Integration of Paleoseismic Data from Multiple ... - GeoScienceWorld

Bulletin of the Seismological Society of America, Vol. 101, No. 6, pp. 2765–2781, December 2011, doi: 10.1785/0120110102

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Integration of Paleoseismic Data from Multiple Sites to Develop an Objective Earthquake Chronology: Application to the Weber Segment of the Wasatch Fault Zone, Utah by Christopher B. DuRoss, Stephen F. Personius, Anthony J. Crone, Susan S. Olig, and William R. Lund

Abstract

We present a method to evaluate and integrate paleoseismic data from multiple sites into a single, objective measure of earthquake timing and recurrence on discrete segments of active faults. We apply this method to the Weber segment (WS) of the Wasatch fault zone using data from four fault-trench studies completed between 1981 and 2009. After systematically reevaluating the stratigraphic and chronologic data from each trench site, we constructed time-stratigraphic OxCal models that yield site probability density functions (PDFs) of the times of individual earthquakes. We next qualitatively correlated the site PDFs into a segment-wide earthquake chronology, which is supported by overlapping site PDFs, large per-event displacements, and prominent segment boundaries. For each segment-wide earthquake, we computed the product of the site PDF probabilities in common time bins, which emphasizes the overlap in the site earthquake times, and gives more weight to the narrowest, best-defined PDFs. The product method yields smaller earthquake-timing uncertainties compared to taking the mean of the site PDFs, but is best suited to earthquakes constrained by broad, overlapping site PDFs. We calculated segment-wide earthquake recurrence intervals and uncertainties using a Monte Carlo model. Five surface-faulting earthquakes occurred on the WS at about 5.9, 4.5, 3.1, 1.1, and 0.6 ka. With the exception of the 1.1-ka event, we used the product method to define the earthquake times. The revised WS chronology yields a mean recurrence interval of 1.3 kyr (0.7–1.9-kyr estimated two-sigma [2σ] range based on interevent recurrence). These data help clarify the paleoearthquake history of the WS, including the important question of the timing and rupture extent of the most recent earthquake, and are essential to the improvement of earthquake-probability assessments for the Wasatch Front region. Online Material: Weber segment time–stratigraphic models in OxCal format and earthquake-timing probability density functions.

Introduction Paleoseismic trench-site data are a critical component of regional earthquake forecasts such as the Uniform California Earthquake Rupture Forecast (Field et al., 2008) and the forecast being developed for the Wasatch Front by the Working Group on Utah Earthquake Probabilities (WGUEP) (Wong et al., 2011). However, integrating paleoseismic data from different sites and studies presents significant challenges related to trenching and dating methods that have evolved over three decades, the correlation of earthquakes among trench sites that is not always straightforward, and the difficulty in objectively distilling and integrating disparate and variable-quality

paleoseismic records from multiple sites into surface-faulting chronologies that apply to an entire segment or fault (e.g., Biasi and Weldon, 2009). For example, on the Wasatch fault zone (WFZ) in northern Utah (Fig. 1), paleoseismic studies range from a single, shallow fault trench with a few bulk radiocarbon (14 C) ages (e.g., Swan et al., 1980) to trenching campaigns with earthquake times supported by numerous luminescence and accelerator mass spectrometry (AMS) 14 C ages on charcoal (e.g., Machette et al., 2007; DuRoss et al., 2008, 2009, 2010; Olig et al., 2011). These differences complicate site-to-site earthquake correlation, but because of the

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C. B. DuRoss, S. F. Personius, A. J. Crone, S. S. Olig, and W. R. Lund

Figure 1. Central five segments of the WFZ in northcentral Utah, which have evidence of multiple Holocene surface-faulting earthquakes. Horizontal white lines indicate segment boundaries; dashed box shows area of Figure 2. Traces of the WFZ and other Quaternary faults are from Black et al. (2003). Base map is a 90-m digital elevation model (DEM) (http://gis .utah.gov/agrc).

limited paleoseismic data for each segment (generally 2–4 sites per 35–60-km-long fault segment), all available data should be considered in regional earthquake analyses. Thus, we present a method to systematically and objectively interpret paleoseismic-site data and combine those data into a segment-wide surface-faulting earthquake chronology from which mean earthquake recurrence can be calculated. We apply this method to the Weber segment (WS) of the WFZ and present revised earthquake timing and recurrence estimates derived from integrating data from four paleoseismic studies completed over three decades. The WS is the second longest segment of the 343-kmlong WFZ and trends through the heavily urbanized northern Wasatch Front (Fig. 1). Paleoseismic data show that three or four surface-faulting earthquakes have occurred on the WS since the middle Holocene (Swan et al., 1980, 1981; Nelson, 1988; McCalpin et al., 1994; Nelson et al., 2006) (Table 1), but important questions remain regarding the timing and extent of the most recent earthquake rupture, the correlation of earthquakes between paleoseismic sites, and the average Holocene earthquake recurrence interval. These issues motivated DuRoss et al. (2009) to conduct a paleoseismic investigation at Rice Creek, where they acquired additional constraints on the timing of mid- to late-Holocene earthquakes on the northernmost part of the segment (Fig. 2). The purpose of this work is to objectively integrate the results of the Rice Creek investigation with previous (1980s vintage) paleoseismic data to refine estimates of surfacefaulting earthquake timing and recurrence for the WS. To accomplish this, we (1) considered common limitations in dating paleoearthquake event horizons, (2) examined the original paleoseismic site reports and associated trench maps to evaluate geologic and chronologic evidence for interpreted events, (3) constructed time-stratigraphic OxCal models for each site, (4) qualitatively correlated events between sites to develop a segment-wide earthquake history, (5) computed composite probability density functions (PDF) for each earthquake, and (6) used the segment-wide PDF data to

