantle and exhibits
a very dark grayish-brown surface layer and is somewhat darker than
Rupert sand. Its calcareous subsoil is usually dark grayish-brown at
about 45 cm (18 in.).

**Burbank Loamy Sand**. Burbank loamy sand is a dark-colored,
coarse-textured soil underlain by gravel. Its surface soil is usually
about 40 cm (16 in.) thick but may be as much as 75 cm (30 in.) thick.
The gravel content of its subsoil ranges from 20 percent to 80 percent.

**Ephrata Sandy Loam**. Ephrata sandy loam is found on level topography
on the Hanford Site. Its surface is darkly colored and its subsoil is
dark grayish-brown medium-textured soil underlain by gravelly material
that may continue for many feet.

**Lickskillet Silt Loam**. Lickskillet silt loam occupies the ridge
slopes of Rattlesnake Hills and slopes greater than 765 m (2509 ft)
elevation. It is similar to Kiona silt loam except the surface soil is
darker. Lickskillet silt loam is shallow over basalt bedrock and
exhibits numerous basalt fragments throughout the profile.

**Ephrata Stony Loam.** Ephrata stony loam is similar to Ephrata sandy
loam. It differs in that many large hummocky ridges are made up of
debris released from melting glaciers. Areas of Ephrata stony loam
located between hummocks contain many boulders several feet in diameter.

**Pasco Silt Loam**. Pasco silt loam is poorly drained very dark
grayish-brown soil formed in recent alluvial material. Its subsoil is
variable, consisting of stratified layers. Only small areas of Pasco
silt loam are found on the Hanford Site, located in low areas adjacent
to the Columbia River.

![](media/image6.png){width="6.190277777777778in"
height="8.254861111111111in"}

**Figure 4.3-6**. Map of Soil Types at the Hanford Site, Washington
(based on Hajek 1966)

**Kiona Silt Loam**. Kiona silt loam occupies steep slopes and ridges.
Its surface soil is very dark grayish-brown, is about 10 cm (4 in.)
thick, and has dark-brown subsoil containing basalt fragments 30 cm (12
in.) and larger in diameter. Many basalt fragments are found in its
surface layer and basalt rock outcrops are often present. Kiona silt
loam is a shallow stony soil normally occurring in association with
Ritzville and Warden soil.

**Warden Silt Loam**. Warden silt loam is dark grayish-brown soil with a
surface layer usually 23 cm (9 in.) thick. Its silt loam subsoil becomes
strongly calcareous at about 50 cm (20 in.) and becomes lighter colored.
Granitic boulders are found in many areas. Warden silt loam is usually
greater than 150 cm (60 in.) deep.

**Scootney Stony Silt Loam**. Scootney stony silt loam developed along
the north slope of the Rattlesnake Hills and is usually confined to
floors of narrow draws or small fan-shaped areas where draws open onto
plains. It is severely eroded with numerous basaltic boulders and
fragments exposed and surface soil is usually dark grayish-brown grading
to grayish-brown within the subsoil.

**Esquatzel Silt Loam**. Esquatzel silt loam is a deep dark-brown soil
formed in recent alluvium derived from loess and lake sediment. Its
subsoil grades to dark grayish-brown in many areas, but the color and
texture of the subsoil are variable because of the stratified nature of
the alluvial deposits.

**Riverwash**. Riverwash is wet, periodically flooded areas of sand,
gravel, and boulder deposits that make up overflowed islands in the
Columbia River and adjacent land.

**Dunesand**. Dunesand is a miscellaneous land type that consists of
hills or ridges of sand-sized particles drifted and piled up by wind.
They are either actively shifted or so recently fixed or stabilized that
no soil layers have developed.

### Seismicity

The historic record of earthquakes in the Pacific Northwest dates from
about 1840. The early part of this record is based on newspaper reports
of human perception of shaking and structural damage as classified using
the Modified Mercalli Intensity (MMI) scale; the early record is
probably incomplete because the region was sparsely populated. The
historical record appears to be complete since 1905 for MMI V and since
1890 for MMI VI (Rohay 1989). Seismograph networks did not start
providing earthquake locations and magnitudes of earthquakes in the
Pacific Northwest until about 1960. A comprehensive network of seismic
stations that provides accurate locating information for most
earthquakes of magnitude greater than 2.5 was installed in eastern
Washington during 1969. DOE (1988) provides a summary of the seismicity
of the Pacific Northwest, a detailed review of the seismicity in the
Columbia Plateau region and the Hanford Site, and a description of the
seismic networks used to collect the data.

