Fault Segmentation: New Concepts from the Wasatch 10.1002/2015JB012519 Fault Zone, Utah, USA Key Points: Christopher B

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Journal of Geophysical Research: Solid Earth

RESEARCH ARTICLE Fault segmentation: New concepts from the Wasatch 10.1002/2015JB012519 Fault Zone, Utah, USA Key Points: Christopher B. DuRoss1, Stephen F. Personius1, Anthony J. Crone2, Susan S. Olig3,4, • Wasatch fault segmentation evaluated 5 6 7 via synthesis of late Holocene Michael D. Hylland , William R. Lund , and David P. Schwartz

paleoearthquakes 1 2 3 • Complex ruptures shorter or longer U.S. Geological Survey, Golden, Colorado, USA, U.S. Geological Survey-Emeritus, Golden, Colorado, USA, Olig Seismic 4 than the primary segment lengths Geology, Inc., Martinez, California, USA, Formerly at URS Corporation, Seismic Hazards Group, Oakland, California, USA, are possible 5Utah Geological Survey, Salt Lake City, Utah, USA, 6Utah Geological Survey-Emeritus, Cedar City, Utah, USA, 7U.S. • Wasatch fault compared to other Geological Survey, Menlo Park, California, USA multisegment normal faults

Abstract The question of whether structural segment boundaries along multisegment normal faults Correspondence to: such as the Wasatch fault zone (WFZ) act as persistent barriers to rupture is critical to seismic hazard C. B. DuRoss, analyses. We synthesized late Holocene paleoseismic data from 20 trench sites along the central WFZ to [email protected] evaluate earthquake rupture length and fault segmentation. For the youngest (<3 ka) and best-constrained earthquakes, differences in earthquake timing across prominent primary segment boundaries, especially for the Citation: most recent earthquakes on the north-central WFZ, are consistent with segment-controlled ruptures. However, DuRoss, C. B., S. F. Personius, A. J. Crone, broadly constrained earthquake times, dissimilar event times along the segments, the presence of smaller-scale S. S. Olig, M. D. Hylland, W. R. Lund, and D. P. Schwartz (2016), Fault segmentation: (subsegment) boundaries, and areas of complex faulting permit partial-segment and multisegment (e.g., New concepts from the Wasatch Fault spillover) ruptures that are shorter (~20–40km)orlonger(~60–100 km) than the primary segment lengths Zone, Utah, USA, J. Geophys. Res. Solid – – (35 59 km). We report a segmented WFZ model that includes 24 earthquakes since ~7 ka and yields mean Earth, 121,1131 1157, doi:10.1002/ – – 2015JB012519. estimates of recurrence (1.1 1.3 kyr) and vertical slip rate (1.3 2.0 mm/yr) for the segments. However, additional rupture scenarios that include segment boundary spatial uncertainties, ﬂoating earthquakes, and multisegment Received 10 SEP 2015 ruptures are necessary to fully address epistemic uncertainties in rupture length. We compare the central WFZ to Accepted 6 JAN 2016 paleoseismic and historical surface ruptures in the Basin and Range Province and central Italian Apennines Accepted article online 11 JAN 2016 ﬁ Published online 18 FEB 2016 and conclude that displacement pro les have limited value for assessing the persistence of segment boundaries but can aid in interpreting prehistoric spillover ruptures. Our comparison also suggests that the probabilities of shorter and longer ruptures on the WFZ need to be investigated.

1. Introduction Multisegment normal faults in areas of active horizontal extension, such as the Wasatch fault zone (WFZ) in the Basin and Range Province (BRP) of the western United States, pose a conundrum for seismic hazard assessments: will the individual faults that linked to form these structures continue to act independently, or will they eventually form a continuous, mature fault zone that experiences throughgoing ruptures across relict structural complexities [Scholz and Gupta, 2000; Mirabella et al., 2005]? That is, are prominent structural complexities along multisegment normal faults (e.g., step overs or gaps in faulting) effective barriers to rupture [e.g., Aki, 1979; Schwartz and Coppersmith, 1984; King and Nabelek,1985;Schwartz,1989;Wesnousky, 2008], or do the faults ultimately interact, link, and bypass the complexities [e.g., Peacock and Sanderson, 1991; Willemse et al., 1996; Scholz and Gupta,2000;Chang and Smith,2002;Manighetti et al., 2007, 2015]? The question of what drives rupture initiation and termination is important to seismic hazard analyses. Controlling factors include a fault’s geometry, nearby crustal rheology, failure criterion (friction law), and initial stress conditions that stem from the history of strain accumulation and moment release on the fault, ﬂuid pressure, and the degree of mechanical interaction and stress triggering from nearby structures [Harris, 1998, 2004]. Mechanically interacting faults are those that interact and inﬂuence each other’s distribution of stresses, earthquake timing, and displacement [e.g., Peacock and Sanderson, 1991; Willemse et al., 1996; Gupta and Scholz, 2000; Scholz and Gupta, 2000]. However, evidence of mechanical interaction between faults or segments does not necessarily imply synchronous rupture, but only that the faults do not act independently. The potential for throughgoing rupture may relate to this degree of interaction [Scholz and Gupta, 2000]. Fault maturation or normal fault growth as a function of alternating periods of displacement accumu-

©2016. American Geophysical Union. lation and lateral propagation may also control the process of fault segmentation [Manighetti et al., 2015]. All Rights Reserved. Ultimately, for faults lacking historical surface-faulting earthquakes, the question of what controls rupture

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length is a critical one as it drives estimates of seismic moment release, earthquake recurrence, and moment magnitude for individual rupture sources [e.g., Wells and Coppersmith, 1994; Stirling et al., 2002; Manighetti et al., 2007; Wesnousky, 2008] and thus has the potential to impact local and regional probabilistic seismic hazard assessments [e.g., Working Group on California Earthquake Probabilities (WGCEP), 2003, 2008; Field et al., 2014; Petersen et al., 2014; Wong et al., 2016]. In this study, we consider the degree to which paleoseismic data can help resolve prehistoric rupture boundaries. Even the best-constrained paleoearthquakes have timing uncertainties of tens [e.g., Scharer et al., 2007, 2014; Shennan et al., 2014] to hundreds [e.g., Lund, 2005; DuRoss, 2008] of years. Thus, it is not possible to unequivocally differentiate between synchronous rupture of two adjacent segments and the occurrence of separate earthquakes spaced closely in time (i.e., minutes to decades) [DuRoss and Hylland, 2015]. Further, although per event displacements and along-strike rupture proﬁles can limit rupture dimensions and magni- tudes [e.g., Hemphill-Haley and Weldon, 1999; Scholz and Gupta, 2000; Biasi and Weldon, 2006; Wesnousky, 2008; Hecker et al., 2010], the results are nonunique. For example, an anomalously large displacement near a segment boundary could be evidence of either interacting faults [e.g., Willemse et al., 1996] or natural slip variability on an isolated fault [e.g., Hecker et al., 2013]. Thus, paleoseismologists must rely on a combination of surface-fault geometry and complexity, comparison of paleoseismic earthquake-timing and displacement data, probabilistic estimates of rupture length, and historical surface-faulting analogs to quantify past rupture behavior and provide input data for seismic hazard assessments. In this paper, we examine the issues of fault segmentation and rupture length through the lens of the WFZ—one of the best-studied, multisegment normal faults in the world. Our goal is to evaluate the segmentation of the WFZ using a compilation of late Holocene earthquake-timing and displacement data for the central, most active, part of the fault, and to compare our results with those for other well-studied normal faults in extensional provinces.

1.1. The Wasatch Fault Zone The WFZ is an approximately 370 km long, multisegment normal fault that forms a prominent structural and physiographic eastern boundary (the Wasatch Front) to the ~700 km wide BRP [Gilbert, 1884, 1928; Cluff et al., 1970, 1973]. For the central WFZ (Figure 1), faulting began ~18 Ma, based on ﬂuid inclusions in fault rock exhumed from ~11 km depth [Parry and Bruhn, 1987]. Vertical slip rates on the WFZ are higher than those for nearby normal faults west of the BRP margin [e.g., Lund, 2005] and apparently have varied through time. Slip rates based on range-front exhumation data are about ~0.7 mm/yr since ~18 Ma [Parry and Bruhn, 1987], mostly ~0.2–0.4 mm/yr since ~5 Ma [Armstrong et al., 2004] and ~0.5–1.0 mm/yr since ~0.4–0.8 Ma [Mayo et al., 2009]. In contrast, displaced late Pleistocene geomorphic surfaces yield vertical slip rates of ~0.1–0.3 mm/yr [Machette et al., 1992]. Slip rates based on latest Pleistocene to Holocene paleoseismic trenching data are ~1–2mm/yr [e.g., Machette et al., 1992; Friedrich et al., 2003; Lund, 2005]. Contemporary horizontal extension across the WFZ is ~1.6–3.2 mm/yr [Chang et al., 2006; Kreemer et al., 2010], which is about 50–90% of the strain budget for the 200 km wide eastern BRP [Dixon et al., 2000; Chang et al., 2006]. The WFZ has been divided into 10 structural segments based on along-strike changes in fault geometry, displacement, and timing of most recent movement [Swan et al., 1980; Schwartz and Coppersmith, 1984; Machette et al., 1992; Wheeler and Krystinik, 1992]. Five of these segments (from Brigham City to Nephi; herein the central WFZ; Figure 1) have abundant stratigraphic and chronologic evidence of recurrent Holocene surface-faulting earthquakes derived from paleoseismic investigations of the fault conducted over nearly four decades (summarized by Machette et al. [1992], McCalpin and Nishenko [1996], Lund [2005], and DuRoss [2008]). For the central WFZ, the interpretation of persistent segmentation stems from observations that the most prominent along-strike changes in fault geometry (e.g., fault orientation and complexity) and long-term displacement (e.g., range-front relief and the presence of unique hanging wall basins bounded by bedrock) coincide with differences in the timing of late Holocene earthquakes determined from paleoseismic investigations on the fault [Schwartz and Coppersmith,1984;Machette et al., 1992]. Although several studies have evaluated the potential for ruptures outside of the segmented fault model [e.g., Chang and Smith,2002; DuRoss,2008;Personius et al., 2012], most treat the segments as seismogenically independent (e.g., Working Group on Utah Earthquake Probabilities (WGUEP)) [Wong et al., 2016]. That is, although a small degree (few kilometers) of rupture overlap on adjacent segments (e.g., spillover rupture) [Crone and Haller, 1991] may occur, they are essentially considered to function as separate and independent sources of large earthquakes. Less is known regarding the segmentation of the northern and southern ends of the WFZ, which include three

