Structure number:
2351.
Comments:
Structure name:
Wasatch fault zone (WFZ).
Comments:
Synopsis:
The WFZ is one of the longest and most tectonically active normal faults
in North America. The fault zone shows
abundant evidence of recurrent Holocene surface faulting and has been the
subject of detailed studies for over three decades (for example, Schwartz and
Coppersmith, 1984; Machette and others, 1991, 1992). Half of the estimated 50 to 120 post-Bonneville surface-faulting
earthquakes in the Wasatch Front region have been on the WFZ (Hecker, 1993).
Date of compilation: 8/01.
Compiler and affiliation:
Bill D. Black, Mike Hylland, and Greg N. McDonald (Utah Geological
Survey), and Suzanne Hecker (U.S. Geological Survey).
State:
Utah, Idaho.
County:
Box Elder, Weber, Davis, Salt Lake, Utah, and Juab; Oneida.
1°
x 2° sheet:
Brigham City, Ogden, Salt Lake City, and Price.
Province:
Basin and Range.
Geologic setting:
Generally north-trending range-bounding normal fault along the western
side of the Malad Range (Clarkston Mountain), Wellsville Mountains, Wasatch
Range, and San Pitch Mountains.
The WFZ marks the eastern boundary of the Basin and Range in northern
Utah. Allluvial-fan sediment and
deposits of Pleistocene Lake Bonneville dominate the surficial geology along
the fault zone. The Wasatch Range is
several kilometers higher than valleys to the west and is the result of
repeated fault movement in Cenozoic time.
Number of sections:
10.
Comments: The WFZ is divided into 10 discrete, independent sections,
totaling 343 kilometers in length. The
sections are thought to represent segments (Schwartz and Coppersmith, 1984;
Machette and others, 1991). The
southern eight sections are wholly in Utah; the Clarkston section straddles
Idaho and Utah, and the northernmost (Malad City) section is in Idaho. The chronology of surface-faulting
earthquakes on the fault is one of the better dated in the world and includes
16 earthquakes since 5.6 ka, with an average repeat time of 350 years (McCalpin
and Nishenko, 1996). Four of the
central five sections (2351E-H) ruptured between 600 and 1,250 years ago; the
remaining section (Brigham City, 2351d) has not ruptured in the past 2,125
years (McCalpin and Forman, 1994: McCalpin and Nishenko, 1996). Slip rates of 1‑2 millimeters/year are
typical for the central sections during Holocene time. In contrast, late Quaternary (<150-250
ka) slip rates on these sections are about 0.1‑0.3 millimeters/year, an
order of magnitude lower than the Holocene rates. This suggests a causal relation between increased slip rates and
isostatic rebound/crustal relaxation following deep lake cycles such as
Bonneville (Machette and others, 1986, 1992).
Based on comparisons with historical surface fault ruptures in the
region, the central fault sections may produce up to magnitude (Ms)
7.5-7.7 earthquakes. McCalpin and
Nishenko (1996) suggest the probability for a surface-faulting earthquake
somewhere on the fault is 13 and 25 percent in the next 50 and 100 years,
respectively.
Length: End
to end (km): 308
Cumulative trace (km): 566
Average strike (azimuth): N6oW
2351b, CLARKSTON MOUNTAIN SECTION
Section number:
2351b.
Section name:
Clarkston Mountain section.
Comments: Hecker's (1993) fault number 6-16.
Reliability of location:
Good.
Comments: Mapped or discussed by
Cluff and others (1974), Machette and others (1992), and Biek and others
(2000). Mapping from Cluff and others
(1974). Part of the section is in
Idaho.
Sense of movement:
N.
Comments:
Dip: No data.
Comments:
Dip direction:
W.
Geomorphic expression:
Steep linear escarpment along the western flank of the Malad Range
(Clarkston Mountain). Section
boundaries are based on structural and geomorphic relations only and are more
tenuous than the boundaries of other WFZ sections to the south. Machette and others (1992) place the
boundary between the Clarkston Mountain and Collinston sections at Short
Divide, which is marked by a prominent east-striking down-to-the-south normal
fault that places Tertiary lacustrine sediment against Paleozoic rock.
Age of faulted deposits:
Late Pleistocene.
Paleoseismology studies: None.
Timing of most recent paleoevent:
(3) Late Quaternary (<130 ka).
Comments: Regressional shorelines below the Provo level wrap around, and
thus postdate, a probable fault escarpment, and deep‑water sediments of
Lake Bonneville are not faulted.
Recurrence interval: No data.
Comments:
Slip rate:
Unknown, probably <0.2 mm/yr.
Comments:
Length: End
to end (km): 10
Cumulative trace (km): 16
Average strike (azimuth): N5oW
Section number:
2351c.
Section name:
Collinston section.
Comments: Hecker's (1993) fault number 6-5.
Reliability of location:
Good to poor.
Comments: The location of the
Collinston section north of the Wellsville Mountains is poorly constrained and
subject to interpretation. Gravity data
and topography suggested to Machette and others (1992) that the fault is west
of the Junction Hills (the low hills north of the Wellsville Mountains), and
ends where it intersects the prominent east-trending Short Divide fault at the
south end of Clarkston Mountain. Cluff
and others (1974) mapped the WFZ as continuous around the south end of
Clarkston Mountain, whereas Machette and others (1992) recognized a 7-kilometer
left step and gap in late Pleistocene faulting between the Collinston and
Clarkston Mountain sections, and interpreted Cluff and others' (1974) supposed
connecting fault as a shoreline. Goessel
and others (1999) suggest that at the northern end of the Wellsville Mountains,
displacement on the Collinston section is transferred several kilometers
eastward to the Beaver Dam fault; they tentatively suggest the northern
boundary of the Collinston section is in the area between the northern
Wellsville Mountains and the Cache Butte divide. Mapping from Doelling (1980), Machette and others (1992), Oviatt
(1986a,b), and Personius (1990).