Table 1 Summary of Earthquake-Timing Data from Previous Studies on the Weber Segment, Wasatch Fault Zone, Utah Kaysville Site (ka)*

East Ogden Site (ka)†

Garner Canyon Site (ka)†

Not exposed K4: 5.7–6.1 (3.8–7.9 possible) K3: 2.1–3.5 K2: 0.6–0.8§ K1: No evidence§

Not exposed EO4: 2.8–4.8 EO3: 2.4–3.9 EO2: 0.5–1.7 EO1: 0.2–0.6

Not exposed GC4: > 2:8 GC3: > 2:1–2:8 GC2: 1.4–2.8 GC1: 0.6–1.5

Rice Creek Site (ka)‡

RC5: RC4: RC3: RC2: RC1:

5.5–7.5 3.7–5.4 1.8–3.7 0.8–1.4 0.5–0.6

*Summarized from McCalpin et al. (1994). Nelson et al. (2006). For GC3 and GC4, times are based on minimum-limiting ages. Nelson et al. (2006) reported a GC3 time of 2.3–4.0 ka based on a possible correlation with East Ogden EO4. ‡ DuRoss et al. (2009). §The original Kaysville trench investigation (Swan et al., 1980, 1981) found evidence for two earthquakes younger than 1580 180 14 C yr B.P. (1.2–1.8 ka; calendar calibration by DuRoss et al., 2009). †

Integration of Paleoseismic Data to Develop Earthquake Chronology: Wasatch Fault Zone, Utah

Figure 2.

Holocene surface trace of the WS of the WFZ (black) showing locations of paleoseismic study sites (I shapes); ball-andbar is on downthrown side of fault. Pleistocene traces of the WFZ are shown in gray. Horizontal white lines indicate segment boundaries. Base map is 2006 National Agriculture Imagery Program orthophotography overlain on a 30-m DEM (http://gis.utah.gov/agrc).

calculate mean recurrence and associated uncertainties. Our revised WS earthquake data have improved models of fault segmentation (e.g., earthquake rupture length and energy release) and will be used by the WGUEP to generate timedependent earthquake probabilities for the WS and the central WFZ. Ultimately, this information helps characterize the earthquake hazard in the Wasatch Front region, which can promote improved earthquake education and preparedness and help guide risk-reduction strategies. Weber Segment of the Wasatch Fault Zone The WFZ (Fig. 1) is the general structural boundary in north-central Utah between the actively extending Basin and Range Province to the west and the relatively more stable Middle Rocky Mountain and Colorado Plateau provinces to the east. The fault accommodates about 50 percent of the east-west extension across the eastern 200 km of the Basin

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and Range Province from easternmost Nevada to northcentral Utah (Chang et al., 2006) and releases strain in large-magnitude (M ∼ 7) surface-faulting earthquakes along seismogenic fault segments that are thought to generally function independently (Schwartz and Coppersmith, 1984). The WFZ comprises ten segments, which are defined on the basis of structural, geomorphic, geological, geophysical, and seismological data and characteristics (Machette et al., 1992; Wheeler and Krystinik, 1992). Of these, the five central segments (Brigham City to Nephi; Fig. 1) have abundant evidence of recurrent Holocene surface-faulting earthquakes with an average composite recurrence interval of 350– 400 years for all segments (Machette et al., 1992; McCalpin and Nishenko, 1996; Lund, 2005). Along strike, variations in the Holocene rupture history of the fault, the late Quaternary fault-trace geometry, and prominent salients forming segment boundaries define these five segments (Swan et al., 1980; Schwartz and Coppersmith, 1984; Machette et al., 1992). The 56-km-long WS has a mostly linear fault trace that extends from North Salt Lake to North Ogden (Fig. 2), but the pattern of faulting is more complex near the boundaries with the adjacent Brigham City and Salt Lake City segments. At the segment boundaries, multiple overlapping fault traces coincide with shallowly buried bedrock (Nelson and Personius, 1993) suggesting a long-term history of slip dissipation (and likely rupture termination). However, DuRoss et al. (2010) documented rupture across the Weber–Brigham City segment boundary in a late Holocene earthquake on the WS. Based on the segment length and estimated mean displacement per earthquake (2:1 1:3 m; DuRoss, 2008), empirical magnitude regressions for both normal- and all-fault types (Wells and Coppersmith, 1994) suggest that the WS is capable of generating earthquakes of moment magnitude 7.0–7.2. The majority of paleoseismic data for the WS are from two studies conducted in the 1980s (Fig. 2): one at Kaysville, where Swan et al. (1980, 1981) and McCalpin et al. (1994) conducted trenching studies a decade apart (1978 and 1988, respectively), and the other at East Ogden (Nelson, 1988; Forman, Nelson, et al., 1991; Nelson et al., 2006). Study of a cut-slope excavation at Garner Canyon (Fig. 2) provided additional earthquake information for the northern part of the segment (Machette et al., 1992; Nelson and Personius, 1993; Nelson et al., 2006). Each of these investigations yielded evidence of at least three large-displacement, surface-faulting earthquakes during the Holocene (Table 1), but important questions remained regarding the correlation of events between sites and the completeness of the earthquake record. In 2007, DuRoss et al. (2009) excavated trenches on the northern part of the WS at Rice Creek (Fig. 2) to clarify the timing and correlation of events and to extend the earthquake record into the early Holocene. Sources of Dating Uncertainty Radiocarbon Dating. Radiocarbon (14 C) dating of organicrich sediment is the primary method for establishing

