Figure 4. Scarp profiles from the Willow Creek site, Nephi segment of the Wasatch

fault zone. Only the fault-scarp portion of the profiles are shown. Profile locations

are shown on figures 3 and 5.

0

2

4

6

8

10

12

14

0

0

2

4

6

8

10

12

0

0

2

4

6

8

10

12

0 40

0

2

4

6

8

10

0 5 10 15 20 25 30

5 10 15 20 25 30 35

405 10 15 20 25 30 35

40

5 10 15 20 25 30 35

Profile m83-34

Profile m83-36

Profile m83-33

Profile m83-35

Distance along profile (m)

Distance along profile (m)

Hei

gh

t (m

)

Distance along profile (m)

Hei

gh

t (m

)

Distance along profile (m)

Hei

gh

t (m

)H

eig

ht

(m)

Lower surface

Lower surface

Lower surface

Lower surface

Willow Creek fan

Willow Creek fan

Willow Creek fan

Fan from side canyon

Lower surface is youngerthan upper surface, thusSO is minimum value

Lower surface is younger than uppersurface, thus SO is minimum value

Lower surface is youngerthan upper surface, thusSO is minimum value

Lower surface is youngerthan upper surface, thusSO is minimum value

SO = 6.0 m

SO = 5.2 m

SO = 5.8 mSH = 6.8 m

SO = 6.7 mSH = 8.7 m

SH = 6.3 m

SH = 6.9 m

SO (surface offsets)are 5.2–6.7 m

(minimum)

SUMMARY

SH (scarp heights)are 6.3–8.7 m

Figure 3. Index map of the Willow Creek site showing fault scarp, trenches, and

other cultural and geographic features.

NN

0

100 m

100 m

429700mE

429600mE

429900mE 430000mE429800mE

4406000mN

4405900mN

4405800mN

4405700mN

4405600mN

4405500mN

4405400mN

4405300mN

4406100mN

Figure 5Figure 5

Mt. Nebo WSA

Mona Irrigation Co. canal

Willow Creek FS Road (gravel)

Will

ow C

reek

WCN trenchWCN trench

m83-36m83-36

m83-34m83-34

m83-37m83-37

m83-38m83-38

U.S

. Fo

rest

S

ervi

ce L

and

U.S

. Fo

rest

Ser

vice

Lan

d (

excl

usi

on

)

Mt. Nebo WSA

m83-39 m83-39

m83-40m83-40

WCStrenchWCS

trench

m83-33m83-33

UTM Coordinates (m E)

UT

M C

oo

rdin

ates

(m

N)

m83-35m83-35

Mt. Nebo WSAMt. Nebo WSA

058

057

055

056

054

053052

051050

059

WPT m E m N

059 429997 4405355058 429994 4405408057 429926 4405489056 429927 4405520055 429930 4405570054 429932 4405806053 429913 4405869052 429866 4405900051 429853 4406025050 429782 4406034

GPS STATION LOCATIONS

NAD 27, UTM zone 12, Nephi 7.5' quad

EXPLANATION

Young alluvial fanTrace of active fault–Bar and ball on downthrown sideGPS Station and way point (WPT) numberLocation of trenchLocation of scarp profile and profile no.CanalsStreamsRoads (unimproved)Forest boundary fence

052

429600mE

63-3

8m

elifo

rP

53-3

8m

elifo

rP

33-38m

eliforP

43-38m

eliforP

N

Relative Elevation (m)

91 - 93

93 - 95

95 - 97

97 - 99

99 - 101

101 - 103

103 - 105

105 - 107

107 - 109

109 - 111

111 - 113

EXPLANATION

Fault

Fence

Unsurveyed areas:

Borrow pit

Area of dense trees

Willow

Creek N

orth

Will

ow C

reek

Sou

th

Survey base station (100 N, 100 E)

Other surveyingstationsDiversion canal

Spoil pile

Trench wall (logged) and excavation

Forest ServiceRoad

Contour Interval: 0.5 m,

labels at 2 m intervals

Fault scarpprofile

wolli

W

keerC

daoR

tseroF

ecivreS

atadyevrusfotimiLLimit of survey data

Limit of survey data

ata

dy

evrus

fo

timiL

ata

dy

ev r

usf

oti

miL

Base camp

230 220 210 200 190 180 170 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20