Sense of movement:
N.
Comments:
Dip: No data.
Comments:
Dip direction:
W.
Geomorphic expression:
West-facing escarpment along the western flank of the Malad Range
(Clarkston Mountain) and Wellsville Mountains.
Machette and others (1992) place the boundary between the Collinston and
Brigham City sections at a reentrant near the mouth of Jim May Canyon, 2
kilometers northeast of Honeyville, where the trend of the fault and amount of
pre-Bonneville displacement changes.
Age of faulted deposits:
Late Pleistocene.
Paleoseismology studies: None.
Timing of most recent paleoevent:
(3) Late Quaternary (<130 ka).
Comments: Faulting generally predates the transgressive phase of Lake
Bonneville (~30 ka). A 2-kilometer-long
scarp in alluvium (equivalent in age to the Provo level of Lake Bonneville) at
the south end of the section is probably related to sympathetic or subsidiary
rupturing from faulting on the Brigham City section. The remainder of the section shows no evidence of Holocene
faulting (Machette and others, 1992).
Recurrence interval: No data.
Comments:
Slip rate:
(D) <0.2 mm/yr.
Comments: At the south end of the
section, alluvium estimated from soil-profile development to be several hundred
thousand years old is displaced 12 meters.
Length: End
to end (km): 30
Cumulative trace (km): 37
Average strike (azimuth): N21°W
Section number:
2351d.
Section name:
Brigham City section.
Comments: Hecker's (1993) fault number 6-6.
Reliability of location:
Good.
Comments: Mapping from Personius
(1990).
Sense of movement:
N.
Comments:
Dip: 65-80oW (Personius,
1991).
Comments: 65-80oW at
Bowden Canyon trench in fluvial and debris-flow deposits; 70-80oW at
Pleasant View trench in lacustrine deposits.
Dip direction:
W.
Geomorphic expression:
West-facing scarps along the western base of the Wellsville Mountains
and Wasatch Range. Scarps on the valley
floor between Willard and Brigham City may be associated with incipient lateral
spreads, but have orientations and relief consistent with a faulting origin. In the southern part of the section,15- to
20-meter-high scarps on a Provo-level delta suggest as many as 6-10
surface-faulting events occurred since about 16 ka (assuming an average
displacement per event of 2+ meters).
However, only a few short, discontinuous scarps are in upper Holocene
deposits near the southern section boundary, which is in contrast to the
abundance of Holocene scarps on the Weber section to the south. Displacement per event is from 1.0 to 2.5
meters.
Age of faulted deposits:
Holocene.
Paleoseismology studies:
A trench excavated in 1986 across a fault scarp on the Bowden Canyon
alluvial fan (Brigham City trench; site 2351d-1), east of Brigham City, exposed
evidence for three surface-faulting earthquakes (Personius, 1991). The oldest of these events could not be
directly dated. Radiocarbon age estimates
on bulk-soil samples from paleosols and the base of the modern soil provide
limiting ages for the penultimate and most recent events on the fault. Timing of the penultimate event may
correlate with the timing of the most recent surface-faulting earthquake
documented in a trench across a fault scarp on the Pleasant View salient (Pole
Patch trench; site 2351d-3), about 20 kilometers south of the Brigham City
trench (Personius, 1991). This scarp is
associated with a short, off-trend fault in the area of the boundary between
the Brigham City and Weber sections of the Wasatch fault. The trench exposed evidence for three
surface-faulting earthquakes. Although
a bulk-soil sample yielded a radiocarbon age estimate that closely approximates
the age of the most recent event, other radiocarbon ages only poorly constrain
the timing of the older events.
In 1992 and 1993, 14 additional
trenches were excavated across scarps of the Brigham City section to lengthen
the paleoseismic record for this fault (McCalpin and Forman, 1994). These trenches were on the Provo-level delta
at the mouth of Box Elder Canyon (site 2351d-2). In total, the trenches exposed evidence for five, or possibly six,
surface-faulting events. Earthquake
timing is constrained by limiting ages obtained from radiocarbon and
thermoluminescence dating of bulk soil samples. The most recent event here (event Z) is younger than the most
recent event at the Brigham City trench; the timing of that earthquake correlates
well with event Y here. Evidence was
not observed for an earthquake correlative with the penultimate event at the
Brigham City trench. A relatively wide
range in limiting ages for event V suggests either a poorly constrained single
event, or two separate events. McCalpin
and Nishenko (1996) prefer the two-event interpretation, resulting in a total
of seven paleoearthquakes on the Brigham City section since 13 ka.
Timing of most recent paleoevent:
(2) Latest Quaternary (<15 ka).
Comments: The Brigham City section is the northernmost WFZ section that
exhibits clear evidence of recurrent Holocene faulting along its entire length
(Personius, 1988; Machette and others, 1992).
McCalpin and Nishenko (1996) show seven paleoearthquakes on the section
since 13 ka; mean ages of these events are 2.1 ka (event Z), 3.4 ka (event Y),
4.7 ka (event X), 6.0 ka (event W), 7.3 ka (event V), 8.5 ka (event U), and
13.0 ka (event T).
Recurrence interval:
1.3 ky (<8.5 ka).
Comments: The overall temporal pattern since 13 ka is one earthquake every
1-2 ky since 8.5 ka, with a long seismic gap between 8.5 and 13 ka. McCalpin and Forman (1994) indicate the ~5
ky seismic gap may have been influenced by changes in crustal stress regime
associated with Lake Bonneville dessication, possibly causing crustal rebound to
suppress extensional movements. Elapsed
time since the last earthquake (2,125 years) exceeds the mean recurrence and
suggests that the section is due for a surface‑faulting earthquake,
unless strain‑accumulation rates have declined during the late Holocene. McCalpin and Nishenko (1996) suggest that
the Brigham City section has a time-dependent earthquake probability that
approaches or exceeds regional and fault-specific probabilities.