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earthquake times in paleoseismic investigations of the WFZ (Machette et al., 1992). However, 14 C ages obtained over the past 30 years must be carefully evaluated due to differences in sampling and dating methods over time. For example, the type of sediment sampled and dated (e.g., bulk soil versus charcoal), the sample location and extent, and the dating pretreatment methods all impact 14 C ages. For all of the paleoseismic studies on the WS, we evaluated the sampling and dating methods, as well as the structural and stratigraphic context of the samples, to identify those ages that are most critical for accurately bracketing individual earthquake times. Many ages from the 1980s studies are from bulk-soil (generally A-horizon) samples, whose ages must be modified to compensate for the mean residence time (MRT) of carbon in the soil (Geyh et al., 1971; Matthews, 1980; Machette et al., 1992). Estimating the MRT of carbon in soils is problematic because it varies according to (1) the rate that carbon accumulates, decomposes, and is mixed in the soil; (2) the part of the A-horizon (e.g., the top or base) that is sampled; and (3) the component of the organic fraction (e.g., total, humic acid, and residue) that is dated (Matthews, 1980; Stafford and Forman, 1993; Martin and Johnson, 1995). Nelson et al. (2006) reported that, for central Utah trench sites, the MRT can range from hundreds to thousands of years, but is typically 100–500 years, which both Nelson et al. (2006) and McCalpin et al. (1994) subtracted (using slightly different methods) from their uncalibrated WS A-horizon ages. A-horizon soil ages may be affected by contamination from both young and old carbon. Anomalously young carbon can be added by bioturbation from burrowing organisms and roots and the addition of post-1950s bomb carbon from thermonuclear testing. Anomalously old ages may be caused by carbon from detrital (i.e., recycled) charcoal, reworked old soils, or soils in which the youngest carbon has decayed or been stripped (Matthews, 1980; Geyh et al., 1983; Taylor, 1987; Machette et al., 1992; Martin and Johnson, 1995). At Rice Creek, 14 C ages are from discrete fragments of soil charcoal (DuRoss et al., 2009), which can also yield problematic ages. Anomalously young charcoal ages can come from A-horizon samples that contain young charcoal mixed by burrowing or rootlets that decayed or burned in place. In contrast, anomalously old charcoal ages generally indicate carbon reworked from soils or sediment that predates the sampled soil horizon. Charcoal fragments can also have an inherited (inbuilt) age from the age of the plant when it burned or the period of time when the charcoal resided on the landscape before it was incorporated into the deposit that was sampled and dated. For example, charcoal from coarse, woody debris (e.g., a dead standing tree or snag) can have an inherited age of hundreds to possibly thousands of years depending on the wood species, tree diameter, and decay rate (Harmon et al., 1986; Gavin, 2001). To help minimize the potential for dating detrital charcoal at Rice Creek, DuRoss et al. (2009) had bulk A-horizon samples processed to identify the type of charred plant material in the charcoal. DuRoss et al. (2009) posited that

preferential dating of locally derived charcoal (e.g., sagebrush) rather than nonlocal charcoal (e.g., conifer transported from higher elevations) would reduce dating uncertainties. Charcoal fragments from local plants are more likely burned in place or very close by, and are therefore less likely to have an inherited, older age, which would nearly always be the case with nonlocal detrital charcoal fragments (Puseman and Cummings, 2005). About one-half of the Rice Creek bulk samples contained locally derived charcoal fragments (e.g., Artemisia, flowering plants such as sagebrush; Quercus, oak; and Juniperus, juniper). The remaining samples only produced collections of small, unidentified charcoal fragments. These fragments were combined into samples of at least ∼0:5 mg, which then yielded composite charcoal ages. Although detrital charcoal could have been present in either the unidentified or identified samples, DuRoss et al. (2009) concluded that the stratigraphic consistency of the ages and the similar ages between the unidentified and identified charcoal fragments (from the same A horizons) indicate minimal age uncertainty related to a detrital signal or postdepositional modification of the dated material. Luminescence Dating. Both thermoluminescence (TL) and optically stimulated luminescence (OSL) dating rely on the cumulative dose of in situ natural radiation in sediment to estimate the time when the sediment was last exposed to sunlight (Huntley et al., 1985). Ideally, the sunlight exposure was sufficiently long (about 8 hours for TL and less than 1 hour for OSL) during deposition to fully reset or zero any preexisting luminescence signal in the grains, and thus the luminescence age should represent the time when the sediment was deposited (Aitken, 1994). If the sediment’s exposure to sunlight was not long enough (e.g., because of rapid deposition or filtered light in turbid water) to fully zero the sediment, then it may retain an inherited luminescence signal (Forman, Pierson, et al., 1991), which results in an overestimated (maximum) age for the deposit. For the 1980s-vintage WS studies, the TL ages were measured from fine-grained fluvial or lacustrine deposits, or from A horizons. At Rice Creek, the OSL ages are from fine-grained silt and sand lenses in alluvial-fan deposits (DuRoss et al., 2009).

Weber Segment Paleoseismic Site Data OxCal Modeling Methods To evaluate earthquake timing and associated uncertainties at individual WS paleoseismic sites, we used OxCal radiocarbon calibration and analysis software (version 4.1) (Bronk Ramsey, 1995, 2001) using the IntCal09 calibration curve of Reimer et al., 2009. OxCal probabilistically models the timing of undated events (e.g., earthquakes) by weighting the time distributions of chronological constraints (e.g., 14 C, luminescence ages, and historical constraints) included in a stratigraphic model (Bronk Ramsey, 2008; Lienkaemper and