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northernmost and two southernmost “end” segments [Machette et al., 1992]. Compared to the central WFZ segments, the end segments have much longer elapsed times since their most recent faulting (e.g., >18 kyr for the northernmost segments), much lower vertical slip rates (~0.01–0.6 mm/yr), and limited to no individual earthquake-timing data [Machette et al., 1992; Hylland, 2007; Hylland and Machette, 2008]. In this paper, we evaluate the fundamental issue of normal fault segmentation using central WFZ paleoseismic data. Paleoseismic trench investigations of the central WFZ provide information on the timing and displacement of surface-faulting (M ~7) earthquakes; however, individual rupture lengths remain uncertain. For example, both Schwartz and Coppersmith [1984] and Machette et al. [1992] considered the individual segments likely to have dis- crete surface-rupturing earthquakes but could not discount the possibility of partial-segment or multisegment ruptures. So how should epistemic uncertainties in rupture length be accounted for? Possible approaches include adding a spatial uncertainty to each segment boundary [e.g., WGCEP, 2003], constructing rupture models that include combinations of single-

Figure 1. Central segments of the WFZ (red), which have evidence of repeated Holocene surface-faulting earthquakes. Circles indicate sites with data that we reanalyzed using OxCal (abbreviations shown in Table 2); triangles indicate sites where data or documentation was inadequate for reanalysis (HC, Hobble Creek; PP, Pole Patch; WC, Water Canyon; WH, Woodland Hills). Other Quaternary faults in northern Utah (white lines) include the ECFZ, East Cache fault zone; OGSLFZ, Oquirrh Great Salt Lake fault zone; ULFF, Utah Lake faults and folds; WVFZ, West Valley fault zone. Fault traces are from Black et al. [2003]. Horizontal bars mark primary segment boundaries. Inset map shows the trace of the WFZ in northern Utah and southern Idaho. Base map is 10 m digital elevation model [Utah Automated Geographic Reference Center, 2015].

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Figure 2. Primary segments, secondary subsegments, and rupture types for part of a multisegment normal fault. Primary segments are those based on the most prominent, first-order segment boundaries (e.g., Figure 3). Less prominent structural complexities (e.g., a fault step or change in strike) within a primary segment define secondary subsegments. Rupture modes highlight five possible ruptures of the segments and/or subsegments that we categorize as either single-segment, partial-segment, or multisegment ruptures.

segment and multisegment ruptures [e.g., Chang and Smith, 2002; WGCEP, 2008; Wong et al., 2016], and utilizing earthquakes of variable rupture length that are allowed to ﬂoat along the fault irrespective of the segment boundaries (herein referred to as ﬂoating ruptures) [e.g., Petersen et al., 2008, 2014]. Further, for the WFZ, which paleoseismic data are most critical to address questions of how structural features control surface rupture? Finally,howdoestherupturebehavioroftheWFZcomparetonormalfaultsinotherareasofactiveextension? The WFZ provides a unique natural laboratory for addressing questions of normal fault structural segmentation and rupture length. Here we (1) evaluate the segmentation of the WFZ using a compilation of late Holocene paleoseismic data, (2) evaluate methods of treating the WFZ in both a segmented and unsegmen- ted structure, (3) evaluate WFZ displacement and segmentation in the context of several historical BRP surface ruptures, and (4) compare the WFZ paleoseismic data to that of a well-studied multisegment normal fault system in the central Apennines of Italy.

2. Rupture Behavior of the Central Wasatch Fault Zone 2.1. Definition of Terms and Data Sources In this paper, we refer to primary segments, subsegments, and several rupture types (Figure 2). We refer to the first-order segment boundaries as primary boundaries. Less prominent structural complexities within a primary segment define secondary or subsegment boundaries (similar to the major and secondary boundaries of Manighetti et al. [2015]). In terms of rupture types, an individual or single-segment rupture refers to the entire rupture of a single segment, where the rupture ends are at primary boundaries. A single-segment rupture that continues across a primary boundary and onto part of an adjacent segment is considered to be a spillover rupture. Partial-segment rupture refers to the rupture of only part of one primary segment, where at least one rupture end does not extend to a primary boundary. Multisegment ruptures are those that extend beyond at least one primary boundary and involve rupture of two or more segments. These ruptures could end at primary boundaries (full multisegment ruptures) or subsegment boundaries (spillover rupture). Primary segments along the central WFZ are derived from Machette et al. [1992] and Wheeler and Krystinik [1992] and are 35 to 59 km long (Table 1). Figure 3 shows structural, topographic, and geophysical anomalies along the fault (north-south distance) interpreted by Wheeler and Krystinik [1992]. We stacked the minimum and maximum ranges in the extents of these anomalies [Wheeler and Krystinik 1992, Figure 5], which correspond well with areas of general fault complexity that we defined (e.g., fault branches, gaps, steps, and changes in strike; Figure 3). These data show that the Weber-Salt Lake City and Provo-Nephi segment boundaries are most prominent.

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Table 1. Primary and Secondary Segments of the Central WFZ Subsegment a b c d e Primary Segment Lprimary (km) Subsegment Lsubseg (km) Lsubseg /Lprimary Boundary Brigham City 35 Northern 18 0.51 60° fault bend Southern 17 0.49 Weber 56 - - - - Salt Lake City 40 Warm Springs fault 10 0.25 3 km fault step East Bench fault 12 0.30 2 km fault step Cottonwood fault 20 0.50 Provo 59 American Fork 25 0.42 1.5 km fault step Central 21 0.36 60° fault bend Spanish Fork 22 0.37 Nephi 43 Northern 17 0.40 4 km fault step Southern 26 0.58 aPrimary WFZ segments corresponding with Figures 1 and 3. bPrimary segment linear length. cSubsegment based on Personius [1990], Personius and Scott [1992], Machette [1992], and Harty et al. [1997]. dSubsegment linear length. eRatio of subsegment length to primary fault length.

The Brigham City-Weber and Salt Lake City-Provo segment boundaries are less prominent, but multiple lines of evidence (e.g., bedrock salients and complex faulting) support their existence. Importantly, the segment boundaries have broad, ~10 to 20 km wide spatial dimensions, and minor anomalies and areas of fault complexity occur between them (e.g., the branches and steps in faulting within the Provo segment, Figure 3). We mapped subsegment boundaries along the central WFZ using two criteria: fault steps and along-strike changes in fault strike. Outside of the primary segment boundaries, we identiﬁed three fault steps ranging from about 1.5 to 4.0 km in fault-normal width. Along-strike changes in fault orientation are based on at least 3 km long

Figure 3. Geologic and geophysical anomalies along the central WFZ from Wheeler and Krystinik [1992]. Light and dark shading show maximum and minimum extents of anomalies, respectively, which we stacked in the panel labeled “sum.” Lowermost panel shows fault complexity, where we identiﬁed changes in fault strike, hanging wall (HW) bedrock, and fault gaps, steps, and branch points. The stacked anomalies and areas of greatest fault complexity correspond well with the four main segment boundaries of Machette et al. [1992] (white vertical dashed lines on trace map). Arrows along trace map indicate subsegment boundaries (Table 1).

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Figure 4. Late Holocene surface-faulting earthquakes identiﬁed at trench sites along the central WFZ. Circles with labels indicate sites with data that were reanalyzed using OxCal, and unlabeled white triangles indicate sites where data or documentation was inadequate for reanalysis. Distance is measured along simpliﬁed fault trace (dash dotted line) shown in top panel. Segment boundaries correspond to Figure 3 and Machette et al. [1992]. Individual earthquake-timing probability density functions (PDFs) and mean times are derived from OxCal models for the paleoseismic sites; number in brackets is event number, where one is the youngest. See Table 2 for site abbreviations and earthquake times and text for discussion.

fault sections forming a 60° or greater bend (inside angle of 120° or less). Although other fault complexities such as branch points, fault intersections, and bedrock salients are present along the fault, we do not use them to define subsegments. The role of branch points and high-angle fault intersections in rupture propagation is generally unknown, but possibly minor if the features are not persistent over multiple earthquake cycles [Machette et al., 1992]. Bedrock salients may be more deep-rooted features capable of arresting or modulating slip, but they are only observed at the primary segment boundaries [e.g., Wheeler and Krystinik, 1992] (Figure 3). We consider other major structural complexities not discussed here (e.g., on the northern Salt Lake City segment or between the Provo and Nephi segments) to be part of the uncertainty in the primary segment boundaries. Four of the five central WFZ segments have at least one subsegment boundary, or significant structural complexity (Figure 3), yielding subsegments that are 10 to 26 km long (Table 1). The Weber segment does not have subsegments using our criteria. On the Brigham City segment, a prominent fault bend [Personius, 1990] defines an 18 km long northern subsegment and 17 km long southern subsegment. The Salt Lake City segment includes three 10 to 20 km long subsegments—north to south, the Warm Springs, East Bench, and Cottonwood faults [Bruhn et al., 1992; Personius and Scott, 1992]. Three subsegments along the Provo segment are 21 to 25 km long—the American Fork, central, and Spanish Fork subsegments [Machette et al., 1992; Machette, 1992]. On

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Table 2. Earthquake-Timing and Displacement Data for Site Earthquakes on the Central WFZ EQ Timed (cal yr B.P.) Per Event Vertical Displacemente (m)