Slip rate:
(C) 0.2-1 mm/yr (<4.7 ka), (B) 1-5 mm/yr (<15 ka).
Comments: 0.75 millimeters/year since 4.7 ka and 1.0-1.3 millimeters/year
for the past 15 ky. Net slip gradually
decreases to the north. The middle to
late Holocene slip-rate estimate is based on trench data from near the center
of the section at Brigham City. The
late Pleistocene estimate is based on the post-Provo displacement in the
southern part of the section.
Length: End
to end (km): 37
Cumulative trace (km): 81
Average strike (azimuth): N9°W
Section number:
2351e.
Section name:
Weber section.
Comments: Hecker's (1993) fault number 11-22.
Reliability of location:
Good.
Comments: Mapping from Nelson and
Personius (1993).
Sense of movement:
N.
Comments:
Dip: 68-78oW (Swan and
others, 1980); 68-70oW (McCalpin and others, 1994).
Comments: Measured in Kaysville
trench A (Swan and others, 1980) and reopened Kaysville trench A (McCalpin and
others, 1994) in lacustrine deposits juxtaposed against slope colluvium.
Dip direction:
W.
Geomorphic expression:
West-facing scarps along the western base of the Wasatch Range. The Weber section is the longest WFZ
section. The southern boundary of the
section is at the prominent Salt Lake salient, but fault scarps in this area
are easily confused with nontectonic features and scarp distribution is less
certain (Machette and others, 1992).
The northern section boundary is at the Pleasant View salient. Scarp heights in the northern part of the
Weber section suggest a higher rate of late Holocene faulting than on the
Brigham City section to the north (Machette and others, 1992). Net displacement measured at three sites
(Garner Canyon, East Ogden, and Kaysville), which are about 20 kilometers
apart, is from 1.0 to 3.5 meters per event.
Individual faulting events may not have ruptured the entire section,
although the general timing of events at the three sites appears similar.
Age of faulted deposits:
Holocene.
Paleoseismology studies:
One of the earliest detailed paleoseismic investigations of the Wasatch
fault (1978) was conducted on the Weber section between Bair (or Baer; variant
spelling) and Shepard Creeks, about 3 kilometers east-southeast of
Kaysville. This study was particularly
important from the standpoint of developing two fundamental concepts of modern
paleoseismology: colluvial-wedge stratigraphy (Swan and others, 1980), and
characteristic earthquakes (Schwartz and Coppersmith, 1984). The Kaysville site (site 2351e-3) included
five test pits and seven trenches, which exposed evidence for at least three
surface-faulting earthquakes since about 12 ka (Swan and others, 1980; Swan,
Schwartz, and others, 1981). A single
radiocarbon age estimate on detrital charcoal indicated that two earthquakes
had occurred since 1,580 ± 150 yr B.P.
The Kaysville site was reoccupied in 1988 to refine the paleoearthquake
chronology; a trench was excavated parallel to and slightly deeper than the
1978 trench A. Detailed stratigraphic
analysis and geometrical reconstructions based on data from both the 1978 and
1988 trenches indicate five or six surface-faulting earthquakes since 13 ka
(McCalpin and others, 1994).
Radiocarbon and thermoluminescence age estimates on bulk-soil samples
from paleosols constrain the timing of the three youngest faulting events. The earlier two or three events could not be
directly dated because stratigraphic evidence for these postulated events was
not exposed.
In 1986, five trenches were
excavated across multiple scarps of the Weber section at the East Ogden site
(site 2351e-2), about 1.5 kilometers north-northwest of the mouth of Ogden
Canyon. The trenches exposed evidence
for three, and possibly four, surface-faulting earthquakes since middle
Holocene time (Nelson, 1988; Machette and others, 1992). Radiocarbon age estimates on bulk-soil
samples from paleosols developed on colluvial wedges provided poor constraint
on paleoearthquake timing (Nelson, 1988), but thermoluminescence age estimates
on fault-related colluvial sediments reduced some of the uncertainty (Forman
and others, 1991). One of the trenches
(EO-3) showed stratigraphic evidence for a small-displacement event within the
past 500-600 years, but obscure stratigraphic relations and problems with
radiocarbon age estimates make interpretation of this event tenuous (Nelson,
1988). Although Nelson and Personius
(1993) characterize this event as “probable,” it has not been observed in any
other trenches on the Weber section.
At Garner Canyon (site 2351e-1), 2
kilometers south of Coldwater Canyon near North Ogden, an artificial exposure
across the fault revealed stratigraphic evidence for four surface-faulting
earthquakes (Nelson and others, 1987; Machette and others, 1992; Nelson and
Personius, 1993). Radiocarbon age
estimates on bulk-soil samples from paleosols developed on colluvial wedges
constrain the timing of the two youngest paleoearthquakes.
Timing of most recent
paleoevent: (2)
Latest Quaternary (<15 ka).
Comments: McCalpin and Nishenko (1996) show four paleoearthquakes on the
section since 6.1 ka; mean ages of these events are 1.0 ka (event Z), 3.1 ka
(event Y), 4.4 ka (event X), and 6.1 ka (event W)
Recurrence interval:
1.7 ky (<6.1 ka).
Comments:
Slip rate:
(B) 1-5 mm/yr.
Comments: 0.9 to 1.9 millimeters/year for the past 15 ky for the central
three-quarters of the section. Slip
rates are highest in the central and northern part of the section, and decrease
toward the ends.
Length: End
to end (km): 56
Cumulative trace (km): 81
Average strike (azimuth): N6°W
Section number:
2351f.
Section name:
Salt Lake City section.
Comments: Hecker's (1993) fault number 12-6.
Reliability of location:
Good.
Comments: Mapping from Personius and
Scott (1992) with the following exceptions:
(1) The southernmost extent of the Warm Springs fault is from Scott and
Shroba (1985). (2) The northernmost
middle and late Quaternary trace (Virginia Street fault) is from Van Horn and
Crittenden (1987; City Cemetery fault).