Large earthquakes (magnitude M greater than or equal to 7) in the
Pacific Northwest have occurred near Puget Sound, Washington, and near
the Rocky Mountains in eastern Idaho and western Montana. Two large
earthquakes occurred beneath Vancouver Island. The first occurred during
1918 and had a maximum MMI VII (estimated magnitude M 7.0). The second
earthquake occurred in 1946 and had a maximum MMI VII (over a wider
area) and magnitude M 7.3. The depth of these early, large earthquakes
beneath Vancouver Island is uncertain. Another large earthquake occurred
at a depth of 53 km (33 mi) near Olympia, Washington, in 1949 that had a
maximum MMI VIII and a magnitude M equal 7.1. A smaller (M 6.5), deep
(63 km \[39 mi\]) earthquake occurred during 1965 between Seattle and
Tacoma. These events may all be related to deformation within the
subducting Juan de Fuca Plate at depth beneath the Vancouver
Island/Puget Sound region.

Two large events occurred on the eastern boundary of the Pacific
Northwest, in the Rocky Mountains. These were the 1959 Hebgen Lake
earthquake in western Montana, which had a Richter magnitude of 7.5 and
an MMI X, and the 1983 Borah Peak earthquake in eastern Idaho, which had
a Richter magnitude of 7.3 and an MMI IX.

Closer to the Hanford Site, a significant, large earthquake of uncertain
location occurred in north-central Washington in 1872. This event had an
estimated maximum MMI ranging from VIII to IX and an estimated Richter
magnitude of approximately 7.4. The distribution of intensities suggests
a location within a broad region between Lake Chelan, Washington, and
the British Columbia border. Evidence of landslides near Lake Chelan
suggests a location near there. A recent study indicates that the
maximum MMI for this event was VIII and its magnitude was 6.8, with a
location at the south end of Lake Chelan (Bakun *et al*. 2002).

On February 28, 2001, there was a moderate (M less than 7), deep
earthquake near Olympia in Puget Sound called the Nisqually earthquake.
This earthquake occurred at a depth of 52 km (32 mi) and had a magnitude
of 6.8; reported ground-shaking effects reached MMI VIII. This event is
similar to those in 1949 and 1965 described above. The Nisqually
earthquake was recorded by a network of strong motion accelerographs at
the Hanford Site. Peak horizontal accelerations measured from 0.0016 to
0.0055g (1g is the acceleration of gravity). These levels of ground
shaking are considerably less than design and evaluation values (PNNL
Seismic Monitoring Team 2001).

The seismicity of the Columbia Plateau, as determined by the rate of
earthquakes per area and the historical magnitude of these events, is
relatively low compared with other regions of the Pacific Northwest, the
Puget Sound, and western Montana/eastern Idaho. The largest known
earthquake in the Columbia Plateau occurred in 1936 near
Milton-Freewater, Oregon. This earthquake had a Richter magnitude of
5.75 and a maximum MMI of VII, and it was followed by a number of
aftershocks indicating a northeast-trending fault plane. Bakun *et al*.
(2002) concluded that the magnitude of this earthquake was a lower value
of 5.35.

Other earthquakes with Richter magnitudes greater than or equal to 5
and/or MMIs of VI occurred along the boundaries of the Columbia Plateau
in a cluster near Lake Chelan in 1872 and extended into the northern
Cascade Range, northern Idaho and Washington, and along the boundary
between the western Columbia Plateau and the Cascade Range. Three MMI VI
earthquakes have occurred within the Columbia Plateau, including one in
the Milton-Freewater, Oregon region in 1921; one near Yakima,
Washington, in 1892; and one near Umatilla, Oregon, in 1893 (Figure
4.3-7). In the central portion of the Columbia Plateau, the largest
earthquakes near the Hanford Site are two that occurred during 1918 and
1973. These two events were magnitude 4.4 and intensity V and were
located north of the Hanford Site near Othello.

Earthquakes often occur in spatial and temporal clusters in the central
Columbia Plateau and are termed "earthquake swarms." The region north
and east of the Hanford Site is a region of concentrated earthquake
swarm activity, but earthquake swarms have also occurred in several
locations within the Hanford Site. The frequency of earthquakes in a
swarm tends to gradually increase and decay with no one outstanding
large event within the sequence. Roughly 90 percent of the earthquakes
in swarms have Richter magnitudes of 2 or less. These earthquake swarms
are generally shallow; 75 percent of the events are at depths less than
4 km (2.5 mi). However, several recent swarm sequences have occurred at
greater depths (PNNL Seismic Monitoring Team 2003). Each earthquake
swarm typically lasts several weeks to

![](media/image7.png){width="6.165277777777778in"
height="7.965277777777778in"}

**Figure 4.3-7**. Historical Earthquake Activity of the Columbia Basin,
Washington and Surrounding Areas. All earthquakes between 1890 and 1970
with a Modified Mercalli Intensity V or larger and/or a magnitude 4 or
larger are shown (Rohay 1989).

months, consists of several to 100 or more earthquakes, and is clustered
in an area 5 to 10 km (3 to 6 mi) in lateral dimension. Often, the
longest dimension of the swarm area is in an east-west direction.
However, detailed locations of swarm earthquakes indicate that the
events occur on fault planes of variable orientation and not on a single
throughgoing fault plane.

