IDAHO EARTHQUAKES
Idaho Earthquakes and Environmental Hazards
Matthew Chapin
Geology 344
University of Idaho
(Historic EQs M≥4.5. Taken from,
http://www.idahogeology.org/services/GeologicHazards/Earthquakes/historiceqs.html).
Abstract
Each year nearly 300 earthquakes occur in Idaho. Certainly, earthquakes are more than
common and precautions need to be taken into account. This research looks at Idaho’s tectonic
environment, Basin and Range type earthquakes, volcanic related earthquakes, along with
Idaho’s earthquake history. Historical earthquakes include the 1959 Hebgen lake earthquake,
and the 1983 Borah Peak earthquake, as well as specific details about the 1983 earthquake.
Additionally, a brief overview of the future outlook in Idaho is discussed.
IDAHO EARTHQUAKES
I. Introduction & Tectonic Environment
Earthquakes (EQs) are amongst nature’s most feared and damaging geologic hazards.
EQs frequently remind society that nature can strike without warning and be the cause of many
deaths. Accordingly, it is essential for society to become aware of these hazards and to take
action to protect civilization. Undoubtedly, Idaho’s greatest natural hazard is from EQs. The
majority of Idaho’s EQs are associated with Basin and Range extension, as well as volcanism in
relation to the Yellowstone Hot Spot (“Idaho State University”).
Basin and Range EQs
Basin and Range provinces are a byproduct of crustal extension, which often consist of a
series of parallel mountain ranges and valleys formed by normal faulting. The Basin and Range
province extends from California to Colorado (West to East), and from Mexico to central Idaho
(South to North). Idaho outlines the eastern boundary of the western U.S. Basin and Range
province; thus, the majority of faults are normal faults. Normal faults develop from crust
undergoing extension (1 is vertical), where the footwall is the up-thrust block and the hanging
wall is the down-drop block. EQs occur in this region of Idaho because a branch of the
Intermountain Seismic Belt (ISB) cuts through central Idaho. This branch is known as the
Centennial Tectonic Belt (CTB). The CTB forms a doglegged parabola in central Idaho that is
connected to the ISB. The ISB is further connected to the Wasatch Mountains (Northern UT) in
the South. Today, the Basin and Range province is a remnant of mantle upwelling, where many
of the faults are active, and historically known to produce large EQs.
Volcanic related EQs
Volcanic related EQs in Idaho are associated with adjustments of the Yellowstone Hot
Spot (YHS) magma chamber; occurring before, during, as well as after eruptions (“Idaho State
University”). Accommodations of the magma chamber presses upward on the earth’s crust,
creating extensional stresses associated with normal faulting (Fig. 1-1). Alongside of magma
chamber adjustments, countless EQs originate from regional tectonic extension. Each year
nearly 3000 EQs take place in Yellowstone (Lowenstern et. al., 2005). The majority of which are
too small to be noticed; however, one of the largest EQs to occur in the lower 48 is volcanic
related, and associated with the YHS. This earthquake is known as the Hebgen Lake earthquake,
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IDAHO EARTHQUAKES
which took place slightly north of the Idaho border in the vicinity of Yellowstone National Park,
Montana. The EQ also caused significant damage in parts of Idaho. During the EQ, a landslide
was triggered which blocked flow of the Madison River creating Quake Lake (Fig 1.-2). The EQ
also caused several dozen fatalities and millions of dollars in damage.
II. Earthquake History
Of the many earthquake prone localities around the world, Idaho is one of the least
expected regions because of its intraplate setting; however, Idaho is ranked the fifth highest for
earthquake hazards in the lower 48 (“Idaho Geological Survey”). In spite of this, Idaho has had a
relatively interesting earthquake history including several large and damaging EQs. The first
documented damaging EQ was in southeastern Idaho on November 9, 1884, resulting in
significant damage to houses (“USGS”). The epicenter of this earthquake was near Paris, Idaho,
and knocked down several chimneys. This earthquake was felt as far away as Salt Lake City, and
minor damage was extended just across the Idaho-Utah border (“Idaho Geological Survey”).
The majority of Idaho’s EQ history consists of small repeat events such as the prior EQ;
however, several larger EQs have occurred as well.
Hebgen Lake Earthquake
The M7.1 Hebgen Lake, Montana EQ on August 17, 1959, also caused significant damage
in Idaho. The Big Springs and Island Park region near Henry’s Lake Idaho was affected with
considerable damage to buildings and modification of springs (“USGS”). Big springs flow rate
increased nearly fifteen percent, and once again many chimneys fell. The earthquake also
produced a landslide that created a lake later to be named Quake Lake, which was formed
when an enormous landslide blocked flow of the Madison River. The majority of the damage
from the earthquake was in Montana near Hebgen Lake where property was significantly
damaged (“USGS”). Furthermore, the EQ took 28 lives, as well as causing $11 million in damage
(“USGS”).