(3) The central middle and late Quaternary trace (University Hospital
fault) is from Hecker (1993), based on interpretation of data contained in
Everitt (1980). (4) The southernmost
middle and late Quaternary trace is from Crittenden (1965) and Van Horn (1972).
Sense of movement:
N.
Comments:
Dip: 60-86°W (Black and others, 1996).
Comments: Measurements taken at the
South Fork Dry Creek site in Holocene alluvial-fan and Pleistocene Lake
Bonneville deposits.
Dip direction:
W.
Geomorphic expression:
The Salt Lake City section is divided into three en-echelon subsections
(from north to south): the Warm Springs fault, East Bench fault, and Cottonwood
subsection (Personius and Scott, 1992).
The Warm Springs fault forms a prominent escarpment along the western
flank of the Salt Lake salient, then trends south into basin fill and dies
out. Location of the southern end of
the Warm Springs fault beneath urbanized Salt Lake City is subject to
interpretation (Schlenker and others, 1999).
Pre‑urbanization studies of the Warm Springs fault at Jones Canyon
by G.K. Gilbert (1890, in Hunt, 1982) showed evidence for three post‑Bonneville
events, with displacements totaling 9 meters.
However, these estimates are probably minima; perhaps six to eight
latest Quaternary events with displacements totaling 14-16 meters have occurred
on this part of the section (Personius and Scott, 1992). Robison and Burr (1991) estimated a maximum
displacement of about 12 meters at a site (Washington Elementary School) at the
south end of the Warm Springs fault.
Based on variations in elevation of correlative Lake Bonneville deposits
indicating paleolake level, Currey (1992) infers the presence of three faults
(Capitol Hill fault zone) east of the Warm Springs fault, having a maximum
cumulative offset of about 21 meters since 20,400 yr B.P.
In northeastern Salt Lake Valley,
faulting activity shifted westward from the range front to the East Bench fault
during the late Quaternary (Personius and Scott, 1992).
The East Bench fault forms prominent
northwest- to southwest-facing intrabasin fault scarps from Salt Lake City
(about 2 kilometers east of the southern end of the Warm Springs fault) along
about 1100 East Street and Highland Drive south to Big Cottonwood Creek. A trench site at the north end of the East
Bench fault revealed evidence for 7 meters of deformation in transgressive Lake
Bonneville deposits, including 3 meters of monoclinal folding that occurred
prior to 12.5 ka and 4 meters of Holocene-age brittle faulting (Machette and
others, 1992). A Quaternary(?) fault
(Rudys Flat fault) cutting bedrock of the Salt Lake salient east of the Warm Springs
fault appears to connect the East Bench fault with the Weber section of the
Wasatch fault zone (2351E), but has no conclusive evidence of Quaternary
movement.
The Cottonwood subsection forms a
prominent (often wide and complex) zone of faulting along the range front from
just north of Big Cottonwood Canyon to the Traverse Mountains. At the mouth of Little Cottonwood Canyon,
the fault zone forms a 50-meter-wide graben with a 25-meter-high main scarp and
10-meter-high antithetic scarp. Farther
south at South Fork Dry Creek, the graben is 400 meters wide, and six
en-echelon scarps comprise the main fault zone. The complexity of the fault zone and poor exposure of antithetic
faults has precluded accurate determination of net tectonic displacement. However, profiling of moraine surfaces
across the fault zone in the Little Cottonwood Canyon area indicates
approximately 14-14.5 meters of net vertical tectonic displacement (Madsen and
Currey, 1979; Swan, Hanson, and others, 1981).
The main fault zone shows stratigraphic evidence for seven events since
15 ka (McCalpin and Nelson, 2000), and estimates of displacement per event
range from 1.5 to 5 meters (Schwartz and Lund, 1988; Black and others,
1996). Based on structural geology,
distribution and size of fault scarps, and comparisons with large historical
earthquakes elsewhere, surface rupture on the Salt Lake City section may
initiate at the southern end of the Cottonwood subsection and propagate
northward (Bruhn and others, 1987; Personius and Scott, 1992).
Age of faulted deposits:
Holocene.
Paleoseismology studies:
Most of the paleoseismic record for the Salt Lake City section comes
from two sites on the Cottonwood subsection of the fault: the mouth of Little
Cottonwood Canyon, and the South Fork of Dry Creek. Four trenches were excavated in 1979 across fault scarps just
north of the mouth of Little Cottonwood Canyon (site 2351f-4). One trench did not expose the fault and was
not logged. The other trenches exposed
evidence for two or three surface-faulting earthquakes during the Holocene
(Swan, Hanson, and others, 1981).
Accelerator-mass-spectrometry radiocarbon age estimates on charcoal
provided a minimum limiting age for the penultimate event, but the most recent
event could not be dated (Schwartz and Lund, 1988). Because of the width and complexity of the fault zone at this
site, not all fault traces were trenched.
To improve the paleoseismic record
of the Salt Lake City section, trenches were excavated in 1985 in another
complex zone of faulting at the South Fork Dry Creek site (site 2351f-5), about
4 kilometers south of the Little Cottonwood Canyon site. Four trenches were excavated across three of
the six scarps present at the site; the trenches exposed evidence for two
surface-faulting earthquakes, and radiocarbon age estimates on bulk-soil
samples from paleosols beneath colluvial wedges provided maximum limiting ages
for the events (Schwartz and Lund, 1988).
Still, the late Quaternary paleoseismic record of the Salt Lake City
section remained incomplete, as was shown by a trench excavated in 1991 as part
of a geotechnical study at Dry Gulch (site 2351f-6), a few hundred meters south
of the South Fork Dry Creek site. This
trench also exposed evidence for two surface-faulting earthquakes; however,
although radiocarbon age estimates on bulk-soil samples showed that the most
recent event in this trench coincided with the age determined in the South Fork
Dry Creek trenches, the penultimate event was previously unrecognized (Lund,
1992).