Earthquakes in the central Columbia Plateau also occur to depths of
about 30 km (18.6 mi). These deeper earthquakes are less clustered and
occur more often as isolated events. Based on seismic refraction surveys
in the region, the shallow earthquake swarms occur in the Columbia River
basalts and the deeper earthquakes occur in crustal layers below the
basalts.

The spatial pattern of seismicity in the central Columbia Plateau
suggests an association of the shallow swarm activity with the east-west
oriented Saddle Mountain anticline. However, this association is
complex, and the earthquakes do not delineate a throughgoing fault plane
that would be consistent with the faulting observed on this structure.

Earthquake focal mechanisms in the central Columbia Plateau generally
indicate reverse faulting on east-west planes, consistent with a
north-south-directed maximum compressive stress and with the formation
of the east-west oriented anticlinal folds of the Yakima Fold Belt
(Rohay 1987; 2003). However, earthquake focal mechanisms indicate
faulting on a variety of fault plane orientations.

Earthquake focal mechanisms along the western margin of the Columbia
Plateau also indicate north-south compression, but here the minimum
compressive stress is oriented east to west, resulting in strike-slip
faulting (Rohay 1987). Geologic studies indicate an increased component
of strike-slip faulting in the western portion of the Yakima Fold Belt.
Earthquake focal mechanisms in the Milton-Freewater region to the
southeast indicate a different stress field, one with maximum
compression directed east-west instead of north-south.

Estimates for earthquake potential of structures and zones in the
central Columbia Plateau have been developed during the licensing of
nuclear power plants at the Hanford Site. In reviewing the operating
license application for the Washington Public Power Supply System
Washington Nuclear Plant (WNP)-2 (now Energy Northwest Columbia
Generating Station), the U.S. Nuclear Regulatory Commission (NRC)
concluded that four earthquake sources should be considered for seismic
design: the Rattlesnake-Wallula alignment, Gable Mountain, a floating
earthquake in the tectonic province, and a swarm area (NRC 1982).

For the Rattlesnake-Wallula alignment, which passes along the southwest
boundary of the Hanford Site, the NRC estimated a maximum Richter
magnitude of 6.5; for Gable Mountain, an east-west structure that passes
through the northern portion of the Hanford Site, a maximum Richter
magnitude of 5.0 was estimated. These estimates were based on the
inferred sense of slip, the fault length, and/or the fault area. The
floating earthquake for the tectonic province was developed from the
largest event located in the Columbia Plateau, the Richter magnitude
5.75 Milton-Freewater earthquake. The maximum swarm earthquake for the
Columbia Generating Station seismic design was a Richter magnitude 4.0
event based on the maximum swarm earthquake in 1973 (Figure 4.3-8). (The
NRC concluded that the actual magnitude of this event was smaller than
previously estimated.)

Probabilistic seismic hazard analyses have been used to determine the
seismic ground motions expected from multiple earthquake sources, and
these are used to design or evaluate facilities on the Hanford Site. The
most recent Hanford Site-specific hazard analysis (Geomatrix 1994, 1996)
estimated that 0.10g horizontal acceleration would be experienced on
average every 500 years (or with a 10 percent

![](media/image8.png){width="6.165277777777778in"
height="7.645138888888889in"}

**Figure 4.3-8**. Earthquake Activity of the Columbia Basin, Washington,
and Surrounding Areas as Measured by Seismographs. All earthquakes
between 1970 and 2005 with Richter magnitudes of 3 or larger are shown
(ANSS 2005).

chance every 50 years). This study also estimated that 0.2 g would be
experienced on average every 2500 years (or with a 2 percent chance in
50 years). These estimates are in approximate agreement with the results
of national seismic hazard maps produced by the U.S. Geological Survey
(USGS 1996, 2002a). A site-specific ground response model for the Waste
Treatment Plant being constructed at the Hanford Site (Rohay and Reidel
2005) increased ground motions for the design basis of this facility by
up to 40 percent to include greater conservatism.

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[^1]: (?) Technical contact person

[^2]: (?) The soil classifications given in Hajek (1966) have not been
    updated to reflect current reinterpretations of soil
    classifications. Until the surface soil on the Hanford Site is
    resurveyed, the descriptions presented in Hajek (1966) will continue
    to be used.