Segmenta Trench Siteb Earthquake no.c Site EQc Mean 2σ Mean/Pref. ± Range

BCS Kotter Canyon (KC) 1 KC1 2450 290 2.1 0.2 BCS Bowden Canyon (BC) 1 BC1 2380 990 - - BCS Box Elder Canyon (BEC) 1 BEC1 2240 570 >0.9 0.4 BCS Pearsons Canyon (PC) 1 PC1 1240 50 0.45 0.35 BCS KC 2 KC2 3510 330 2.1 0.2 BCS BC 2 BC2 3400 550 1.0 0.1 BCS BEC 2 BEC2 3250 490 >1.0 0.1 BCS BC 3 BC3 4490 610 2.5 0.3 BCS BEC 3 BEC3 4440 1080 - - BCS BC 4 BC4 5770 1640 2.5 0.3 BCS BEC 4 BEC4 5680 730 >1.1 0.2 WS Rice Creek (RC) 1 RC1 570 80 2.0 0.7 WS Garner Canyon (GC) 1 GC1 640 420 1.2 0.2 WS East Ogden (EO) 1 EO1 460 220 0.7 0.2 WS Kaysville (K) 1 K1 590 210 1.8 0.1 WS RC 2 RC2 1220 250 3.2 0.5 WS GC 2 GC2 1510 470 1.5 0.7 WS EO 2 EO2 890 380 2.6 0.3 WS K 2 K2 940 510 - - WS RC 3 RC3 3370 730 1.1 0.3 WS GC 3 GC3 3240 570 1.0 0.1 WS EO 3 EO3 2960 410 4.2 0.4 WS K 3 K3 2820 1700 2.9 0.6 WS RC 4 RC4 4560 490 2.0 0.4 WS GC 4 GC4 4400 560 - - WS EO 4 EO4 3950 920 4.2 0.4 WS K 4 K4 5750 1310 1.4 0.1 WS RC 5 RC5 5990 1010 2.0 0.4 SLCS Penrose Drive (PD) 1 PD1 4000 520 1.4 0.4 SLCS Little Cottonwood Cyn. (LCC) 1 LCC1 1340 40 1.8 0.2 SLCS South Fork Dry Cr. (SFDC) 1 SFDC1 1350 230 2.0 0.5 SLCS PD 2 PD2 5890 710 10 0.3 SLCS LCC 2 LCC2 2110 280 1.8 0.2 SLCS SFDC 2 SFDC2 1980 460 2.0 0.5 SLCS LCC 3 LCC3 4440 550 1.8 0.2 SLCS SFDC 3 SFDC3 3760 600 - - SLCS LCC 4 LCC4 5530 810 1.8 0.2 SLCS SFDC 4 SFDC4 4980 550 - - PS American Fork (AF) 1 AF1 390 210 2.5 0.3 PS Rock Canyon (ROC) 1 ROC1 600 70 3.3 0.3 PS Mapleton North (MN) 1 MN1 570 70 4.7 0.5 PS Mapleton South (MS) 1 MS1 680 670 - - PS AF 2 AF2 2010 780 2.5 0.3 PS MN 2 MN2 1480 380 1.4 0.9 PS MS 2 MS2 2210 820 - - PS AF 3 AF3 4310 1480 2.5 0.3 PS MN 3 MN3 3200 1560 - - PS AF 4 AF4 6180 1030 - PS MN 4 MN4 4710 290 - - PS MN 5 MN5 5600 530 - - NS Spring Lake (SL) 1 SL1 880 220 1.1 0.3 NS Santaquin (SQ) 1 SQ1a 460 110 3.0 0.2 NS SQ 1 SQ1b 340 240 3.0 0 NS North Creek (NC) 1 NC1 220 100 2.1 0.7 NS Willow Creek (WIC) 1 WIC1 210 90 2.2 0.4 NS Red Canyon (REC) 1 REC1 490 530 1.4 0.3 NS SL 2 SL2 2950 720 0.8 0.2 NS NC 2 NC2 1170 120 2.1 1 NS WIC 2 WIC2 1220 120 2.2 0.4 NS REC 2 REC2 1170 350 1.5 0.2

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Table 2. (continued) EQ Timed (cal yr B.P.) Per Event Vertical Displacemente (m)

Segmenta Trench Siteb Earthquake no.c Site EQc Mean 2σ Mean/Pref. ± Range

NS SL 3 SL3 3970 520 1.0 0.25 NS NC 3 NC3 2590 890 2.1 1.2 NS WIC 3 WIC3 1940 530 2.2 0.4 NS REC 3 REC3 4700 2670 1.7 0.3 NS SL 4 SL4 4770 750 0.9 0.2 NS NC 4 NC4 4020 90 2.1 0.7 NS WIC 4 WIC4 4690 1780 - - NS SL 5 SL5 5720 760 1.0 0.25 NS NC 5 NC5 4670 710 2.1 0.9 NS SL 6 SL6 6640 730 1.1 0.3 aCentral WFZ segment abbreviations: BCS, Brigham City; WS, Weber; SLCS, Salt Lake City; PS, Provo; and NS, Nephi. bAbbreviations correspond to site earthquake names and Figures 1 and 4. cSite earthquake number, where 1 is the youngest, and name (e.g., KC1). Compilation includes all earthquakes younger than 7 ka on the central WFZ. dSite earthquake times (rounded to the nearest decade) derived from OxCal models; see text for source information. ePer event vertical displacements for site earthquakes; see text for source information.

the Nephi segment, two subsegments are separated by a 4 km wide fault step in bedrock [Machette et al., 1992]. A 17 km long northern subsegment overlaps 13 km with the southernmost Provo segment (forming the primary Nephi-Provo segment boundary), and a 26 km long southern subsegment extends to the southern terminus of the Nephi segment [Harty et al., 1997; DuRoss and Bruhn, 2005]. The number and lengths of the central WFZ subsegments are consistent with the results of Manighetti et al. [2015], who evaluated ~900 normal faults in Afar, East Africa. The five primary central WFZ segments each have zero to three subsegments, which are on average 19 km long and yield ratios of subsegment length to primary segment length of about 0.3 to 0.6 (mean of 0.4). Manighetti et al. [2015] found that about 90% of the Afar normal faults studied have two to five primary segments. Of the primary segments, ~70% are divided further and have two to four subsegments, with most having two. Ratios of subsegment length to primary segment length depend on the number of subsegments and have peak values at about 0.2 (four to five subsegments) and 0.3–0.5 (two to three subsegments). We used existing paleoseismic data to assess the persistence of segment and subsegment boundaries along the central WFZ. These data are derived from 20 paleoseismic sites investigated between 1978 and 2012 (Figure 4 and Table 2), excluding results from incomplete or unpublished investigations or those having insufficient numerical data for evaluating earthquake timing (Figure 1). We compiled timing data for 67 earthquakes younger than ~7 ka at these individual sites (Figure 4). The majority of these data are from trench investigations across <10 m high fault scarps in Holocene alluvial fan deposits [e.g., Crone et al., 2014]. Some sites targeted larger (~10 to 20 m high) scarps in latest Pleistocene Lake Bonneville lacustrine (or Bonneville-age alluvial fan) deposits [e.g., McCalpin, 2002; McCalpin and Forman, 2002; Olig et al., 2011; DuRoss and Hylland, 2015]. Most earthquake times are numerically constrained by radiocarbon and lumines- cence ages, the limitations of which are described by Nelson et al. [2006] and DuRoss et al. [2011]. Here we compiled the per site earthquake-timing distributions (probability density functions, PDFs) exported from OxCal models [Bronk Ramsey, 2001, 2008] constructed by DuRoss et al. [2011] for the Weber segment, Personius et al. [2012] for the Brigham City segment, Crone et al. [2014] for the Nephi segment, and DuRoss and Hylland [2015] for the Salt Lake City segment. Earthquake-timing data for the Provo segment are derived from Machette et al. [1992], Lund and Black [1998], and Olig et al. [2011]. We also include new results for the Spring Lake and North Creek sites on the Nephi segment [DuRoss et al., 2014a], which are not included in the Crone et al. [2014] synthesis. Wong et al. [2016] used data predating the Spring Lake and North Creek investigations to provide input data for the WGUEP; however, in this study we include the most up-to-date paleoseismic data and critically evaluate evidence that facilitates evaluation of fault segmentation. Estimates of per event vertical displacement used in this report are from DuRoss [2008], DuRoss et al. [2009, 2012, 2014a, 2014b], Olig et al. [2011], and DuRoss and Hylland [2015]. Displacement data are generally spar- sely distributed along the fault, and values range from ~0.5 to 4.7 m, though most are ~1–3 m. Limitations in

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the displacement data are related to assumptions of how colluvial wedge thickness relates to fault displacement and the reporting of per site average displacements (e.g., the total displacement divided by the number of events observed) [DuRoss, 2008].