Five additional trenches were
excavated at the South Fork Dry Creek site in 1994, across scarps that had not
previously been trenched, to establish a complete late Holocene paleoseismic
chronology for the Salt Lake City section.
Stratigraphic relations exposed in the trenches, combined with limiting
ages from radiocarbon age estimates on bulk-soil samples, provided evidence for
yet another previously unrecognized surface-faulting earthquake, increasing the
number of events since 6 ka to four (Black and others, 1996). This study also provided insights into the
variable nature of recurrent surface faulting along existing fault traces in
complex fault zones.
The Little Cottonwood Canyon site
was reoccupied in 1999 as part of an investigation to document all of the post-Bonneville
surface-faulting earthquakes on the Salt Lake City section. A “megatrench” and accompanying auger hole
exposed a 26-meter vertical sequence that revealed evidence for seven
surface-faulting events, including three in the period 6-15 ka (McCalpin and
Nelson, 2000). Radiocarbon age
estimates on bulk-soil samples and charcoal provided timing constraints for the
paleoearthquakes. A significant finding
from this study is that the average earthquake recurrence interval is much
longer between 6 and 15 ka than it is after 6 ka. The megatrench also documented a prolonged early Holocene
aseismic interval on the Wasatch fault, consistent with observations made by
McCalpin and Forman (1994) at their trench site (2351d-2) on the Brigham City
section.
Geotechnical studies have provided
information on the late Quaternary paleoseismic history of the Warm Springs
fault, at the northern end of the Salt Lake City section of the Wasatch
fault. Trenches at the Washington
Elementary School property at 400 North and 200 West Streets in northern Salt
Lake City (site 2351f-1) revealed evidence of faulting; however, lack of
correlative hanging-wall and footwall strata precluded measurement of offset,
and no radiometric dating was done.
Based on a minimum 12-meter displacement of Lake Bonneville sediments
and assumed 2- to 3-meter displacement per event, four to six faulting events
were estimated to have occurred since 15 ka (Robison and Burr, 1991). At the Salt Palace Convention Center at 200
South West Temple Street in downtown Salt Lake City (site 2351f-2),
construction excavations and exploratory trenches revealed evidence for three
faulting events; radiocarbon age estimates on bulk-soil samples provide a
minimum limiting age for the oldest event and a maximum limiting age for the
two younger events (Simon and Shlemon, 1999).
The observed deformation at the Salt Palace was alternatively
interpreted as being related to liquefaction-induced lateral spread (Korbay and
McCormick, 1999).
Two trenches were excavated across
the East Bench fault as part of a geotechnical study at the Dresden Place site
at 550 South 900 East Street in eastern Salt Lake City (site 2351f-3). Although the number and timing of individual
faulting events were not determined, the trenches did reveal intriguing
evidence for contrasting deformational styles (including monoclinal warping and
planar fault rupture) associated with faulting before and after regression of
Lake Bonneville to an elevation below that of the site (results summarized by
Machette and others, 1992).
Timing of most recent
paleoevent: (2)
Latest Quaternary (<15 ka).
Comments: Combined observations at two sites on the Cottonwood section in
1979 and 1985 showed a total of three events since 8-9 ka, but not all scarps
at either site were trenched. Complete
trenching at the South Fork Dry Creek site in 1994, combined with observations
at Dry Gulch in 1991 and at South Fork Dry Creek in 1985, showed four
paleoearthquakes occurred since 6 ka and at least five events occurred since 8-9
ka (Black and others, 1996). McCalpin
and Nishenko (1996) indicate mean ages for the last four events are 1.2 ka
(event Z), 2.5 ka (event Y), 3.9 ka (event X), and 5.4 ka (event W). Results from the “megatrench” at the mouth
of Little Cottonwood Canyon indicated an additional three events in the time
period 6-15 ka, rather than the six or seven events that would be expected if
the post-6 ka recurrence interval had continued throughout latest Quaternary
time (McCalpin and Nelson, 2000). The
East Bench fault has evidence for at least two events in the past 26 ka (during
and after the Bonneville lake cycle).
The earliest documented event appears as 3 meters of monoclinally
warped, deep‑water sediments and probably occurred subaqueously, before
the lake level dropped below the site about 12.5 ka. Subsequent events occurred as brittle, presumably subaerial,
deformation. Simon and Shlemon (1999)
found evidence for three events on a possible southern extension of the Warm
Springs fault in downtown Salt Lake City, one between ~7.1 and 8.1 ka and two
events after ~6.4 ka, alternatively interpreted by Korbay and McCormick (1999)
as liquefaction-related deformation (lateral spreading).
Recurrence interval:
1.4 ky (<5.4 ka).
Comments: Based on the mean interevent time between events Z through
W. McCalpin and Nelson (2000) indicate
that recurrence has been irregular, with a much longer recurrence interval in
the period 6-15 ka following and perhaps attributable to the drying up of Lake
Bonneville.
Slip rate:
(B) 1-5 mm/yr.
Comments: 1.4 millimeters/year (<5.4 ka) based on an average net slip
per event of 2 meters and a recurrence interval of 1.4 ky. Hecker (1993) indicates a post-19 ka slip rate
of 0.8 millimeters/year and recurrence interval of 2.4-3.0 ky based on
2-meter-displacement events and evidence from the Little Cottonwood Canyon
site. A rough latest Quaternary slip‑rate
estimate of 1 millimeter/year for the East Bench fault is significantly greater
than a rough long‑term (Quaternary) estimate of 0.04‑0.14
millimeters/year based on shallow seismic-reflection data (Crone and Harding,
1984).
Length: End
to end (km): 43
Cumulative trace (km): 92
Average strike (azimuth): N4°W
Section number:
2351g.