tion; and (2) depending on the trench and dateable material present, the sampling densities for pre- and postfaulting deposits can differ considerably. Thus, in our OxCal models, sequences depict trench stratigraphy, with numerical ages that are distributed throughout the strata depending on the locations of buried soils, organic-bearing deposits, or sediments suitable for luminescence dating, and undated earthquakes (Boundaries) that represent major changes in deposition at the site. OxCal is well suited to trench studies characterized by numerous, overlapping minimum- and maximum-limiting ages (e.g., Lienkaemper and Williams, 2007) because overlapping limiting ages for an earthquake generally produce narrow, well-constrained PDFs. Unfortunately, for earthquakes without overlapping limiting ages (typical of WFZ trench studies) the resulting PDFs may have broad shapes, uniform probability distributions between the nonoverlapping ages, and geologically meaningless mean values. For example, as illustrated in Figure 3, earthquake A (based on earthquake W of McCalpin, 2002) has a maximum age limited by detrital charcoal from prefaulting alluvial-fan sediments dated at 7:5 0:3 ka (sample R1; calibrated age two sigma 2σ) and a minimum age from a soil formed on postearthquake A fissure fill dated at 5:2 0:2 ka (sample R2; calibrated age 2σ). The challenge is to account for the logical inference made by the original authors that the minimum age from the fissure fill is a better approximation of the earthquake time than the maximum-limiting age from the fan sediments. Simply modeling these ages with OxCal (Fig. 3, Sequence 1) yields an earthquake time of 6:6 1:4 ka. The large (1:4 kyr) uncertainty in the earthquake time 0.04

Limiting ages R1 7.5 ± 0.3 ka (2σ)

R2 5.2 ± 0.2 ka (2σ)

0.02

mean ± 2σ mode

0 5.0

6.0

7.0

8.0

Calibrated age (ka)

0.02

Probability

Bronk Ramsey, 2009). The program generates a PDF for each event in the model, or the likelihood that an earthquake occurred at a particular time, using the chronologic and stratigraphic constraints and a Markov Chain Monte Carlo (MCMC) sampling method (Bronk Ramsey, 2008). Our OxCal models use depositional models (simple sequences; Bronk Ramsey, 2008) that are not depth dependent; that is, the ages of undated events are not influenced by burial depth. We removed numerical-age outliers on the basis of (1) geologic judgment (knowledge of sediments, soils, and sample contexts), (2) inconsistency with other ages in the model for comparable deposits (e.g., stratigraphically inverted ages), and (3) the agreement index (Bronk Ramsey, 1995)—a measure of the agreement between the modeled numerical ages and the original (unmodeled) data (considered to be in poor agreement if below 60%; Bronk Ramsey, 2008). Based on historical records, no large surface-faulting earthquakes (M ∼ 6:5) have occurred since Mormon pioneers settled in Salt Lake Valley in about 1847, which we use as a historical constraint in our models. For sites with multiple trenches, we correlated units between trenches and constructed a single OxCal model. To account for published values of the MRT of bulk 14 C ages, we used the Delta_R OxCal command, which offsets the 14 C age for the MRT correction and increases its uncertainty prior to calendar calibration (Bronk Ramsey, 2010). We report site-specific earthquake time ranges (e.g., for K1—the most recent earthquake in the Kaysville OxCal model) and the supporting 14 C and luminescence ages from the OxCal models as the mean with two-sigma (2σ; 95.4%) uncertainty rounded to the nearest century. Although OxCal is a common tool used to refine the timing of undated events, there is little agreement in whether earthquakes should be modeled as Boundaries (e.g., Fumal et al., 2002; Yen et al., 2008) or Dates (e.g., Lienkaemper and Bronk Ramsey, 2009; Fraser et al., 2010). Both commands are used to generate PDFs for undated events; however, C. Bronk Ramsey (written commun, 2010) explained that Boundaries should be used when the sampling density and deposition type are different within different sections of a sequence. For example, given a sequence of (1) soil development (constrained by many numerical ages), (2) earthquake E, and (3) colluvial-wedge sedimentation (constrained by few numerical ages), the Date and Boundary commands will yield different results for earthquake E. The Date command will be biased by the different sample densities; that is, the numerous age constraints for the soil relative to the colluvium, whereas the Boundary command will not. As opposed to using Date to model earthquake E, a Boundary will not assume that the increased sample density for the soil represents more deposits and thus more elapsed time, which would unjustifiably affect the time of earthquake E. We modeled WS earthquakes using Boundaries because (1) fault trenches clearly show that these earthquakes have generated as much as several meters of surface displacement, and thus, represent a major depositional change from soil development to colluvial-wedge sedimenta-

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0.01

Sequence 1 R_Date(R1) Boundary(A) R_Date(R2)

Earthquake A (boundary) 6.6 ± 1.4 ka (2σ) (mode: 7.3 ka)

0 0.02

Sequence 2 R_Date(R1) Gap(2000) Boundary(A) R_Date(R2)

Earthquake A (boundary; offset using gap)

0.01

5.4 ± 0.3 ka (2σ) (mode: 5.3 ka)

0 0.02 0.01

Sequence 3 R_Date(R1) Zero_boundary(R2-R1) Boundary(A) R_Date(R2)

Earthquake A (boundary; paired with zero_boundary) 6.0 ± 1.1 ka (2σ) (mode: 5.3 ka)

0 5.0

6.0

7.0

8.0

Modeled age (ka)

Figure 3. Example PDFs for the time of an earthquake A (see text for discussion) constrained by limiting ages R1 and R2. Sequences 1–3 are OxCal models that include a Boundary for earthquake A (sequence 1), a 2-kyr Gap preceding the earthquake-A Boundary (sequence 2), and the pairing of Zero Boundary with the earthquake-A Boundary (sequence 3).