2.2. Observations and Results 2.2.1. Site Earthquake Data We draw two observations from our compilation of the timing of earthquakes at individual sites (site PDFs; Figure 4 and Table 2). First, the individual site earthquakes have moderate timing uncertainties that are mostly related to the number and type of limiting ages used to constrain earthquake timing (e.g., bulk soil versus charcoal 14C dating) [Nelson et al., 2006; DuRoss et al., 2011]. The general pattern is that the youngest site earthquakes are better constrained than older events. For example, the youngest (post ~2.6 ka) earthquakes have a mean 2 sigma timing uncertainty of 370 years, which is about 45% of that for the older (pre ~2.6 ka) earthquakes (830 years). These younger records are also more complete and in some cases have per event displacement measurements [e.g., DuRoss, 2008]. Second, the site data are unevenly distributed, both tempo- rally and spatially, along the central WFZ. Some primary segments and segment boundaries have clusters of data (e.g., the central Brigham City segment and Brigham City-Weber segment boundary; Figure 4), and each segment has at least one ~15 to 30 km long data gap. Further, several sites only identified a single earthquake. We identified 36 earthquakes postdating 3 ka from 19 separate sites, in contrast to 31 earthquakes predating 3 ka from 16 sites (Figure 4). These data indicate that the youngest (post 3 ka) earthquakes are the best constrained and most abundant and therefore are best suited to evaluate along-strike segmentation. 2.2.2. Primary Segment Boundaries Significant differences are apparent in the timing of the most recent earthquakes (MREs) along the three northern primary segments of the central WFZ (Figure 5a). Specifically, sites on the Weber segment have earthquakes at ~0.5–0.6 ka, including a well-constrained earthquake at 0.6 ± 0.08 ka immediately south of the segment boundary with the Brigham City segment (Rice Creek site) [DuRoss et al., 2011]. On the Brigham City segment, no earthquakes postdate ~1.2 ka near the segment boundary (Pearsons Canyon site) or ~2.4 ka near the center of the segment (Kotter Canyon, Bowden Canyon, and Box Elder Canyon sites) [Personius et al., 2012]. Similarly, on the Salt Lake City segment, the youngest earthquake occurred at ~1.3 ka on the southern part of the segment (Little Cottonwood Canyon and South Fork Dry Creek sites) and possibly ~4.0 ka on the northern part (Penrose Drive site) [DuRoss and Hylland, 2015]. Sites on the Weber segment yield a broad range (0.7–1.7 ka) for the time of the penultimate earthquake (PE). The best-constrained earthquake (1.2 ± 0.3 ka) was documented at the Rice Creek site and likely corresponds with an earthquake dated at 1.2 ± 0.05 ka at the Pearsons Canyon site on the southern Brigham City segment [DuRoss et al., 2012; Personius et al., 2012]. A 1.2 ka earthquake on the Weber segment, with spillover rupture to the Brigham City segment, clearly postdates the MRE of ~2.2–2.6 ka at sites on the central Brigham City segment. However, the southern extent of this rupture is unclear. The PEs at the East Ogden and Kaysville sites are ~0.9 ka but have large (±0.4–0.5 kyr) uncertainties, which prevents us from determining whether these earthquakes correlate with the ~1.2 ka earthquake at Rice Creek or are a separate rupture at ~0.9 ka [DuRoss et al., 2011]. Farther south, the 1.2 ka time of the Rice Creek PE overlaps with the ~1.3 ka Salt Lake City segment earthquake identified at the Little Cottonwood Canyon and South Fork Dry Creek sites. Although no earthquakes younger than ~4 ka were exposed at the Penrose Drive site near the north end of the Salt Lake City segment [DuRoss and Hylland, 2015], the site is on the northernmost East Bench fault, in an area of overlap with the Warm Springs fault (Figure 1), and thus, the late Holocene record of earthquakes on the Salt Lake City segment at this latitude could be incomplete. Ultimately, these data show clear evidence of rupture arrest near the Brigham City-Weber segment boundary, but with a spillover rupture that extends about 8 km onto the Brigham City segment. Synchronous rupture of the Weber and Salt Lake City segments is possible, but we consider the evidence in support of this interpretation to be weak considering the lack of overlap in the two-sigma time ranges for the Kaysville PE and the MREs at the Little Cottonwood Canyon and South Fork Dry Creek sites, and the lack of post 4 ka earthquakes at the Penrose Drive site (Figure 5a and Table 2). Older earthquakes on the Salt Lake City and Weber segments generally have broad timing distributions and thus are less useful for making along-strike correlations (Figure 4). For example, four sites on the Weber segment identified earthquakes at ~2.8–3.4 ka that have broad time ranges (2 sigma uncertainties of 0.4–1.7 ka;

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Figure 5. Possible correlation of the youngest and best-constrained site earthquakes along the central WFZ, showing (a) signiﬁcant differences in the timing of the youngest earthquake along the northern part of the central WFZ and (b) overlapping earthquake times for the southern part of the central WFZ. Numbers denote relative earthquake timing at each site (abbreviations shown in Table 2). Boxes highlight site earthquakes that may correspond to single-segment ruptures (with the exception of a spillover rupture from the Weber to Brigham City segment) and approximate earthquake- timing uncertainties; lighter shading indicates rupture extent poorly constrained or unknown. Dash dotted line is simpliﬁed fault trace for measuring distance south along fault.

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Table 2). Although a Weber segment earthquake at ~2.8–3.4 ka clearly predates the Brigham City MRE at ~2.2–2.6 ka, it does overlap with the PE on the Brigham City segment at ~3.2–3.5 ka. The ~2.8–3.4 ka Weber segment earthquake also overlaps with the PE on the Salt Lake City at ~2.0–2.1 ka, but we have low confidence in this correlation because the time of the earthquake on the southernmost Weber segment is poorly constrained (2.8 ± 1.7 ka at the Kaysville site). These data allow for multiple interpretations, including single-segment and synchronous ruptures of the Brigham City and Weber segments. For the southern part of the central WFZ, which includes the primary segment boundaries between the Salt Lake City, Provo, and Nephi segments (Figure 5b), the youngest earthquakes are clustered so closely in time that differences in timing between them do not consistently exceed the individual earthquake-timing uncertainties. The best example of segment boundary control is the youngest earthquakes identified at sites along the Provo segment at ~0.4–0.7 ka, which clearly postdate the youngest Salt Lake City segment earthquake at ~1.3 ka. The extent of the 0.4–0.7 ka Provo rupture to the south is less certain because of earthquakes on the Nephi segment that have similar time ranges. However, using the best-constrained Provo segment earthquakes of 0.6 ± 0.07 ka at Rock Creek and 0.6 ± 0.07 ka at Mapleton North to define the time of this Provo segment rupture, it predates earthquakes constrained to ~0.2–0.5 ka on the Nephi segment (Santaquin, North Creek, and Willow Creek sites) [DuRoss et al., 2008; Crone et al., 2014]. Although an earthquake at ~0.4 ka at the American Fork site (northern Provo segment) overlaps with the 0.2–0.5 ka Santaquin, North Creek, and Willow Creek events, we have low confidence in this possible correlation given the well-constrained, youngest ruptures (at ~0.6 ka) at the Rock Creek and Mapleton North sites. The Provo segment MRE may be an example of a single-segment rupture, but alterna- tive interpretations are possible, such as partial-segment rupture at the American Fork site at ~0.4 ka, separate from the rest of the Provo segment, and rupture of the Santaquin site on the northern Nephi segment separate from the southern Nephi segment, and possibly synchronously with the Provo segment at ~0.5 ka. The PDFs for older earthquakes on the southern part of the central WFZ overlap significantly across the segment boundaries (Figure 5b). For example, the ~2.1–2.2 ka PE on the Salt Lake City segment corresponds well with the 2.0 ± 0.8 ka earthquake at the American Fork site on the Provo segment. Further support for this correlation comes from a consultant trench site within the Salt Lake City-Provo segment boundary (Fort Canyon site) [Western GeoLogic, 2004], yielding evidence of a surface-faulting earthquake shortly after 2.5 ka with 1.8 m of displacement. Sites on the central and southern Nephi segment have earthquakes at ~1.2 ka, which could correspond with either the Spring Lake MRE at ~0.9 ka or the Mapleton North PE at 1.5 ± 0.4 ka. Alternatively, the 0.9 ka Spring Lake earthquake, which does not correspond well with any event on the Provo or Nephi segments (Figure 5b), could be interpreted as a short rupture confined to the overlapping part of the segment boundary zone. The 1.5 ka Mapleton North event time is similar to the 1.3 ka Salt Lake City segment earthquakes; however, no earthquakes of similar timing have been documented in the ~50 km distance between the Mapleton North and South Fork Dry Creek sites. Site earthquakes on the Salt Lake City segment (Little Cottonwood Canyon and South Fork Dry Creek sites), Provo segment (American Fork and Mapleton North sites), and Nephi segment (North Creek and Willow Creek sites) all have earthquakes close to ~2–2.5 ka (Figure 5b). These overlapping earthquake times allow for multiple rupture scenarios, including separate segment wide earthquakes, or synchronous rupture of two or all three segments. 2.2.3. Subsegment Boundaries Limited paleoseismic data restrict our ability to evaluate rupture continuity across the subsegment boundaries. On the Brigham City segment, the earthquake record south of the subsegment boundary is incomplete. Only a single earthquake has been identified on the southern part of the segment (at ~1.2 ka at the Pearsons Canyon site), with a rupture that likely terminated about 5 km south of the subsegment boundary [DuRoss et al., 2012; Personius et al., 2012]. On the Salt Lake City segment, there are significant differences in the timing of the youngest earthquakes across the subsegment boundary between the East Bench and Cottonwood faults (e.g., MREs at ~4.0 versus ~1.3 ka, respectively); however, DuRoss and Hylland [2015] noted that similar earthquake times exist across the complexity at ~4 and ~5 ka (Figure 4). Although slightly different earthquake times (~0.4 versus 0.6 ka) exist across the boundary between the American Fork and central subsegments of the Provo segment, these events overlap at the two-sigma level of uncertainty and may correlate. Ultimately, we have low confidence in the interpretation of separate events across this subsegment boundary because of the broadly constrained American Fork MRE time that is based on legacy bulk soil radiocarbon ages and limited documentation for this site. Similar earthquake times (~0.6 ka) both north and south of the branch and change in fault strike between the central and Spanish Fork subsegments on the southern Provo segment suggest that this

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fault geometry has not significantly impeded ruptures; however, an older event at ~1.5 ka, which has been observed south but not north of this boundary, could indicate a rupture that was confined to the Spanish Fork subsegment. Broad uncertainties in the earthquake times and limited data permit multiple interpretations of ruptures across, or limited by, the Nephi subsegment step. For example, earthquakes at ~0.2–0.5 ka on both northern and southern parts of the segment (Figure 5b) have overlapping 2 sigma uncertainties and large (2–3m) displacements, indicating that the subsegment boundary may not be a significant barrier to rupture. In contrast, the 0.9 ka Spring Lake earthquake on the northern Nephi segment has a smaller (<1 m) displacement and overlaps only minimally with the ~1.2 ka earthquakes on the southern part of the segment and thus is supportive of separate rupture of the Nephi subsegments.