21

01

22

02

23

03

24

04

25

05

26

06

27

07

28

08

29

09

20

1

00

1

21

1

01

1

22

1

02

1

23

1

03

1

93

91

93

9595

95

97

97

97

97

95

99

99

99

99

99

103

103 103

103

101

101

101

101

101

105

105

105

105

107

107

107

107

109

109

109109

109111

230 220 210 200 190 180 170 160 150 140 130

NORTHING (meters)

120 110 100 90 80 70 60 50 40 30 20

NORTHING (meters)

)sr

ete

m(G

NIT

SA

E

)sr

ete

m(G

NIT

SA

E

0 41

04 1

Suggested citation: Machette, M.N., Crone, A.J. Personius, S.F., Mahan, S.A., Dart, R.L., Lidke, D.J., and Olig, S.S., 2007,

Paleoseismology of the Nephi segment of the Wasatch fault zone, Juab County, Utah—Preliminary results from two

large exploratory trenches at Willow Creek: U.S. Geological Survey Scientific Investigations Map 2966, 2 sheets.

Willow Creek trench site

Figure 3

Red Canyon trench site

North Creek trench site

Santaquintrench site

Figure 2. Index map for the Nephi segment showing location of paleose ismic trench sites

(marked by asterisks). Map and shaded-relief digitial elevation model (DEM) created by Chris

DuRoss (UGS, November 2006).

Nephi

Santaquin

Payson

JUA

BV

ALL

EY

0 10 205km

*

*

*

*

PROVOSEGMENT

NEPHISEGMENT

Northernstrand

Southernstrand

40°

39°

45’

39° 45’

EXPLANATION

Fault; bar and ball on downdropped side

Trench site*

Hansel V alley fault

Towanta F lat graben

B ear R iverfault

E as tern B earLake fault,

s outhern section

Fish S pringsfault

Hogsback fault,s outhern section

J oes Valleyfault

Morganfault

North P romontory fault

Oquirrhfault

S outhern OquirrhMountains fault

S trawberryfault

S ugarvillea rea faults

Great S

alt Lake

fault

Wa

sa

tc

hfa

ult

zo

ne

WFZ

E. Cache fault,Logan s ection

0 10050 km

B righam C itys egment

Webers egment

S LC

Fayettesegment

WVF Z

Nephis egment

Levans egment

P rovos egment

UTAH

J ames P eaksection

Index Map (Fig. 2)

E. Cache fault,

INTRODUCTIONThe Nephi segment is one of the most prominent, yet least understood parts of the Wasatch fault zone (WFZ) in Utah

(fig. 1) (see Machette and others, 1992a, 1992b). Initial studies of this segment of the WFZ by Woodward-Clyde geolo-

gists (Hanson and others, 1981, 1982; Schwartz and others, 1983, 1984) reported evidence for multiple Holocene

surface ruptures at North Creek with only broad bounds of the timing of each event. Machette (USGS) mapped the

surficial geology along the segment in 1983–84 (see Harty and others, 1997), and Michael Jackson (Jackson, 1991)

trenched the southern part of the segment at Red Creek and reconfirmed evidence of three Holocene earthquakes. How-

ever, Jackson’s results did little to refine the original, now two-decade-old chronology. With designation of the Mt. Nebo

Wilderness area, the possibility of further trenching on the segment seemed problematic. However, the Utah Geological

Survey (UGS) recently re-evaluated the segment’s paleoseismic record for seismic hazards (Lund, 2005), and DuRoss and

Bruhn (2005) estimated new slip rates based on the scarp’s morphology. As a result, the UGS identified the Nephi

segment as Utahs’ highest priority for further paleoseismic research (Lund, 2005), and the hunt was on for a new trench-

ing site.