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is appropriate, but because the two limiting ages do not overlap, the earthquake PDF has a broad, roughly uniform distribution that does not reflect the interpretation that the earthquake occurred about 5.3 ka, close to the 5.2-ka minimum age. Alternatively, we can use the OxCal Gap command to specify an elapsed time between elements in a sequence (Bronk Ramsey, 2008; Bronk Ramsey, 2010). In the case of an undated earthquake, a user-defined Gap effectively shifts the earthquake (Boundary) PDF to be closer to the minimumor maximum-limiting age(s). For example, in Sequence 2 (Fig. 3), the elapsed time between ages R1 (7.5 ka) and R2 (5.2 ka) of approximately 2 kyr is modeled as a Gap that occurs between R1 and earthquake A, thus limiting the earthquake age to 5:4 0:3 ka. The mean earthquake time is close to the minimum age of 5.2 ka, but the uncertainty (0:3 ka) is unrealistically small because the earthquake PDF does not include the presumed low probability that the event actually could have occurred near the ∼7:5-ka maximum age. Furthermore, the duration of the Gap significantly impacts the modeled earthquake time (e.g., using a 1-kyr Gap produces a mean earthquake time of 5.9 ka). Our solution is to pair the earthquake Boundary with a separate Zero Boundary command representing the elapsed time between the limiting ages. The Zero Boundary command, which models a time distribution that rises linearly from zero when paired with a Boundary (Bronk Ramsey, 2008; Bronk Ramsey, 2010), produces an asymmetric shape to the earthquake PDF. The Zero Boundary skews the earthquake PDF toward the minimum or maximum-limiting age(s), depending on its placement in the sequence, and is less arbitrary than specifying a fixed Gap time. Alternatively, including a Boundary rather than a Zero Boundary accounts for the unknown elapsed time, but does not strongly skew the earthquake PDF. In Sequence 3 (Fig. 3), the added Zero Boundary shifts the greatest earthquake A timing probability toward the minimum-limiting age, while allowing for the low probability that the earthquake occurred near the maximum age. The mean time is 6.0 ka, but because of the asymmetry of the PDF, the 5.3-ka modal time is more appropriate. With this method, the uncertainty in the earthquake A time (width of the PDF) reflects the full elapsed time between the maximum and minimum ages, but the shape of the PDF is consistent with the interpretation of the original authors that the event occurred closer to the 5.2-ka minimum age. Weber Segment OxCal Models Kaysville. A prominent fault scarp south of Kaysville, about 20 km north of the southern terminus of the WS (Fig. 2), has been the site of two separate trench investigations. In one of the first paleoseismic studies on the WFZ, Swan et al. (1980, 1981) excavated several trenches across the 22-m-high fault scarp and an approximately 40-m-wide graben. The trenches exposed evidence for at least three surface-faulting earthquakes. Swan et al. (1981) described scarp-derived

colluvium related to two earthquakes and an older sequence of graben-fill deposits complexly faulted by one or more earthquakes. A charcoal fragment from the graben fill provided a maximum age for the two youngest earthquakes of 1580 180 14 C yr B:P: (1.2–1.8 ka as calibrated by DuRoss et al., 2009) (Table 1). In 1988, McCalpin et al. (1994) reexcavated the Kaysville site and used 14 C and TL ages to constrain the timing of three earthquakes (K4–K2 in Table 1; nomenclature corresponds with our OxCal model discussed subsequently). McCalpin et al. (1994) concluded that earthquake 3 (Table 1; K4) occurred at 5.7–6.1 ka (possible range of 3.8–7.9 ka) based on TL and bulk 14 C ages for a buried A horizon, and that earthquake 2 (Table 1; K3) occurred at 2.1–3.5 ka using an assumed sedimentation rate in the graben (Table 1). Earthquake 1 (Table 1; K2) occurred shortly before 0.6–0.8 ka, based on limiting ages for a thick buried soil A horizon. McCalpin et al. (1994) discounted the occurrence of the second earthquake younger than ∼1:5 ka that was inferred by Swan et al. (1981). Our OxCal model for the Kaysville site (Fig. 4; see the Data and Resources section) includes three earthquakes at 5:7 1:3 ka (K4), 2:8 1:7 ka (K3), and 0:6 0:2 ka (K1), similar to the interpretation of McCalpin et al. (1994), as well as an additional earthquake (K2) at 0:9 0:5 ka (Fig. 4, Table 2) that stems from our review and synthesis of stratigraphic and structural data from both investigations and chronological constraints from McCalpin et al. (1994) (discussed subsequently). For 14 C ages from bulk-soil samples, we modeled an MRT correction of 300 200 yr (McCalpin et al., 1994) using the OxCal Delta_R command. Large uncertainties in the timing of earthquakes K4 (1:3 ka) and K3 (1:7 ka) reflect our use of a Zero Boundary to model an unknown elapsed time between these events. Earthquake K1 occurred at 0.6 ka; overlapping 14 C and TL ages contribute to the small timing uncertainty (0:2 ka) for this event. Using stratigraphic and structural evidence, we interpret a possible additional earthquake K2 that occurred after deposition of colluvium associated with earthquake K3, but before the formation of a thick cumulic soil A horizon (soil unit S3 of McCalpin et al., 1994; unit S2 of Swan et al., 1981), which was clearly faulted by the most recent earthquake, K1. The strongest arguments for earthquake K2 are related to prominent fissures filled with organic sediment that extend downward several meters into older deposits from the base of soil S3. Swan et al. (1981) and McCalpin et al. (1994) interpreted the fissures as postdating soil S3 and thus forming in earthquake K1; however, the trench logs show that the fissure boundaries do not extend through soil S3, and thus, could be older than K1. Swan et al. (1981) show minor faults with less than 20 cm of vertical displacement in pond deposits that overlie soil S3, and these faults may be related to the fissures (D. Schwartz, U.S. Geological Survey, verbal commun., 2010). However, the pond deposits are continuous across the zone of fissuring, and thus, we consider it unlikely that the fissures formed by open cracks at the ground surface (McCalpin et al., 1994) in the same event that produced minor faulting

Integration of Paleoseismic Data to Develop Earthquake Chronology: Wasatch Fault Zone, Utah E1

E3

E2 RC1

E5

E4

mean

0.04

2σ RC2 0.02

RC3 RC5

RC4

Rice Creek 0 E2a 0.02 GC2

Probability

GC1

GC3

GC4

0

Garner Canyon not exposed

EO1

0.02

EO3

EO2

East Ogden

EO4 0

not exposed ?