Gaps in the spatial distribution of trench sites allow for the possibility that the subsegment boundaries act as barriers to partial-segment ruptures associated with smaller-magnitude earthquakes, particularly near and across the primary segment boundaries (Figure 6). For example, a ~20 to 30 km long rupture of the southern Brigham City and northern Weber segments is possible if the ~1.2 ka spillover rupture from the Weber to Brigham City segment [Personius et al., 2012] terminated north of the East Ogden site and thus did not rupture the entire Weber segment [DuRoss et al., 2011] (Figure 6a). Paleoseismic data also permit ~30 to 40 km long ruptures of the southern Salt Lake City segment and northern Provo segment. On the southern Salt Lake City segment, earthquakes at ~2 ka were documented at the Little Cottonwood Canyon and South Fork Dry Creek sites but not the Penrose Drive site north of the subsegment boundary between the East Bench and Cottonwood faults (Figure 6b). Similarly, the PE at the American Fork site on the northernmost Provo segment occurred at ~2 ka (Figure 6b), and data from the Fort Canyon site [Western GeoLogic, 2004] within the Salt Lake City-Provo segment boundary indicate that this primary boundary ruptured shortly after ~2.5 ka. A similar, younger rupture is permitted by the data if the 0.4 ka American Fork MRE is interpreted as a separate rupture of the American Fork subsegment, extending north to the Salt Lake City segment (but terminating south of the South Fork Dry Creek site). However, this potential partial rupture is constrained by a single site and only allowable because of the large spatial gap between the South Fork Dry Creek and American Fork sites. On the southern Provo and northern Nephi segments, the broad primary segment boundary, subsegment complexities, and complicated late Holocene earthquake histories allow for ~30 to 40 km long ruptures across (or limited to) this segment boundary (Figure 6b). The best example of such a rupture within the limits of the earthquake-timing data is the 0.9 ka earthquake at the Spring Lake site, which has a 2 sigma time range that overlaps only minimally (<50 years) with those for the youngest and best- constrained earthquakes on the Provo and Nephi segments (Figure 6b and Table 2). Based on the spatial gaps between sites, the 0.9 ka earthquake could have ruptured the Spanish Fork subsegment and northern part of the northern Nephi subsegment. The ~1.5 ka Provo segment earthquake may have had a rupture extent similar to that for the 0.9 ka Nephi segment event, but the 1.5 ka earthquake has only been identified at the Mapleton North site on the southernmost Provo segment. Ultimately, these data show that although the subsegment complexities may not impede rupture of an entire segment, they may play a role in imped- ing slip that is localized near or across the segment boundaries. 2.2.4. Along-Strike Displacement Profiles Although we have limited data, along-strike displacement profiles show evidence of segments mechanically interacting across their primary boundaries (Figure 7). The best examples include displacement profiles for the Weber and southernmost Brigham City segments and the Provo and Nephi segments. On the Weber segment, displacements for the PE (at ~0.9–1.2 ka) increase to the north, toward the segment boundary with the Brigham City segment, before decreasing steeply on the southernmost Brigham City segment. This pattern is consistent with observations of mechanically interacting faults that have their peak displacements and maximum displacement gradients shifted toward the fault-overlap zone [e.g., Willemse et al., 1996] as well as the interpretation of a throughgoing rupture across this primary boundary [Personius et al., 2012]. In contrast, displacements for the ~0.6 ka MRE on the Weber segment do not markedly increase to the north, toward the segment boundary. On the Provo and Nephi segments, along-strike displacements for the ~0.4–0.6 ka MRE on the Provo segment and ~0.2–0.5 ka MRE on the Nephi segment increase toward the primary boundary between these segments. Peak displacements for these segments are close to the zone of overlapping faults, rather than at the segment centers. However, the displacement profile for the PE on the Nephi segment does not show a similar pattern. Other per event displacement observations (e.g., on the Salt Lake City segment) are too sparse to use in interpreting along-strike displacement profiles.

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Figure 6. Possible partial-segment and spillover ruptures (hachured boxes) permitted by the earthquake-timing data, showing (a) the northern part and (b) the southern part of the central WFZ. Partial-segment and spillover ruptures are extended along fault strike and terminated at the closer of either (1) the next site where no evidence for the rupture exists or (2) the next subsegment boundary (black arrows). Ruptures also terminate at a primary segment boundary unless a possible correlation exists across it, in which case the rupture is extended according to rules above.

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Figure 7. Summary of earthquake-timing and displacement data for the central WFZ. (a) Simpliﬁed plot showing the mean timing of the most recent (dark gray) and penultimate (light gray) earthquakes at the sites, where changes in earthquake timing along the fault occur midway between sites having dissimilar times. Horizontal white dashes indicate sections of the fault where earthquake-timing data may be incomplete. (b) Per event vertical displacement data, which we grouped to form along-strike displacement proﬁles for the ruptures shown. Triangles and diamonds indicate correlation of per displacements in the most recent and penultimate earthquakes, respectively. Pluses are per event displacement observations that we were unable to correlate along strike. Small arrow indicates that displacement estimates at site are minima.

The displacement profiles may signify mechanically interacting segments, but limited and poorly understood data prevent us from using them to characterize segment boundary behavior. For example, the nature of the per event displacement observations remains uncertain. Issues affecting the displacement measurements, which are difficult to quantify, include site preservation and a bias toward investigating the largest scarps along a rupture, how colluvial wedge thickness relates to per event displacement, measurements that are averages over several earthquake cycles, and which methods are most suitable for assessing per event displacement uncertainties [e.g., Hemphill-Haley and Weldon, 1999; DuRoss, 2008]. Further, we are unsure of how to interpret the prominent differences in the displacement profiles that we observed for subsequent ruptures on the Weber and Nephi segments. Possible explanations include inaccurate displacement data, an insufficient number of displacement points used to construct the profiles, changes in rupture propagation direction, or highly variable slip per event in subsequent earthquakes. Finally, evidence of mechanical interaction does not imply throughgoing rupture. For example, similar displacement patterns could result from spillover rupture or separate earthquakes on the segments closely spaced in time. Thus, in the absence of more detailed and accurate per event displacement profiles, we do not use the displacement data to test segmentation.

2.3. Modeling Wasatch Fault Zone Ruptures We outline three approaches to address prehistoric and future rupture behavior of the WFZ: (1) treat the WFZ as segmented [e.g., Machette et al., 1992; Wong et al., 2016], by relying primarily on the most recent earthquake data and applying the segmentation model to all timing and displacement data, (2) deﬁne spatial

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Figure 8. Segmented model for the central WFZ based on the correlation and integration of surface-faulting earthquakes identiﬁed at sites along the segments (lightly shaded PDFs). Dark-shaded PDFs are those combined from the individual site data. Shaded horizontal bands indicate correlation of site earthquakes, with the width corresponding to the 2 sigma uncertainty for the segment earthquake. See text and Table 2 for discussion of source data. Segment earthquake labels and mean times correspond with Table 3. Vertical bands indicate primary segment boundaries consistent with our spatial uncertainties shown in the upper panel.

uncertainties for the segment boundaries to allow for a minor degree of partial and spillover rupture [e.g., WGCEP, 2003], and (3) model a ﬂoating earthquake that allows for a full range of possible rupture lengths. Here we discuss these approaches, updating a similar treatment by Wong et al. [2016], which also includes an analysis of the potential for multisegment (e.g., two-segment) ruptures of the fault. An additional approach consists of objectively constructing suites of rupture models. For example, for the southern San Andreas fault, Biasi and Weldon [2009] used paleoseismic data to build suites of rupture models and evaluated these models using criteria such as how well the modeled cumulative displacement for the fault compared to slip rate observations. We considered this to be a worthwhile task but deem that it is outside the scope of the present study. 2.3.1. A Segmented Wasatch Fault Zone The timing of the most recent and penultimate earthquakes along the central WFZ and per event displacement data for several ruptures highlight along-strike differences in the paleoseismic histories that are the basis for a segmented fault model (Figure 7). This approach uses the entire site PDF data set and relies heavily on the youngest earthquakes as a basis for the segmentation model. To achieve the segment earthquake times (Figure 8 and Table 3), site PDFs are correlated along the segment and combined using either a mean [Biasi and Weldon, 2009] or product [DuRoss et al., 2011] of their site PDF probabilities over common time bins.

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Table 3. Paleoseismic Data for the Central WFZ Based on a Segmented Fault Model Vertical Mean Displacementb Recurrencec Slip Rated Segment SB Uncertainty SB Uncertainty Segmenta EQ Timinga (ka) (m) (kyr) (mm/yr) Lengthe (km) (km)f (N end) (km)f (S end)

BCS B1: 2.4 ± 0.3; B2: 3.5 ± 0.2; 1.7(1.2–2.1) 1.1 ± 0.2 1.6(0.9–2.5) 35 ±3 +3, À8 B3: 4.5 ± 0.5; B4: 5.6 ± 0.6 (B4–B1) WS W1: 0.6 ± 0.1; W2: 1.1 ± 0.6; 2.4(1.1–4.1) 1.3 ± 0.1 1.8(0.8–3.4) 56 +8, À3±7 W3: 3.1 ± 0.3; W4: 4.5 ± 0.3; W5: 5.9 ± 0.5 (W5–W1) SLCS S1: 1.3 ± 0.2; S2: 2.2 ± 0.2;S3: 1.7(1.2–2.2) 1.3 ± 0.1 1.3(0.9–1.8) 40 ±7 ±6 4.1 ± 0.2; S4: 5.3 ± 0.2 (S4–S1) PS P1: 0.6 ± 0.05; P2: 1.5 ± 0.4; 2.6(1.3–3.6) 1.3 ± 0.2 2.0(0.8–3.3) 59 ±6 +4, À13 P3: 2.2 ± 0.4; P4: 4.7 ± 0.3; P5: 5.9 ± 1.0 (P5–P1) NS N1: 0.2 ± 0.07; N2: 1.2 ± 0.08; 2.0(1.5–2.7) 1.1 ± 0.2 1.8(1.2–2.8) 43 +5, À17 ±6 N3: 2.4 ± 0.1; N4: 4.0 ± 0.09; N5: 4.7 ± 0.5; (N6–N1) N6: 5.7 ± 0.8 aPer segment earthquake timing, based on integration of site earthquake data younger than 7 ka; see text for discussion and references. Segment abbreviations: BCS, Brigham City; WS, Weber; SLCS, Salt Lake City; PS, Provo; and NS, Nephi. bMean vertical displacement per segment, calculated by ﬁtting ellipse-shaped displacement proﬁles [after Chang and Smith, 2002; Biasi and Weldon, 2009; and DuRoss and Hylland, 2015] to the per event displacements observed at the paleoseismic sites. Displacement range is in parentheses. cClosed mean recurrence intervals calculated using individual earthquakes in parenthesis. 2σ uncertainties based on the earthquake times are shown; however, because of the small sample sizes (n =4–6) uncertainties stemming from the sample size would likely exceed these values. dMean vertical slip rate based on mean vertical displacement (and range) per segment divided by mean recurrence (and 2σ range). eEnd-to-end (straight line) segment length. fSegment boundary spatial uncertainties for the northern and southern ends of the segments based on the geometry and extent of faulting, paleoseismic data and site locations, and the degree of spillover rupture if observed.