Section name:
Provo section.
Comments: Hecker's (1993) fault number 12-3.
Reliability of location:
Good.
Comments: Mapping from Machette
(1992).
Sense of movement:
N.
Comments:
Dip: 77oW (Lund and Black,
1998); 79oW (Lund and others, 1991); 58oW (Swan and
others, 1980).
Comments: Measurements are from the
main fault in the trench at Rock Canyon in alluvium and debris-flow deposits
(Lund and Black, 1998), south trench at Mapleton in lacustrine deposits (Lund
and others, 1991), and Hobble Creek trench HC-1 in Provo fan-delta deposits.
Dip direction:
W.
Geomorphic expression:
Based on fault geometry and apparent recency of movement as indicated by
scarp morphology, Machette and others (1986) tentatively subdivided the Provo
section (as originally proposed by Schwartz and Coppersmith, 1984) into three
subsections (from north to south, the American Fork, Provo "restricted
sense," and Spanish Fork).
However, based on the timing of the last two events deciphered from
trench studies, the entire length of the WFZ in Utah Valley appears to be a
single section (Machette, 1992; Lund and others, 1991; Machette and others,
1991; Lund and Black, 1998). Lund and
Black (1998) measured 3.3 meters of displacement from the most recent event at
Rock Canyon in correlative geologic units across the fault zone, which is one
of the most accurate available for the WFZ.
The Woodland Hills splay of the Spanish Fork subsection has evidence for
three or four events, totaling 3 meters of displacement, in about the past 130
ka, yielding a slip rate of 0.01‑0.02 millimeters/year and an average
recurrence interval of about 40-65 ky (Machette, 1992). Movement on the splay apparently occurs
during only some of the events on the main fault, although the most recent
event on the splay occurred about 1.0 ka and may be correlative with the most
recent event on the main fault.
Movement on a couple of short subsidiary faults at the northern end of
Utah Valley appears to have occurred during, and may be related to, the
recession of Lake Bonneville.
Age of faulted deposits:
Holocene.
Paleoseismology studies:
Machette and Lund (1987) excavated three trenches in 1986 just south of
the mouth of American Fork Canyon (site 2351g-1) (results also described in
Machette, 1988, 1992; Machette and others, 1992). The trenches crossed several traces of the fault, and
collectively revealed stratigraphic and structural evidence for four
surface-faulting events since 8 ka.
Radiocarbon and thermoluminescence age estimates on charcoal and bulk-soil
samples from colluvial-wedge deposits and buried A horizon soils constrain the
timing of the events.
In 1986, M.N. Machette (USGS) and
W.E. Mulvey (UGS) mapped a natural stream-cut exposure of the fault northeast
of Provo at Rock Canyon (site 2351g-2).
They observed evidence for the most recent surface-faulting event;
radiocarbon age estimates on bulk-soil samples from a colluvial-wedge deposit
and a paleosol buried by post-event deposits constrain fault timing (Machette,
1992; Machette and others, 1992). In
1988, a trench excavated across the fault scarp about 50 meters south of the
stream-cut exposure (Lund and others, 1990; Lund and Black, 1998) also revealed
evidence for the most recent event, and re-examination of the cut identified a
second fault trace. Radiocarbon age
estimates from bulk-soil samples of three buried paleosols exposed in the
trench provided additional constraints on the timing of the most recent
event. Also, additional samples
(collected in 1988 and 1995) of a paleosol that was buried by scarp-derived
colluvial-wedge material, and exposed at the stream cut, yielded a radiocarbon
age estimate that provides a maximum limiting age for the most recent event.
Near Springville, Swan and others
(1980) profiled scarps where the main fault trace crosses a large alluvial-fan
complex near the mouth of Hobble Creek Canyon, and excavated three trenches
about 2 kilometers northwest of Hobble Creek at Deadmans Hollow (site
2351g-3). Collectively, the study area
showed evidence for six or seven post-Provo (14.3 ka) surface-faulting events. The trenches revealed colluvial stratigraphy
indicating three young events, and three or four older events are inferred from
tectonic strath terraces preserved along Hobble Creek upstream from the fault
zone.
In 1987, Lund and others (1991)
excavated a total of five trenches at two closely spaced sites southeast of the
town of Mapleton. The “Mapleton North”
(2351g-4) and “Mapleton South” (2351g-5) sites are 4 and 5.5 kilometers,
respectively, south of the Hobble Creek site.
The north trench site revealed stratigraphic and structural evidence for
the most recent surface-faulting event; radiocarbon age estimates from charcoal
contained in pre- and post-event deposits constrain the timing of this
event. Trench exposures at the south
site revealed evidence for two surface-faulting events. A lack of datable material precluded
determining the timing of the most recent event. Timing of the penultimate event was estimated from radiocarbon
and thermoluminescence age estimates obtained from a buried soil that was
displaced by faulting shortly before the time of burial.
The U.S. Bureau of Reclamation
(USBR) excavated two trenches in 1987 in alluvial-fan deposits at the mouth of
Water Canyon (site 2351g-6), about 8 kilometers south of the town of Spanish Fork (Ostenaa,
1990). The trenches revealed evidence
for at least three Holocene surface-faulting events. Radiocarbon age estimates from charcoal in surface-burn horizons
buried by scarp-derived colluvium provide maximum limiting ages for the two
most recent events. Radiocarbon age
estimates from detrital charcoal in alluvium and colluvium bracket the ages of
the older events. Just prior to the
study at Water Canyon, the USBR excavated two trenches across the Woodland
Hills fault, a splay of the Provo section of the Wasatch fault. This site (2351g-7) is about 2 kilometers
southwest of the Water Canyon site. The
trenches revealed evidence for three or four surface-faulting events since
about 130 ka, based on correlations of faulted alluvial-fan soils with similar
soils in the area (Machette, 1992; Machette and others, 1992). Radiocarbon age estimates on bulk-soil
samples from an A horizon soil buried by scarp-derived colluvium provide an
approximation for the timing of the most recent event.