E2b K1

0.02

K2

K4

K3

Kaysville

0 0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

Thousands of calendar years B.P. (ka)

Figure 4.

Correlation of site PDFs from OxCal models for the Kaysville (K), East Ogden (EO), Garner Canyon (GC), and Rice Creek (RC) paleoseismic sites (see Data and Resources section). Light-gray-shaded areas show how site PDFs correlate to form segment earthquakes E5 to E1: E2a and E2b indicate the possible partial-segment rupture of the WS in two separate earthquakes (see text for discussion). Horizontal bars show 2σ ranges.

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in the overlying pond sediment. The fissures could have formed by shearing, but this interpretation conflicts with organic sediment (fissure fill) extending several meters downward into older deposits, basal gravel clasts mapped by Swan et al. (1981), and a basal fissure-fill sediment 14 C age of ∼1:9 ka (McCalpin et al., 1994), which likely dates sediment eroded from a soil developed on K3 colluvium exposed in a fissure wall. We find it more likely that the thick package of sediment and organics (and fissure fill) in soil S3 was deposited in response to faulting, scarp formation, and fissuring in K2, followed by deposition of pond deposits and their subsequent minor displacement in K1. Additional evidence for K2 includes (1) at least five small-displacement (∼20–30 cm) faults that apparently terminate at the contact between K3 colluvium and the overlying soil S3 (Swan et al., 1981), indicating an event horizon that postdates earthquake K3 and predates K1; (2) possible buried fault scarps formed in K3 colluvium and buried by soil S3 (Swan et al., 1981; McCalpin et al., 1994); (3) 1.0–1.4 m of main-fault displacement of K3 colluvium (soil S3 is not offset based on Swan et al., 1981); and (4) a distinct wedge shape for soil S3, which tapers from ∼1 to 2 m thick near the main scarp to about 0.5 m thick in the graben (Swan et al., 1981; McCalpin et al., 1994). East Ogden. At the East Ogden site, Nelson (1988) excavated five trenches across down-to-the-west scarps that have 5-m and 8-m surface offsets and an antithetic scarp that has a 2-m surface offset. Based on bulk-sediment 14 C, charcoal 14 C, and TL ages, Nelson et al. (2006) interpreted four late Holocene earthquakes (Table 1). Their oldest event (EO4) occurred at 2.8–4.8 ka, based on overlapping 14 C and TL ages between 3 and 5 ka from two trenches. Earthquake EO3 occurred at 2.4–3.9 ka, after a soil formed on EO4 colluvium dated at 3:2 0:5 ka, and before deposition of scarp colluvium from EO3, which contains six stratigraphically consistent TL ages between 2:7 0:6 ka and 1:2 0:2 ka. Earthquake EO2 occurred at 0.5–1.7 ka, based on the maximum-limiting ages

Table 2 OxCal Models for Weber Segment Paleoseismic Sites and Summary of Revised Earthquake Times* Earthquake

Kaysville†

East Ogden‡

Garner Canyon‡

Rice Creek§

UQFPWG Consensus∥

This Study; Mean 2σ#

E5 E4 E3 E2 E1

5:7 1:3 No evidence 2:8 1:7 0:9 0:5 0:6 0:2

Not exposed 4:0 0:9 3:0 0:4 0:9 0:4 0:5 0:2

Not exposed 4:4 0:6 3:3 0:6 1:5 0:5 0:6 0:4

6:0 1:0 4:6 0:5 3:4 0:7 1:2 0:3 0:6 0:1

6:1 0:7 4:5 0:7 3:0 0:7 0:95 0:45 0:5 0:3**

5:9 0:5 4:5 0:3 3:1 0:3 1:1 0:6 0:6 0:1

This Study; Mode (Fifth–Ninety-Fifth)#

5.6 4.5 3.1 1.3 0.5

(5.6–6.4) (4.2–4.7) (2.9–3.3) (0.7–1.7) (0.5–0.6)

*All earthquake times are reported as the mean 2σ in thousands of calendar years B.P. (1950), with the exception of the mode (fifth–ninety-fifth percentile) reported in the last column. †Based on McCalpin et al. (1994); E2 is based on our synthesis of paleoseismic data from Swan et al. (1981) and McCalpin et al. (1994). ‡ Based on Nelson et al. (2006). § Based on DuRoss et al. (2009). ∥Consensus earthquake timing of the UQFPWG (Lund, 2005). # Revised earthquake timing for the WS (this study); E2 time based on simple mean of site PDFs, all other earthquake times based on product of site PDFs. **The UQFPWG concluded that E1 may represent partial rupture of the northern WS (Lund, 2005).