Recent work by DuRoss et al. [2011], Personius et al. [2012], Crone et al. [2014], and DuRoss and Hylland [2015] demonstrates the product approach for the Weber, Brigham City, Nephi, and Salt Lake City segments, respectively. We used this approach to model segment earthquake times for the Provo segment and to update the Nephi segment results (Figure 8). Applying the central WFZ segmentation model to all available paleoseismic data on the segments yields 24 earthquakes younger than ~7 ka (Figure 8). These earthquakes yield similar mean recurrence estimates per segment of 1.1–1.3 kyr (Table 3). Using the mean earthquake times, interevent recurrence times between earthquakes on the segments range from 0.5 to 2.5 kyr and have a mean of 1.2 kyr. Interevent recurrence times are most similar for the Brigham City segment (~1.0–1.1 kyr) and dissimilar for the Provo segment (~0.7–2.5 kyr). Following the site-PDF correlation scheme, we also combined site displacements along the segments by fitting ellipse-shaped displacement curves to the point displacement observations (slip tapering at the segment ends after Chang and Smith [2002] and Biasi and Weldon [2009], see also DuRoss and Hylland [2015]) and reported mean vertical displacement per segment of 1.7–2.6 m (Table 3). Mean interval slip rates for the segments, which are dependent on the per segment displacement and recurrence estimates, range from ~1.3 to 2.0 mm/yr. This interpretation of central WFZ paleoseismic data has historical roots [e.g., Schwartz and Coppersmith,1984; Machette et al., 1992] and relies on the best-constrained earthquakes per segment to define the earthquake times [e.g., DuRoss et al., 2011; Personius et al., 2012] but ignores the potential for more complex ruptures. Advantages of the segmented model include its simplicity, easy construction, use of all available data, and resulting per segment estimates of earthquake recurrence and magnitude, which can be used in regional earthquake forecasts [e.g., Wong et al., 2016] and seismic hazard assessments [e.g., Petersen et al., 2014]. However, as we have demonstrated, not all of the site earthquake data nicely fit the segmentation model. Important nuances, such as the possibility of partial, spillover, and synchronous (e.g., two-segment) ruptures, which could generate ruptures shorter or longer than the lengths of the defined segments, are excluded. 2.3.2. Segment Boundary Uncertainties Along-strike spatial uncertainties in segment boundary locations allow for variability in rupture processes, while honoring available paleoseismic data. For the central WFZ, we defined segment boundary uncertainties (Figure 8) using (1) the geometry and extent of Holocene faulting at the segment boundaries, including areas of fault complexity (Figure 3), (2) paleoseismic data from sites close to the segment boundaries (Figure 4), and (3) the degree of spillover rupture if observed. These uncertainties are intended to account for possible spillover ruptures, such as that observed for the Borah Peak earthquake [Crone et al., 1987] and the Weber

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and Brigham City segments [Personius et al., 2012], rather than all rupture styles and extents. Spatial uncertainties in the segment boundaries are typically 3–8 km but are as large as 13–17 km for the complex Provo-Nephi segment boundary. The northern Brigham City segment has the best-constrained boundary (±3 km) based on the well-defined limit of Holocene surface faulting [Personius, 1990; Personius et al., 2012]. For the Brigham City-Weber and Provo-Nephi boundaries, we defined asymmetric uncertainties (Figure 8) to better reflect the complexity of the fault and paleoseismic data. Using these uncertainties, possible rupture lengths for the segments range from 20–46 km to 41–71 km. The minimum values imply partial ruptures of ~50–80% of the segment lengths, whereas the maximum values signify ruptures of 120–130% of the segment lengths, or spillover ruptures ~3–8 km beyond the segment boundaries. 2.3.3. Floating Ruptures A floating earthquake is a way to capture possible modes of central WFZ rupture that are not identifiable within the limits of the paleoseismic data [e.g., Petersen et al., 2014]. One option is a floating earthquake having a range of rupture lengths based on (1) distances between subsegment boundaries, (2) estimates of minimum fault length required to generate a surface-faulting earthquake, (3) probabilistic estimates of rupture length based on point displacement observations [e.g., Biasi and Weldon, 2006], (4) combinations of segments and subsegments, and (5) the maximum observed historical normal faulting earthquake rupture length. For the central WFZ, ruptures would thus range from about 10–20 km (based on subsegments lengths in Table 1) to ~100 km (based on the 1887 M 7.5 Sonora, Mexico, earthquake) [Suter, 2015]. Some floating ruptures could be centered close to (say within 20 km of) the segment boundaries, consistent with our spatial segment boundary uncertainties and the observation that surface-faulting earthquakes may nucleate at areas of stress concentration such as structural complexities [e.g., Aki, 1989; Nielsen and Knopoff, 1998]. This would allow for the possibility of short (~20 to 40 km long) ruptures at the segment boundaries, but likely limited by subsegment complexities, as well as longer ruptures not centered on the segments, and possibly combining individual faults from multiple segments. Alternatively, these rupture lengths and centers could be randomly assigned to a section of the fault, regardless of the segment boundaries. Floating earthquake models provide a level of certainty that all possible rupture modes have been accounted for.

3. Discussion 3.1. Are Central Wasatch Fault Zone Ruptures Segmented? For the central WFZ, are we able to address the question of segment boundaries and rupture lengths using late Holocene paleoseismic data? Although this question cannot be unequivocally answered given the limitations of the earthquake-timing data, we are able to establish some limits on rupture lengths using a subset of our data that consists of the youngest and best-constrained earthquakes. We infer that these data in large part favor the model of a segmented central WFZ but also permit more complex (e.g., partial-segment and multisegment) ruptures. For the northern segments of the central WFZ, significant differences in MRE timing, which exceed the site-PDF uncertainties and occur close to the segment boundaries (Figure 7), lend confidence to separate independent ruptures on the Brigham City, Weber, and Salt Lake City segments. However, exceptions, such as the Weber segment to Brigham City segment spillover rupture (Figure 5a), the possibility of partial ruptures of the Weber segment at ~0.9 and ~1.2 ka (Figure 6a), and older events that allow for multisegment ruptures, highlight the potential for more complex ruptures. For the southern part of the central WFZ, MREs along the Salt Lake City, Provo, and Nephi segments are consistent with a segmented fault model (Figure 5b), but we are less confident in the persistent segmentation of this part of the WFZ because of overlapping earthquake times, dissimilar earthquake times for adjacent sites that may indicate ruptures limited to the southernmost Provo segment and/or northernmost Nephi segment (Figure 6b), the greater structural complexity of the fault, and perhaps mechanical interaction across the Provo-Nephi segment boundary suggested by the displacement data (Figure 7). These examples show that although the primary segment boundaries influence the extent of surface-faulting earthquakes on the central WFZ, a more nuanced approach to evaluating fault segmentation that accounts for a wide range in possible rupture permutations is required. Our observations indicate that ~20–40 km long partial-segment and spillover ruptures near or across the segment boundaries, but possibly limited spatially by subsegment complexities, are permitted by the data (Figure 6). These ruptures are generally shorter than the defined segment lengths (35–59 km) and would yield smaller-magnitude earthquakes (M ~6.6–6.9) than complete rupture of the segments (~M 6.9–7.1) (using the

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surface rupture length-magnitude regressions of Wells and Coppersmith [1994] and Wesnousky [2008]). It has been proposed that normal fault segment lengths are fundamentally controlled by the thickness of the seismogenic crust [Jackson and White, 1989; Jackson and Blenkinsop, 1997]. In the context of the observations here, this implies that subsegment or partial-segment ruptures of 15–20 km may be typical in the BRP [Jackson and White, 1989]. Although these shorter ruptures may not be the mechanism by which the fault releases signiﬁcant amounts of accumulated strain, they are important for understanding how strain is released at the segment boundaries, modeling possible spillover and multisegment ruptures, and more accu-

rately assessing earthquake hazard. Historical examples of partial ruptures in the BRP include the 1950 ML 5.6 Fort Sage Mountains, California, earthquake [Gianella, 1957] and the 1934 ML 6.6 Hansel Valley, Utah, earthquake [Walter,1934;Shenon, 1936]. The Fort Sage Mountains earthquake ruptured about 8 km of the 20 km long Fort Sage fault and generated scarps about 0.1–0.2 m high [Gianella,1957;Briggs et al., 2013]. The Hansel Valley earthquake included a <10 km long rupture of the southernmost part of the ~28 km long Hansel Valley fault with less than 0.5 m of vertical displacement [McCalpin et al., 1992]; however, strike-slip displacement inferred from waveform modeling may exceed this [Doser, 1989]. Both of these faults have paleoseismic histories in support of longer- and larger-displacement ruptures [McCalpin et al., 1992; Briggs et al., 2013]. We surmise that for the central WFZ, partial ruptures such as these are not fully represented by either the short (~170 years) historical earthquake record or the late Holocene paleoseismic record. That is, for any given trench site, their limited length and small (<0.5 m) vertical displacements, close to the displacement detection thresh- old in an alluvial fan environment, would make them less likely to be observed and reported. What additional paleoseismic data are needed to further refine the central WFZ rupture boundaries? In our analyses, we relied heavily on the youngest, best-constrained site earthquake PDFs and per event displacements along the WFZ. Longer-term records (e.g., on the Salt Lake City segment) [DuRoss and Hylland, 2015] are essential for analysis of possible earthquake clustering and to help refine mean recurrence and slip rate estimates. However, the broader earthquake-timing constraints of most WFZ earthquakes older than ~4 ka (Figure 4) limit the value of these data in refining models of fault segmentation and estimates of rupture length. Thus, for the WFZ and other multisegment normal faults, a focus on the most recent ruptures along the fault and particularly those close to and within prominent structural complexities [e.g., Bennett et al., 2015; DuRoss et al., 2015] would help reduce uncertainties in the extents of the individual ruptures and test whether additional short ruptures (e.g., within the 15–30 km long data gaps) have occurred. For the WFZ, refining the timing of all post 3 ka site earthquakes on the central WFZ to within tens to a few hundreds of years and filling prominent gaps in the temporal and spatial record (e.g., Figure 4) would serve to substantially refine our understanding of how structural complexities impede or modulate slip. Additional paleoseismic data are also needed to refine the timing of older (~middle to early Holocene) earthquakes on the WFZ; however, because fewer sites have records of earthquakes older than ~3 ka, considerable uncertainties in the extents of these earthquake ruptures are likely to remain. We constructed displacement profiles, which are generally consistent with our rupture length interpretations and, in some cases, are consistent with the mechanical interaction of segments across the primary boundaries. However, we recognize that the along-strike displacement profiles alone may not be indicative of rupture length. For example, a displacement profile containing individual peaks over many tens of kilometers could be the result of at least two independent earthquakes, or a pair of triggered events (e.g., the 1954 M 7.2 Fairview Peak and M 6.9 Dixie Valley earthquakes) [Caskey et al., 1996], or a long, multisegment rupture (e.g., the 1887 Sonora, Mexico, earthquake) (Figure 9). However, when combined with earthquake-timing data, displacement data are perhaps most useful for identifying spillover ruptures, where moderate displacement of the spillover section exceeds that expected given the spillover length. For example, as much as ~1 m of displacement occurred along 10 km of the Warm Springs segment adjacent to the Thousand Springs segment of the Lost River fault zone, which ruptured in the 1983 M 6.9 Borah Peak earthquake [Crone et al., 1987] (Figure 9). Biasi and Weldon [2006] show that the rupture responsible for a point displacement of 1 m has a 95% probability of being greater than ~15 km and a 50% probability of being greater than ~35 km. This method can also be used to infer the possible extent of ruptures beyond the trench sites into areas where there are data gaps [e.g., Olig et al., 2011]. Thus, although difficult to obtain, future studies focused on acquir- ing robust measures of per event displacement and constructing detailed displacement profiles, especially close to fault segment boundaries, would improve our understanding of how displacement relates to rupture style and length.