Timing of most recent
paleoevent: (2)
Latest Quaternary (<15 ka).
Comments: The most recent surface-faulting earthquake at the Rock Canyon
site occurred just prior to 650 yr ago (Lund and Black, 1998), which
corresponds well with the timing of the most recent surface-faulting
earthquakes at the American Fork (500 yr ago; Machette, 1990) and Mapleton (600
yr ago; Lund and others, 1991) sites to the north and south, respectively. The penultimate event occurred about 2.6-3.0
ka; based on results from the American Fork site, two prior events also
occurred about 5.3 ka and 5.5-8.0 ka. A
conflicting chronology of faulting from a site near the southern boundary of
the section (at Water Canyon, where two events have occurred in the last 1.0
ka) may be explained by spatial overlap of the Nephi and Provo sections,
whereby events from both sections are recorded at the site (Machette, 1992;
Ostenaa, 1990).
Recurrence interval:
2.4 ky (<5.3 ka).
Comments: Based on data from the American Fork site, where rates of
activity appear to have been constant during post-Bonneville time. Six or seven post‑Provo events are
inferred to have occurred at the Hobble Creek site (east of Spanish Fork) site,
yielding an average recurrence interval of 1.7-2.6 ky.
Slip rate:
(B) 1-5 mm/yr.
Comments: 1.1-1.3 millimeters/year since 5.3 ka and 1.0-1.7
millimeters/year in the past 15 ka based on data from the American Fork
site. However, two‑to‑three
times more displacement is recorded in Bonneville transgressive deposits than
in Provo‑age regressive deposits at the Hobble Creek site. Twenty to thirty meters of displacement in
just a few thousand years represents slip rates as high as 10 millimeters/year
and may be related to the presence of Lake Bonneville.
Length: End
to end (km): 59
Cumulative trace (km): 127
Average strike (azimuth): N14°W
Section number:
2351h.
Section name:
Nephi section.
Comments: Hecker's (1993) fault number 13-21.
Reliability of location:
Good.
Comments: Mapping from Machette
(1992) and Harty and others (1997).
Sense of movement:
N.
Comments:
Dip: 72oW (Jackson, 1991).
Comments: Measured at the Red Canyon
trench in alluvial-fan, mudflow, and fluvial deposits.
Dip direction:
W.
Geomorphic expression:
Scarps in Quaternary lacustrine deposits and alluvium. The northern end of the section overlaps the
Provo section at the Payson salient.
The Benjamin fault forms the west side of the salient and dies out as it
extends northward into Utah Valley (Harty and others, 1997). Sediments of the Provo phase of the
Bonneville lake cycle are only offset up to 2 meters along this fault
(Machette,1992). The southern section
boundary is at a 15-kilometer gap in Holocene and latest Pleistocene surface
faulting where a large alluvial fan (Levan Ridge) extends westward from the San
Pitch Mountains (Harty and others, 1997).
Gravity data suggest the fault continues through and beneath Levan
Ridge, but has been inactive for perhaps tens of thousands of years (Zoback,
1983; Machette and others, 1992).
Faults associated with young scarps north of the town of Nephi are
probably continuous with near‑surface faults in the town identified from
seismic-reflection data (Crone and Harding, 1984). A number of small faults in Quaternary deposits have been
identified on the western flank of the Gunnison Plateau east of Nephi (Biek,
1991). Displacement per event for the
last three surface-faulting earthquakes is 1.4-2.5 meters.
Age of faulted deposits:
Holocene.
Paleoseismology studies:
Hanson and others (1981) excavated three trenches at North Creek (site
2351h-1), about 6 kilometers northeast of the town of Mona (results also
summarized in Schwartz and others, 1983; Schwartz and Coppersmith; 1984;
Machette and others, 1992). Colluvial-wedge
stratigraphy exposed in the trenches indicated two surface-faulting events; an
older, third event was inferred from a tectonic strath terrace incised in an
uplifted part of the North Creek alluvial fan.
Charcoal obtained from deposits exposed in the trenches, although not
from stratigraphically definitive positions, yielded radiocarbon age estimates
that provide a maximum limiting age for the most recent event; scarp morphology
suggests the age of this event may be considerably younger than the limiting
radiocarbon age estimate. An organic-rich
soil that formed on scarp-derived colluvium deposited after the penultimate
event yielded a radiocarbon age estimate that provides a minimum limiting age
for this event. Radiocarbon dating of a
buried, offset burn layer in the North Creek alluvial-fan deposits provides a
maximum limiting age for the antepenultimate event.
Jackson (1991) excavated one trench
at Red Canyon near the southern end of the Nephi segment (site 2351h-2), about
3.5 kilometers north of the town of Nephi.
The trench revealed colluvial-wedge stratigraphy indicating three
surface-faulting events since about 4.5 ka.
Thermoluminescence and radiocarbon age estimates from soil buried by the
youngest colluvial wedge provide a maximum limiting age for the most recent
event. Samples collected from
colluvial-wedge material associated with the penultimate event yielded
radiocarbon age estimates that constrain the upper age of this event. A maximum age for the antepenultimate event
is thought to be close to the oldest radiocarbon age estimate for the
colluvium, about 4-4.5 ka.
Timing of most recent
paleoevent: (2)
Latest Quaternary (<15 ka).
Comments: Scarp morphology and continuity suggest very recent displacement
(~300‑500 years ago), although a combination of 14C and TL
dates suggest an age of about 1.2 ka for the most recent event (event Z). Schwartz and Coppersmith (1984) determined
that the penultimate event (event Y) occurred before about 4 ka, whereas
Jackson (1991) constrained the event between about 3 and 3.5 ka. Event X may have occurred between 4 and 4.5
ka (Jackson, 1991). Thus, actual middle
to late Holocene recurrence intervals may vary from less than 1 to more than 3
ky. Three middle to late Holocene
events post‑date a late Pleistocene(?) fan at the southern end of the
section (at Red Canyon), suggesting a possible hiatus in faulting activity
during latest Pleistocene to early Holocene time (Jackson, 1991).