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from the EO3 scarp colluvium and a minimum age of 0:6 0:1 ka for charcoal in post-EO2 sediment. The youngest East Ogden event (EO1) occurred at 0.2–0.6 ka, based on the 0.6 ka charcoal age as a maximum and a 0:4 0:2 ka age from bulk soil organics in post-EO1 colluvium. Nelson et al. (2006) considered event EO1 to be younger than the youngest Kaysville (K1) or Garner Canyon (GC1) events and thus suggested that it might be evidence of a partial segment rupture on the northern WS. Our OxCal model for the East Ogden site (Fig. 4; see the Data and Resources section) includes four earthquakes at 4:0 0:9 ka (EO4), 3:0 0:4 ka (EO3), 0:9 0:4 ka (EO2), and 0:5 0:2 ka (EO1) (Table 2) and is based on the four-event interpretation of Nelson et al. (2006). We included the limiting ages reported by Nelson et al. (2006) and dismissed those samples described as potentially being reworked or modern (burrowed). We excluded the maximum TL age of 4:0 0:8 ka for EO4 because it was inconsistent with two 14 C ages of 4:6 0:8 ka and 4:8 0:6 ka from the same soil horizon. In addition, we only included ages for EO3 colluvium from trench 1 where the ages were stratigraphically consistent: additional ages from the other trenches are generally consistent and including them does not affect the modeling results. For bulk-soil 14 C ages, Nelson et al. (2006) applied an MRT correction by subtracting 100 yr from the younger end of the 2σ uncalibrated 14 C age range and 500 yr from the older end prior to calibration. Similar to Nelson et al. (2006), we applied this correction (300 200 yr) to all bulk 14 C ages, except the youngest minimum age limit for EO1, which becomes modern if corrected. Garner Canyon. At Garner Canyon, which is about 5 km north of the East Ogden site, Nelson et al. (2006) mapped the exposure created by an excavation in a 4-m-high fault scarp and reported stratigraphic and structural evidence of four earthquakes. Earthquakes GC4 and GC3 have no maximum age constraints; however, Nelson et al. (2006) used minimum-limiting 14 C ages between 1:8 0:5 ka and 2:7 0:5 ka from a soil A horizon developed on event GC3 colluvium and a possible correlation with East Ogden earthquake EO4 to define an earthquake GC3 time of 2.3–4.0 ka. Earthquake GC2 occurred about 1.2–2.8 ka, based on the 1.8– 2.7 ka 14 C ages that provide a maximum and 0:7 0:4 ka to 1:0 0:4 ka ages from a soil A horizon on fault-scarp colluvium from GC2 that define a minimum age limit (Nelson et al., 2006). The most recent earthquake (GC1) occurred at about 0.4–1.0 ka, that is, after GC2 colluvial deposition and soil development at 0.7–1.0 ka, but before development of an A horizon on GC1 colluvium at about 0–0.5 ka (0:2 0:3 ka and 0:2 0:2 ka from two 14 C ages). Nelson et al. (2006) interpreted the youngest Garner Canyon earthquake as older than the youngest, 0.5-ka event, reported at East Ogden. Our OxCal model for Garner Canyon (Fig. 4; see the Data and Resources section) yields the timing of four earthquakes at 4:4 0:6 ka (GC4), 3:3 0:6 ka (GC3), 1:5 0:5 ka

(GC2), and 0:6 0:4 ka (GC1) (Table 2). The timing of GC4 and GC3 is based on a plausible correlation of these events with earthquakes EO4 and EO3 at East Ogden. Geologic mapping shows that the alluvial fans at both Garner Canyon and East Ogden (Nelson and Personius, 1993) are similar in age (late-Holocene), so we used the mid-Holocene age of the fan at East Ogden to constrain the maximum age of GC3 and GC4. We included a Zero Boundary between GC4 and GC3 to reflect an unknown, but possibly substantial amount of time between these events based on a soil A horizon mapped on the GC4 colluvium (Nelson et al., 2006). We applied a 300 200 yr MRT correction to all bulk 14 C ages at Garner Canyon. Rice Creek. In 2007, DuRoss et al. (2009) excavated two trenches at Rice Creek, near the northern end of the WS (Fig. 2). The trenches extended across two down-to-the-west scarps that have surface offsets of 8 and 4 m, and an antithetic scarp that has a surface offset of about 1 m. They found evidence of at least five, and probably six earthquakes since deposition of early Holocene alluvium at the site. Their preferred interpretation of the stratigraphic and structural evidence and the chronologic data from these trenches produced an OxCal model that included five earthquakes since the mid Holocene (RC5 to RC1). In this study, we have modified their OxCal model to use Boundaries to define earthquake times, thus following a consistent format with our other OxCal models. The OxCal model includes the five most recent Rice Creek earthquakes (Fig. 4; see the Data and Resources section). Earthquake RC5 occurred at about 6:0 1:0 ka, shortly after fan deposition ceased at the site, but before four 14 C charcoal ages of 4.7–5.5 ka (∼5 ka) from a soil A horizon developed on fault-scarp colluvium from this event. Earthquake RC4 occurred at 4:6 0:5 ka, after A-horizon development on RC5 colluvium at ∼5 ka, but before deposition of scarp colluvium from this event at ∼2:5 ka. DuRoss et al. (2009) interpreted the ∼5-ka ages as close maximum limits for RC4 and therefore, we used a Zero Boundary command to skew the RC4 PDF toward the maximum ages. Earthquake RC3 occurred at 3:4 0:7 ka, after a soil A horizon formed on alluvial-fan deposits (3:7 0:1-ka age from detrital charcoal), but likely before a soil A-horizon developed on both the RC3 colluvial wedge and the alluvial-fan deposits (1:8 0:2-ka age for the youngest part of the A-horizon). DuRoss et al. (2009) described clear stratigraphic and chronologic evidence for earthquakes RC2 and RC1. The RC2 time is 1:2 0:2 ka based on three maximum-limiting ages between 1.4 and 1.8 ka and three stratigraphically consistent minimum ages between 0.6 and 0.8 ka from RC2 scarp colluvium. We used Zero Boundary commands to skew the RC3 and RC2 PDFs toward the maximum ages to be consistent with the interpretation of DuRoss et al. (2009). Earthquake RC1 occurred at 0:6 0:1 ka based on maximum- and minimum-limiting ages of 0:6 0:1 ka and


Revised Earthquake History for the WS

(a)