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Figure 9. Along-strike vertical displacement profiles for historical Basin and Range Province earthquakes. The profiles depict four major normal faulting rupture styles: (1) single-segment/fault rupture—the 1959 Hebgen Lake earthquake, (2) spillover rupture—the 1983 Borah Peak earthquake, (3) triggered rupture—the 1954 Fairview Peak and Dixie Valley earthquakes, and (4) multisegment rupture—the 1887 Sonora earthquake. Rupture maps and displacement profiles are modified from Wesnousky [2008], with the exception of that for the Sonora earthquake, which is from Suter [2015]. Arrows indicate possible subsegment boundaries along the rupture. Stars indicate earthquake epicenters.

3.2. Comparison to Historical Basin and Range Province Earthquake Ruptures To put our WFZ data in a regional context, we compare central WFZ per event displacements and along-strike proﬁles to those for several historical BRP surface ruptures (Figure 9). For this comparison, we compiled vertical displacement proﬁles of four rupture types: (1) single-segment/fault rupture—the 1959 M 7.3 Hebgen Lake, Montana, earthquake [Witkind, 1964], (2) spillover rupture—the 1983 M 6.9 Borah Peak, Idaho, earthquake [Crone et al., 1987], (3) triggered slip—the 1954 M 7.2 Fairview Peak and M 6.9 Dixie Valley earthquakes [Caskey et al., 1996], and (4) multisegment rupture—the 1887 M 7.5 Sonora, Mexico, earthquake [Suter, 2015].

Possible examples of partial-segment (or subsegment) rupture include the 1950 ML 5.6 Fort Sage Mountains earthquake and the 1934 ML 6.6 Hansel Valley earthquake; however, along-strike displacement proﬁles for

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these ruptures were not available. Finally, although the Hebgen Lake rupture, which consists of two separate faults, may not be an ideal example of a single-segment rupture, we include the rupture because of the significant amount (~10 km) of overlap between the two faults and because the entirety rather than a fraction of the fault zone ruptured. We considered including the 1915 M 7.3 Pleasant Valley, Nevada, earthquake, but the complex rupture pattern includes multiple faults [Wallace, 1980; Wesnousky, 2008]. Displacement profiles (Figure 9) show vertical displacement only and are modified from Wesnousky [2008], with the exception of the Sonoran earthquake rupture, which is from Suter [2015]. Compared to the central WFZ displacement data, these historical profiles offer important insights regarding rupture complexity, subsegment rupture, and along-strike displacement. All of the BRP ruptures are complex and show slip across several structural complexities (Figure 9). Although some ruptures crossed minor structural complexities, several terminated at prominent fault gaps and steps [e.g., Zhang et al., 1999]. Two ruptures terminated along a continuous fault trace (the southern end of the Dixie Valley rupture and northern end of the Borah Peak rupture). dePolo et al. [1991] drew similar conclusions from similar BRP displacement profiles and concluded that “simple earthquake segmentation models may be inadequate for evaluating larger earthquakes in the Basin and Range province.” Further, the profiles exhibit a sawtooth pattern comprising steep displacement gradients between adjacent point observations. This displacement pattern is consistent with several well-studied historical strike-slip ruptures [e.g., Rockwell et al., 2002; Haeussler et al., 2004; Gold et al., 2013] and highlights the difficulty in characterizing rupture displacement using only a few point observations. The profiles, when averaged over several kilometers, have steep displacement gradients of ~0.5 m/km at the rupture ends. The complex BRP ruptures demonstrate that normal fault ruptures can include numerous combinations of subsegments and the boundaries between them. The BRP ruptures each have two to four subsegments (Figure 9), with most having three. This is consistent with most of the primary WFZ segments as well as a compilation of normal fault data from Afar, East Africa [Manighetti et al., 2015]. For most of these BRP ruptures, the subsegment boundaries appear to have modulated rather than completely impeded fault rupture. For example, along the rupture of the Dixie Valley fault in the Dixie Valley earthquake, displacement peaks correspond with the centers of three subsegments, and displacement troughs occur at the subsegment boundaries (Figure 9). Along the Sonora earthquake rupture, the subsegment boundaries are gaps and steps in faulting, across which the rupture jumped. These data suggest that the subsegment definitions are possibly most useful for providing reasonable limits on the extents of ruptures both shorter and longer than the primary segments, rather than inferring fault segmentation behavior in the absence of robust earthquake-timing data. Displacement profiles for several BRP historical earthquakes suggest that prehistoric displacement profiles may aid in interpreting spillover ruptures but are less useful in distinguishing between single-segment, triggered, and multisegment ruptures. First, spillover rupture from the Weber segment to Brigham City segment at ~1.2 ka (Figure 7) is similar to that from the Thousand Springs segment to Warm Springs segment in the Borah Peak rupture (northern section rupture in Figure 9). Displacements on the spillover sections are a fraction of the mean slip in the main rupture (~25% of the Weber segment rupture and ~50% of the Thousand Springs rupture) but may not fit scaling relations between displacement and rupture length (e.g., using Biasi and Weldon [2006]). Displacement profiles along faults triggered by the Fairview Peak earthquake (between the Fairview Peak and Dixie Valley faults; Figure 9) show similar misfits. Similarly, in the Sonora earthquake, the Teras and Otates faults (both ~20 km long) had moderately large, ~1–2 m displacements that are ~50% of the Pitaycachi fault displacement. These examples show that in a paleoseismic context point displacements, together with well-constrained earthquake times, may help identify spillover ruptures, but separate rupture of adjacent segments (e.g., the Teras and Otates faults), with large displacement-length scaling misfits, cannot be fully discounted. Ultimately, displacement profiles for single-segment ruptures, triggered events, and multisegment ruptures are not unique. For example, in a prehistoric setting, the Otates rupture of the Sonora earthquake could just as easily be interpreted as independent rupture as the Fairview Peak and Dixie Valley ruptures could be interpreted as a multisegment (synchronous) rupture.

3.3. Comparison of the Wasatch and Fucino Fault Zones Finally, to put the WFZ into a global context, we compare the WFZ to the Fucino fault zone (FFZ), a well- studied ~100 km long, multisegment normal fault zone (Figure 10) in the complex Lazio-Abruzzo extensional fault network in the central Italian Apennines that accommodates approximately 0.5–2 mm/yr of horizontal

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Figure 10. Summary of earthquake-timing data for the Fucino fault zone (FFZ) of the central Apennines, Italy. The FFZ is broken into two major fault zones separated by the Tre Monti fault. In the northern part, the FN-E and FN-W indicate two prominent eastern and western fault zones, respectively. Similar fault zones are labeled FS-E and FS-W in the southern part. Fault trace and structural complexities (marked by dashed lines) are modiﬁed from Benedetti et al. [2013]. Vertical bars show individual earthquakes, with time ranges derived from paleoseismic trench (circles) and 36Cl analyses (triangles) [Pantosti et al., 1996; Galadini and Galli, 1999; Salvi et al., 2003; Schlagenhauf et al., 2011; and Benedetti et al., 2013]. See text for discussion of paleoseismic and historical earthquake data. Gray boxes and heavy dashed arrows show possible correlation of site earthquakes along the FFZ (this study).

extension (vertical slip rate of ~1–4 mm/yr) [Piccardi et al., 1999; Benedetti et al., 2013]. Similar to the WFZ, the FFZ has abundant earthquake-timing and displacement data along its length, derived from both fault-trench investigations [Pantosti et al., 1996; Galadini and Galli, 1999; Salvi et al., 2003] and in situ 36Cl exposure dating of limestone bedrock exhumed during surface-faulting earthquakes [e.g., Schlagenhauf et al., 2011; Benedetti et al., 2013]. We have compiled those data in Figure 10. Although in some cases the exposure dating and trench results yield discordant earthquake histories (e.g., for the Magnola fault; Figure 10), many of the results agree and we considered it outside the scope of this paper to reconcile these differences. Unlike the WFZ, the FFZ has a much longer historical record (~1500 years versus 170 years for the WFZ) that includes destructive, large-magnitude (M ~6–7) surface-faulting earthquakes in 508 A.D. (1.4 ka), 801 A.D. (1.1 ka), 1340 A.D. (0.6 ka), and 1915 A.D. [Michetti et al., 1996; Pantosti et al., 1996; Galadini and Galli, 1999; Benedetti et al., 2013]. Here we