Recurrence interval:
1.7-2.7 ky (<5.5 ka).
Comments:
Slip rate:
(B) 1-5 mm/yr.
Comments: 0.8-1.3 millimeters/year since 5.5 ka. The range of displacement and slip-rate values reflects a
systematic decrease in slip between the middle (larger values) and southern end
(smaller values) of the section.
Length: End
to end (km): 43
Cumulative trace (km): 80
Average strike (azimuth): N4°E
Section number:
2351i.
Section name:
Levan section.
Comments: Hecker's (1993) fault number 13-22.
Reliability of location: Poor.
Comments: Mapping from Machette and
others (1992).
Sense of movement:
N.
Comments:
Dip: 68oW (Jackson, 1991).
Comments: Measured at the Skinner
Peaks trench in sandstone bedrock in fault contact with Holocene mudflow and
fluvial deposits.
Dip direction:
W.
Geomorphic expression:
A 15‑kilometer‑long gap in Holocene faulting marks the
boundary between the Levan and Nephi sections (Harty and others, 1997). The range front in this area is marked by
old, degraded scarps on middle(?) Pleistocene surfaces, with little evidence
for latest Pleistocene faulting. In contrast
to the planar fault geometry at the southern end of the Nephi section (Zoback,
1992), this section of the fault appears to have a listric subsurface geometry
and/or to terminate at a shallow detachment fault (Standlee, 1982; Smith and
Bruhn, 1984). Displacement from the
most recent event was 1.8 to 2.0 meters.
Age of faulted deposits:
Holocene.
Paleoseismology studies:
Two natural exposures have provided paleoseismic data for the most
recent surface-faulting event on the Levan section (Schwartz and Coppersmith,
1984; Machette and others, 1992). At
Pigeon Creek (site 2351i-1), just east of the town of Levan, radiocarbon age
estimates on charcoal from faulted alluvium provide a maximum limiting age for
the most recent event. At Deep Creek
(site 2351i-2), about 5 kilometers south of Levan, a radiocarbon age estimate
on charcoal from within a debris-flow deposit on the upthrown block broadly
constrains the maximum limiting age for the most recent event. Jackson (1991) also studied the Deep Creek
exposure and obtained a thermoluminescence age estimate from a buried A horizon
that formed on the ground surface prior to faulting. This age estimate is interpreted to closely approximate the
timing of faulting at about 1 ka.
Jackson (1991) also excavated a trench
(Skinner Peaks trench; site 2351i-3) 1.5 kilometers northwest of Skinner Peaks
and about 200 meters east of Utah Highway 26. Although correlation of
stratigraphy exposed in the trench from the footwall block to the hanging-wall
block was problematic, Jackson’s (1991) preferred interpretation indicates
evidence for two surface-faulting earthquakes.
Thermoluminescence and radiocarbon age estimates on samples from a burn
horizon in the footwall block provide a maximum limiting age for the most
recent surface-faulting event.
Similarly derived age estimates on samples (including disseminated
charcoal) from a buried incipient A horizon in the hanging-wall block provide a
minimum limiting age for the penultimate event.
Timing of most recent
paleoevent: (2)
Latest Quaternary (<15 ka).
Comments: Stratigraphic relations indicate that 1 ka is a close maximum
limiting age for the most recent event; TL and 14C dates provide
maximum ages of about 1.5 and 1.7 ka.
An alluvial fan dated at 7.3 ka is crossed by a single‑event scarp
(~1 ka?), providing a minimum time interval since the penultimate event of ~6.3
ka. At one site, the penultimate event,
which is inferred from greater stratigraphic thicknesses on the downthrown side
of the fault, predates TL and 14C dates of 3 to 4 ka.
Recurrence interval: No data.
Comments:
Slip rate:
Unknown, probably <0.2 mm/yr.
Comments: A maximum slip rate of
~0.3 mm/yr is indicated by a maximum displacement of 2.0 meters in a minimum
6.3 ka since the penultimate event, but evidence for only one Holocene event
indicates that the slip rate is probably much lower.
Length: End
to end (km): 30
Cumulative trace (km): 32
Average strike (azimuth): N17°E
Section number:
2351j.
Section name:
Fayette section.
Comments: Hecker's (1993) fault number 13-23.
Reliability of location: Poor.
Comments: Mapping from Machette and
others (1992).
Sense of movement:
N.
Comments:
Dip: No data.
Comments:
Dip direction:
W.
Geomorphic expression:
The Fayette section is the southernmost WFZ section, and is similar in
many aspects to the northern WFZ end-sections except that Lake Bonneville was
shallower and remained below (west of) the main fault trace. The Fayette section consists of a
concave-west southeast-trending eastern trace, and a shorter north-trending
western trace. Fault scarps along the
section are eroded at canyon mouths, but preserved on elevated streams terraces
(2-5 meters above stream level). Some
antiquity is suggested by the lack of scarp preservation at canyon mouths.
Age of faulted deposits:
Late Pleistocene.
Paleoseismology studies: None.
Timing of most recent
paleoevent: (2)
Latest Quaternary (<15 ka).
Comments: Morphologically, the fault scarps plot between regression lines
for data from the Drum Mountains fault scarps and the highest shoreline of Lake
Bonneville, suggesting an age of 10-15 ka.
Recurrence interval:
Comments:
Slip rate:
Unknown, probably <0.2 mm/yr.
Comments: Lack of evidence for
Holocene events indicates a generally lower slip rate than central WFZ
sections, perhaps similar to northern end-sections.
Length: End
to end (km): 16
Cumulative trace (km): 20
Average strike (azimuth): N5°W
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