Key questions about the paleoseismic history of the Nephi segment focused on:

(1) Time of the most recent surface-rupturing earthquake (P1) was known to be younger than 750–1,110 yr from charcoal

in a faulted debris flow, but was considered to be considerably younger (300–400 yr old) based on the fault scarp’s

youthful morphology;

(2) The penultimate (P2) earthquake was either >1,650 or >3,640 yr based on conflicting radiocarbon dates from charcoal

and organics in the buried A horizon on fault-scarp colluvium deposited after P2;

(3) An earlier (P3) earthquake was inferred from a fluvial terrace inset into the uplifted block at North Creek; and

(4) The entire faulting history at North Creek was known to be <4,000±400 yr (or <4,580 yr) based on two ages from

charcoal in fan-head deposits adjacent to the trench (Bucknam, 1978). (All above ages are based on uncorrected

radiocarbon dates; that is, in 14C yr B.P.).

The purpose of this report is to publish detailed stratigraphic, structural, and chronologic data required to interpret

the paleoseismic history of the Willow Creek site. Such interpretations, which require discussing the entire Nephi segment

and its relation to the WFZ, will be made in subsequent publications with reference to this report .

WILLOW CREEK TRENCH SITEIn 2004, we identified a small parcel of U.S. Forest Service land at the mouth of Willow Creek (about 5 km west of

Mona, Utah) that was excluded from the Mount Nebo Wilderness area owing to previously established egress to the forest.

At the Willow Creek site, which is near the middle of the southern strand of the Nephi segment (see index map, fig. 2), the

WFZ has vertically displaced alluvial-fan deposits >6–7 m, forming large, steep, multiple-event scarps (see fig. 4). Both

DuRoss and Bruhn (2005) and Machette (field notes, 1984) had surveyed topographic profiles in the area. In May 2005,

we dug two 4- to 5-m-deep backhoe trenches at the Willow Creek site, identified three colluvial wedges in each trench, and

collected samples of charcoal and A-horizon organic material for AMS (acceleration mass spectrometry) radiocarbon

dating, and sampled fine-grained eolian and colluvial sediment for luminescence dating (table 1). The new radiocarbon

dates (table 2) have been calendar corrected using Calib v. 5.0.2 (http://radiocarbon.pa.qub.ac.uk/calib/). These AMS

dates are from charcoal and herbaceous materials (twigs) that were separated by Kathy Puseman of Paleo Research

Institute, Golden, Colo.

The trenches yielded a stratigraphic assemblage composed of moderately coarse grained fluvial and debris-flow

deposits and discrete colluvial wedges associated with three faulting events (P1, P2, and P3). About one-half of the net

vertical displacement is accommodated by monoclinal tilting of fan deposits on the hanging-wall block, possibly related to

massive ductile landslide deposits that are present beneath Willow Canyon. The top of these landslide deposits are

exposed as small islands surrounded by Holocene alluvium of the Willow Creek fan, west of our study area.

WILLOW CREEK NORTH TRENCH (WCN)The Willow Creek north trench site (WCN) is about 150 m north of the Willow Creek (U.S. Forest Service) road, and

north of an east-west jog in the U.S. Forest Service boundary fence (see fig. 5). This trench crossed a 8.7-m-high scarp

(profile m83-36, figs. 4 and 5) on colluvium and fan alluvium deposited on the north margin of the valley of Willow Creek.

The alluvial package exposed in the WCN trench comprises alternating fluvial, debris-flow, and loess deposits (fig. 6, table

3), all parallel bedded but slightly finer grained than the fan-head alluvium in the Willow Creek South trench (WCS).

Although deposits on both sides of the fault appeared correlative, dating shows that the hanging-wall deposits are substan-

tially younger that those on the footwall. We mapped three discrete colluvial wedges, each related to a major surface-

faulting earthquake. As with the south trench, tilting accommodated about one-half of the net vertical tectonic throw.