Borah Peak Earthquake
The most recognized EQ in Idaho occurred on the morning of October 28, 1983 near
Challis, Idaho. Several eye witnesses saw a 2-3 meter fault scarp (Fig. 1-3) produced before their
eyes within seconds in the Lost River Range. The EQ was complimented with extensive surface
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IDAHO EARTHQUAKES
rupture along the Thousand Springs segment of the Lost River fault. The resulting scarp was a
byproduct from a 𝑀𝑠 7.3 (𝑀𝑏 6.9) in association with Borah Peak (Yeats et al., 1997). The focal
depth of the earthquake was very shallow, only about 16km (“Idaho State University”). This
depth is at or near the base of the seismogenic zone, and typical of normal faults. The ground
rupture produced along Mt. Borah’s western flank is approximately 35km long (22miles), and
produced a net throw that was nearly nine feet (“Idaho State University”). Focal mechanism
solutions for the main earthquake and aftershocks essentially exhibit normal faulting with an
oblique component of left-lateral strike slip (Fig. 1-4) (Yeats et al., 1997). Geomorphic indicators
of left-lateral slip are seen in physical offset of Double Springs Pass Rd. along with rightstepping en echelon fractures. The segment of the Lost River Fault that ruptured during this
earthquake has a slip rate of about 0.3mm/yr. and last ruptured in the early to middle Holocene
(Yeats et al., 1997). Additionally, the recurrence interval of the Lost River fault is estimated to
be between 10-15 thousand years.
Borah Peak is part of the western U.S. Basin and Range province and often produces
sizeable EQs from normal faulting (1 is vertical). Six such sizeable EQs have historically
ruptured in the Basin and Range province (Stein et al., 1985). The 1983 earthquake is the
largest ever recorded in Idaho, which caused two deaths from falling masonry in Challis along
with about $12.5 million dollars in damage (“Idaho Geologic Survey”). The majority of property
damage from the 1983 Borah Peak earthquake happened in the areas closest to the epicenter.
Mackay and Challis were most affected where a dozen business-related buildings, in addition to
several dozen private houses, experienced major damage. Additionally, 200 other homes in the
area experienced minor damage (“Idaho Geologic Survey”). Mackay experienced the most
dramatic damage to buildings, with the majority of the property on Main Street being damaged
and nearly 90 percent of residential area’s chimneys were damaged (“USGS”). The EQ was felt
as far away as Oregon, Washington, in addition to parts of Canada, and produced many
geologic hazards (“USGS”). Subsequent features (geologic hazards) produced from the
earthquake include; ground rupture, landslides, damage to roads and buildings, as well as
alterations in the hydrologic system (“Idaho State University”).
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IDAHO EARTHQUAKES
Borah Peak EQ Secondary (post-seismic) features
Sand boils and springs were created and increased/decreased flow from modifications
of groundwater, remaining disrupted for weeks following the earthquake (“Idaho Geologic
Survey”). Additionally, from these hydrologic adjustments, a temporary lake was formed near
the town of Dickey (“Idaho Geologic Survey”). The Thousand Springs Valley and neighboring
areas experienced the most significant modifications of hydrologic systems; however, changes
also occurred as far away as Yellowstone where the cyclic eruptions of various geysers including
“Old Faithful” were affected (“Idaho State University”). The majority of landslides and rock fall
occurred on the steep rocky gradient of the Challis foothills. Some of these boulders crashed
down the slope and damaged houses. Additionally, 120km (75miles) north of the mainshock on
Highway 93, the road was blocked from rock fall on steep roadcuts (“Idaho State University”).
Thousand Springs fault segment geometries
Several hundred meters of the Thousand Springs fault scarp along the Double Springs
Pass Rd. display geometric complexities such as grabbens and rifts. These features exist as a
result of the valley being arched, and subsequently experiencing a higher level of strain than
other adjacent areas. Segmentation of the Lost River fault occurs on all scales. The Thousand
Springs Segment (EQ segment) is broken into many geometric segments that are further
divided by barriers, for instance; salients and bedrock. The Willow Cr. Hills (salient) mark the
boundary between the Warm Springs Valley and Thousand Springs Valley, where surface
rupture ceased. Surface rupture in the Thousand Springs segment displays many overlapping
segments, as a result of the surface rupture breaking through unidurated alluvium.
Furthermore, below the surface the geometry most likely exhibits a single continuous fault.