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compare the structural expression and timing of surface-rupturing earthquakes of the FFZ to similar data on the WFZ to explore existing models of WFZ segmentation. The WFZ and FFZ are normal faults in areas of broad continental extension that have similar rates of vertical slip and structural complexities that define separate segments and subsegments. In the FFZ, the Tre Monti fault separates the FFZ into two primary, northern (FN-E and FN-W) and southern (FS-E and FS-W) fault zones [Benedetti et al., 2013]. The Tre Monti fault is orthogonal to most other normal faults in the region and, similar to other more prominent cross structures in the region [e.g., Pizzi and Galadini, 2009], likely acts as a barrier to rupture. This is consistent with the northern extent of surface rupture in the 1915 A.D. earthquake [Michetti et al., 1996; Galadini and Galli, 1999] and differences in prehistoric earthquake timing on and across the fault [e.g., Schlagenhauf et al., 2011; Benedetti et al., 2013] (Figure 10). Numerous structural complexities exist along the northern and southern parts of the FFZ, which Benedetti et al. [2013] used to define several, ~10 to 20 km long segments (herein subsegments, defined by dashed lines in Figure 10). Structural complexities that define the subsegments are similar to those present along the WFZ (Figure 3) and include fault steps, gaps, and areas of complex faulting. However, unlike the WFZ, the relatively more youthful FFZ has not formed a single throughgoing fault system but comprises numerous subparallel overlapping faults. The FFZ has only been active since about the Pliocene [Galadini and Galli, 1999] compared to the Miocene (since ~18 Ma) [Parry and Bruhn, 1987] for the WFZ. Thus, the lower level of structural maturity of the FFZ marks an important difference with the WFZ. Similar to the WFZ, uncertainty remains in possible FFZ rupture lengths. If published analyses are correct, then the WFZ and FFZ may be at opposite ends of the spectrum in terms of expected rupture lengths. On the FFZ, subsegment boundaries may act as barriers to rupture, limiting earthquake lengths from ~10 to 20 km [Benedetti et al., 2013], consistent with the historical earthquake data [e.g., Pantosti et al., 1996]. For example,

the 1915 A.D. Ms 7 earthquake ruptured three parallel, overlapping fault strands (the Serrone, Trasacco, and Luco del Marsi faults; Figure 10), but only had a straight line length of 23 km [Michetti et al., 1996]. In contrast, on the WFZ, the primary segments and paleoseismic data for the most recent earthquakes indicate earthquake ruptures between ~35 and 60 km, although we identiﬁed several examples of possible shorter (<40 km long) ruptures focused near the segment boundaries that are permitted by the paleoseismic data. Considering the broadly constrained earthquake times and limited extent of the paleoseismic data, we infer that longer ruptures of the FFZ, linking adjacent and overlapping subsegments, cannot be ruled out (see large red arrows in Figure 10). For example, on the FN-E, rupture of the Campo Felice, P. Di Pezza, and Ovindoli faults at ~3.4–3.8 ka and possibly at 801 A.D. is permitted by the paleoseismic data [Pantosti et al., 1996]. One of these ruptures could correspond with the poorly constrained MRE (0.6–3.7 ka) on the Monte Ocre fault. Similarly, several faults within the FN-W could have ruptured synchronously, including the Velino and Magnola faults at ~1.3 ka. It is possible that this rupture corresponds with one of the young (<2 ka) ruptures of the Fiamignano fault; however, no data exist for the Val Di Malito fault between them. On the FS-E, paleoseismic data suggest that the Serrone and San Sebastiano faults rupture independently, but several adjacent faults remain unstudied and thus uncertainty remains in plausible rupture lengths. Several rupture combinations are also possible on the FS-W. The rupture extent of the ~3.0 ka earthquake on the central part of the Trasacco fault is unconstrained. The rupture could have been limited to the central part, but also overlaps with the rupture of the northwest subsegment at ~3.3–3.5 ka. No paleoseismic data constrain the rupture to the south. Rupture combinations on faults in the FS-E and FS-W or FN-E and FN-W are also possible, as suggested by the faults that ruptured in the 1915 A.D. earthquake, but these combinations would not appreciably increase the possible rupture lengths because of the parallel, overlapping fault geometries. Finally, although some broadly constrained earthquake times are similar across the Tre Monti fault, the lack of earthquakes younger than ~5 ka on this structure and the historical record suggest a low probability of throughgoing ruptures (e.g., from the FN-E to FS-E), although triggered slip across the Tre Monti fault cannot be ruled out.

The paleoseismic correlations discussed above yield variable rupture lengths, depending on how many adjacent subsegments are activated in each earthquake. For example, ruptures limited to individual faults would have lengths of ~10 to 20 km, compared to combinations of two to three faults that would yield ~20–30 km long ruptures. In the unlikely case of complete rupture of one of the separate fault zones (i.e., the FS-E, FS-W, FN-E, or FN-W), ~30–45 km long ruptures would result [Benedetti et al., 2013]. On the FS-E and FS-W, the

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~20 km long 1915 A.D. rupture supports shorter ruptures, which are also consistent with relatively small vertical displacements (mostly <1 m). On the FN-W and FN-E, larger per event displacements (~1–3m) [Pantosti et al., 1996; Schlagenhauf et al., 2011; Benedetti et al., 2013] may signify longer ruptures. These displacements suggest ruptures having a 95% probability of being at least ~15–50 km long and a 50% probability of being ~35–95 km long (based on Biasi and Weldon [2006]). However, ~10 to 20 km long ruptures are inferred to be most common [Benedetti et al., 2013], which would imply that the relatively immaturity and structurally complex FFZ has a large displacement-to-length scaling ratio (consistent with Manighetti et al. [2007]). Ultimately, we infer significant (~20 km) uncertainties in rupture length because of the potential earthquake correlations discussed above and the fact that several faults and subsections remain unstudied. Our analysis of the FFZ has significant implications for the WFZ. Shorter ruptures of the FFZ appear more consistent with the historical record than the longer, ~40–60 km long ruptures inferred on the WFZ. This dissimilar- ity is likely related to the different levels of structural maturation that these faults have obtained. We speculate that the WFZ has reached a level of maturity in which shorter ruptures such as those on the FFZ are less likely because of individual fault growth and linkage. However, individual subsegments may persist over time (e.g., segmentation is generic, irrespective of fault maturity and other factors such as slip rate) [Manighetti et al., 2015], suggesting that subsegment ruptures of the WFZ are still possible. Thus, the relatively short FFZ ruptures lend confidence to the inclusion of shorter-length (<40 km long) and smaller-magnitude earthquakes in WFZ rupture models. Study of the WFZ and FFZ suggests that for these faults, ruptures of variable length based on the subsegment complexities, primary segment boundaries, combinations of segments and subsegments, and rupture of the primary boundaries themselves cannot be ruled out. However, some structural features, such as the Tre Monti fault, may have a combination of structural orientation [e.g., Pizzi and Galadini, 2009], bedrock rheology, and/or fault complexity that allows them to endure as persistent barriers to rupture over many earthquake cycles as the structural maturity of the fault increases. These boundaries may provide an upper limit on rupture length for an immature fault as opposed to a mean or modal limit for a more developed fault zone. The importance of inherited cross structures in the BRP is a promising area of research, especially since they may have had a role in limiting the rupture length of at least one historical surface-faulting earthquake (the Borah Peak earthquake) [Susong et al., 1990].

4. Conclusions Late Holocene paleoseismic data for the central WFZ, one of the best-studied, multisegmented normal faults in the world, are a window into issues of structural segmentation and prehistoric earthquake rupture behavior. Our study demonstrates that the youngest (<3 ka) earthquake-timing and displacement data along the fault are best suited to address questions of fault segmentation. These data in large part support a segmented fault model but also permit more complex ruptures. For example, primary segment boundaries, especially for the northern part of the central WFZ, correspond to clear along-strike differences in MRE timing. However, broadly constrained earthquake times, isolated events that do not overlap in time with earthquakes at adjacent sites, per event displacements that raise the possibility of interaction between segments, and areas of structural complexity (e.g., along the southern part of the central WFZ) permit more complex (e.g., partial-segment, spillover, and multisegment) ruptures of the fault. We also conclude that ~20–40 km long partial-segment ruptures near or across the primary segment boundaries are possible, and that subsegment boundaries—perhaps some not yet identified—may control the lengths of these ruptures as well as spillover ruptures. We present a segmented fault model consisting of 24 earthquakes younger than 7 ka with mean recurrence estimates per segment of 1.1–1.3 ka, but ultimately, additional models of fault behavior, including segment boundary and subsegment boundary uncertainties, floating earthquakes, and suites of possible ruptures, are required to fully address the epistemic uncertainties in the segmentation of the WFZ. A comparison of the central WFZ to other normal faults yields useful insights. Displacement profiles for several BRP historical earthquakes suggest that prehistoric displacement profiles may aid in interpreting spillover ruptures but are less useful in distinguishing between single-segment, triggered, and multisegment ruptures. We also compared the structural expression and prehistoric earthquake-timing and displacement data for the multisegment Wasatch and Fucino (Italy) fault zones. Both are long, continental normal faults having late Holocene paleoseismic data in support of large-magnitude surface-faulting earthquakes. Significant differences include a much longer historical earthquake record for the Fucino fault zone

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(~1500 years versus 170 years) and major differences in the complexity of the fault traces and expected rupture lengths (10–20 km versus 40–60 km for the FFZ and WFZ, respectively). The shorter rupture lengths on the FFZ are likely related to the structural complexity and relative immaturity of the fault zone but lend conﬁdence to our interpretation that <40 km long ruptures of the WFZ are possible. Ultimately, the uneven spatial distribution of trench sites along both faults, variations in possible rupture lengths resulting from subsegment to primary segment rupture, and the complex rupture modes observed for historical BRP earthquakes lead us to conclude that both shorter (~20–40 km) and longer (~60–100 km) rupture scenarios need to be modeled for the WFZ. Further, we suggest that paleoseismic research focusing on the youngest earthquakes at sites close to the primary segment boundaries and investigations of fault maturity and the role of inherited structures in controlling fault surface ruptures are essential lines of research needed to better understand normal fault segmentation.

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