WILLOW CREEK SOUTH TRENCH (WCS)The Willow Creek south trench (WCS) is about 55 m north of the Willow Creek (U.S. Forest Service) road (fig. 5) and

crossed a 6.3- to 6.9-m-high scarp (profiles m83-33, m83-34, and m83-35; figs. 4 and 5) on fan-head alluvium of Willow

Creek. We suspected that the alluvium is middle Holocene in age, based on a 4,580 14C yr B.P. date from charcoal

(Bucknam, 1978) collected from alluvial deposits at North Creek, 6 km to the north (fig. 2). The alluvial package exposed

in the WCS trench comprises alternating fluvial, debris-flow, and loess deposits, all parallel bedded and largely conform-

able (fig. 7, table 4). The trench exposed correlative deposits on both sides of the fault, as demonstrated by dating. Not

finding correlative deposits is a common problem in paleoseismic investigations, often resulting in minimum estimates of

fault displacement and incomplete paleoseismic records.

We mapped three discrete colluvial wedges, each related to a major surface-faulting earthquake. The oldest wedge

(Cw3) is the smallest, but fills a large fissure along the main fault. This suggests the event P3 may have caused both broad

tilting in the footwall block and brittle faulting. We mapped some minor synthetic faults, but no antithetic faults or grabens

were found in the trench. As a result, about one-half of the net vertical tectonic throw in WCS is related to tilting rather

than brittle faulting.

RELATIVE TIMING OF EVENTSCurrently, our conclusions about the absolute timing of surface faulting events at Willow Creek are limited. The

luminescence age estimates (table 1) and radiocarbon dates (table 2) provide broad timing constraints. Two dates from the

WCS trench tightly bracket the most recent faulting event (P1) at between 270 and 315 calendar yr ago (see results of

radiocarbon dating, table 2). Event P2 is probably older than 1,227±54 calendar yr since this is the age of organic

material collected from the A horizon of the soil formed on the P2 colluvium (unit Cw2 in the WCN trench, fig. 7). The

oldest recorded event (P3) in the Willow Creek trenches must be much older than P2 (>1,227 calendar yr), but younger

than faulted deposits in the footwall, which are dated at about 2,500 to 3,150 calendar yr (dates generalized from tables 1

and 2).

Combining our new dates with those previously published for the North Creek and Red Canyon sites, our analyses

suggest new and interesting interpretations. Our preliminary interpretations of the timing of major surface-faulting events

at Willow Creek are as follows:

P1—probably about 300 yr ago,

P2—probably about 1,230 yr ago, and

P3—less than 2,320 yr ago.

These times are much younger than those from the North Creek and Red Canyon sites and will have to be reconciled

in light of our new data. In addition, new studies at the Santaquin site (fig. 2) should reveal timing for the northern strand

for the Nephi segment of the WFZ.

Figure 1. Major Quaternary faults in northern Utah. Red box shows location of Nephi segment of Wasatch fault zone

(fig. 2). Faults from Black and others (2003); shaded-relief digitial elevation model (DEM) created by Chris DuRoss

(UGS, November 2006). Abbreviations: WFZ, Wasatch fault zone; WVFZ, West Valley fault zone; SLC, Salt Lake City.

110°

42°

41°

40°

39°

112°114°

Figure 5. Detailed topographic map of the Willow Creek site showing trenches, profile locations and other features.

PALEOSEISMOLOGY OF THE NEPHI SEGMENT OF THE WASATCH FAULT ZONE, JUAB COUNTY, UTAH—PRELIMINARY RESULTS FROM TWO LARGE EXPLORATORY TRENCHES AT WILLOW CREEK

ByMichael N. Machette1, Anthony J. Crone1, Stephen F. Personius1, Shannon A. Mahan1, Richard L. Dart1, David J. Lidke1, and Susan S. Olig2

1U.S. Geological Survey, Denver, Colorado, and 2URS Corporation, Oakland, California2007

SCIENTIFIC INVESTIGATIONS MAP 2966 SHEET 1 OF 2 Version 1.0

U.S. DEPARTMENT OF THE INTERIORU.S. GEOLOGICAL SURVEY

Manuscript approved for publication April 1, 2007

Any use of trade names is for descriptive purposes only and does not emply endorsement by the U.S. Government

This map was produced on request, directly from digital files, on an electronic plotter

For sale by U.S. Geological Survey Information Services, Box 25286, Federal Denver, Denver CO 80225

1-888-ASK-USGS

A PDF for this map is available at http://pubs.usgs.gov/sim/2007/2966