Geodetic Methods
Geodetic leveling analysis determines variations in height between neighboring
benchmarks (Yeats et al., 1997). Many changes in elevation were attained by the subtraction of
benchmark heights by survey after the 1983 EQ (Stein et al., 1985). The total elevation changes
in the Lost River Range conclude that 3km of dip slip has accumulated at a slip-rate of 0.20.4mm/yr. since the Pliocene (Stein et al., 1985). Geodetic analysis also revealed that highangle faulting extends to the base of the seismogenic zone (Stein et al., 1985) . Commonly large
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IDAHO EARTHQUAKES
normal fault EQs originate at 6-15km, and have dips from 30-60 (Jackson, 1987). The Thousand
Springs segment of the Lost River fault dips ~45 SW, and the EQs in association with this fault
are believed to be characteristic EQs (same size, slip, depth, etc…). Consequently, the depth of
the Borah Peak EQ is consistent with other large magnitude normal faulting EQs that originate
at the base of the seismogenic zone.
III. Future Outlook
The reason that Idaho ranks fifth highest earthquake hazard in the lower 48, is due to
many of Idaho’s cities being built on unconsolidated sediments (“Idaho State University”).
Unconsolidated sediments move effortlessly in response to seismic activity and pose a great
risk to buildings. Liquefaction is also a prominent geologic hazard for unconsolidated sediments
if they are saturated. EQs in Idaho are prone to happen in any seismically active areas and
precautions need to be taken to prevent excessive damage. Idaho has incidentally encountered
two of the four largest EQs in the lower 48, which both caused fatalities and millions of dollars
in damage. Given that many of Idaho’s populated cities are located near or on alluvial deposits
(unconsolidated sediments), Uniform Building Codes (UBC) have become standard practices
and are amongst the greatest in the country (“Idaho Geologic Survey”). Buildings that are built
to UBC maintain an improved chance of surviving EQS; however, being that Idaho’s populated
regions are located on unconsolidated material, seismic risk still exists.
IV. Conclusion
Idaho has experienced several damaging EQs; despite none having occurred since the
1983 Borah Peak EQ. Recurrent EQs of smaller magnitude, numerous active faults, and a history
of damaging EQs demonstrate the likelihood. Consequently, historic EQs expose the state to a
moderate earthquake risk and areas such as; the Lost River Range, and southeastern Idaho at
high risk (“Idaho Geologic Survey”). Also, seismic examination of the state ranks Idaho as fifth
highest earthquake hazard in the lower 48, and much of the state adheres to Uniform Building
Codes. Then again, the majority of populous areas in the state are built on alluvial deposits and
seismic risk nonetheless exists. Ground shaking reports in the course of prior EQs have
produced interpretations of seismic hazard to buildings (Fig. 1-5). Furthermore, records of
Idaho EQs confirm roughly 30,000 EQs in the last century (“Idaho Geologic Survey”). Overall,
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IDAHO EARTHQUAKES
Idaho has had relatively few large damaging EQs, and there have been many frequent lesser
magnitude EQs; thus, seismic hazard and risk coexist.
(Fig. 1-1: Normal faulting in association with magma chamber motion. Taken from,
http://www.explorevolcanoes.com/yellowstone.html).
(Fig. 1-2: Quake Lake. The 1959 M7.1 Hebgen lake earthquake triggered a landslide that blocked flow of the
Madison River creating Quake Lake. Note landslide scarp in background. Taken from, http://www.greateryellowstone.com/graphics/West-Yellowstone/quake-lake.jpg).
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IDAHO EARTHQUAKES
(Fig. 1-3: Borah Peak fault scarp near Double Springs Pass Rd. Note person for scale. Taken from
http://www.nationalatlas.gov/articles/geology/IMAGES/faultScarp.gif)
(Fig. 1-4: 1983 Borah Peak EQ mainshock and aftershock focal mechanism solutions. Note normal faulting with an
oblique component of left-lateral strike slip .Taken from,
http://www.bssaonline.org/content/vol94/issue3/images/large/943_03122_06.jpeg).
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IDAHO EARTHQUAKES
(Fig. 1-5: Idaho ground shaking seismic hazard assessment. Note; red: extreme risk, orange: high risk, yellow:
moderate risk. Taken from http://www.idahogeology.org/Services/GeologicHazards/Earthquakes/eqrisk.html).
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IDAHO EARTHQUAKES
References
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<http://imnh.isu.edu/digitalatlas/geo/quakes/quakes.htm>.
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Jackson, J.A. "Active normal faulting and crustal extension." Geological Society, London, Special
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Stein, Ross S., and Sergio E. Barrientos. "Planar High-Angle Faulting in the Basin and Range:
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