B

Developing an Earthquake Chronology from Paleoseismic Site Data

Fig. 4) using (1) a qualitative assessment based on geologic judgment (our preferred correlation) and (2) a calculation of the degree of overlap between site PDFs. For example, we subjectively interpret the oldest earthquakes at Rice Creek (Fig. 4; RC5 at ∼6 ka) and Kaysville (Fig. 4; K4 at ∼5:8 ka) as evidence of a single-segment-wide earthquake, rather than two possible partial-segment ruptures on the northern and southern parts of the segment. Recognizing that there are multiple correlation possibilities, we quantitatively measured the amount of overlap between different site PDFs (overlap area of Biasi and Weldon, 2009) to help evaluate the quality of our preferred correlation (Fig. 5a). PDF overlap values can range from zero to one, where zero indicates no overlap and one corresponds to full overlap (Fig. 5b). For example, our preferred correlation matches Kaysville earthquake K1 with East Ogden earthquake EO1, although K1 could potentially correlate with EO2 (McCalpin et al., 1994). The correlation of K1 and EO1 takes into account our reasoning that the two most recent earthquakes at both Kaysville and East Ogden are younger than ∼1:2 ka and likely correlate, and the fact that the site PDF for K1 has significantly more overlap with EO1 (0.55) than with EO2 (0.33). In correlating the site PDFs, we assumed that the WS behaves independently of the adjacent Brigham City and Salt Lake City segments, and is prone to large, generally segment-wide characteristic earthquakes (Schwartz and Coppersmith, 1984). With a few exceptions, we did not consider partial- or multi-segment ruptures, or the correlation of site PDFs across segment boundaries (e.g., Biasi and Weldon, 2009). That is, we interpreted the similar earthquake chronologies at the four WS sites and moderate PDF overlap values per earthquake (mean values of 0.35–0.58 per earthquake; Fig. 5b) as evidence of five late- and mid-Holocene earthquakes that ruptured all or most of the length of WS. This rationale of using characteristic behavior is supported by the observation that individual earthquakes at the sites have moderate (∼1 m) to large (∼2–4 m) displacements for each event (DuRoss, 2008; DuRoss et al., 2009) and that the segment is defined by prominent structural boundaries at the north and south ends (Machette et al., 1992; Wheeler and Krystinik, 1992), suggesting a history of dominant singlesegment ruptures. However, because of uncertainties in the Kaysville earthquake chronology, and the potential for noncharacteristic behavior in the second WS earthquake (E2, including possible partial-segment rupture and rupture across

tmin

(b)

tmax

time (t)

1.0 E2 mean: 0.35 K2 EO2

0.8

E4 mean: 0.58 RC4

Overlap in site PDFs

To determine an earthquake chronology for the entire WS, we compared and correlated earthquake PDFs from the OxCal models (site PDFs, e.g., Rice Creek earthquake RC1;

PDF overlap (A,B): sum of minimum probabilities of A and B per time bin in area of overlap (tmin to tmax) (Biasi and Weldon, 2009)

A

probability

0:5 0:1 ka, respectively, which tightly constrain the time of this event.

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E1 mean: 0.45

0.6

K1 K1 RC1 EO1

GC4

E3 mean: 0.40 EO3 GC3

GC1 EO1 K1 GC1

EO4

GC4

E5: 0.54 RC5 K4

RC3 GC3 RC4 EO4

0.4 RC1 EO1

RC2 GC2 RC2 EO2 RC2 K2

RC3 K3 EO3 K3 EO3 RC3 K3 GC3

RC1 GC1

0.2 K2

GC

EO2 GC2

0

E1

E2

E3

E4

E5

Weber segment earthquake

Figure 5.

PDF overlap, showing (a) explanation of PDF overlap calculation (Biasi and Weldon, 2009) for hypothetical PDFs A and B, and (b) overlap in site PDFs contributing to segment-wide earthquakes E5 to E1, where 0 indicates no overlap and 1 is full overlap. Site PDF abbreviations are K (Kaysville), EO (East Ogden), GC (Garner Canyon), and RC (Rice Creek). See Figure 4 for correlation scheme.

the Weber-Brigham City segment boundary; discussed subsequently), we consider exceptions to this rule. We discuss our preferred correlation of earthquakes for the entire segment in the following paragraphs. In our preferred earthquake correlation (Fig. 4), we combined the separate OxCal site PDFs to generate a single PDF for each segment-wide earthquake (after Biasi and Weldon, 2009) (Fig. 6). For example, earthquake E1 comprises site PDFs RC1, GC1, EO1 and K1. We first scaled the site PDFs, where for each PDF the sum of the probabilities for each 5-yr time bin equals 1, and then combined the probabilities into a single PDF that defines the time of the WS earthquake (e.g., Fig. 6; E1). We refer to the combined PDFs (which are derived from two or more site PDFs) as segment PDFs. In general, only two to three site PDFs constrain any given segment-wide earthquake, with the shape of the site PDFs varying according to the number and quality of the limiting ages, the age of the event, and the vintage of the study (reflecting the evolution in sampling and dating techniques). For a more objective treatment of the limited and variable-quality data, we chose to use all of the site data rather than, say, to arbitrarily discard the most broadly shaped PDFs. We tested several approaches for combining site PDFs, and with one exception, favor a product solution, computed

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C. B. DuRoss, S. F. Personius, A. J. Crone, S. S. Olig, and W. R. Lund 0.06

E1 time 560 ± 70 (product) 560 ± 290 (mean)

probability density

0.05

0.03 0.025

0.04 0.02 0.03

0.015

0.02

0.01

0.01

0.005

0 0.2

0.4

0.6

0.8

0

1.0

0.5

0.015

probability density


1.0

1.5

2.0

2.5

0.015


0.01

0.005

E4 time 4470 ± 300 (product) 4310 ± 840 (mean) 0.01

0.005

0

0 2.0

3.0

4.0

5.0

6.0

3.0

3.5

4.0

4.5

5.0

5.5

Thousands of calendar years (ka)


probability density

0.01 0.008 0.006

Product of site PDFs Simple mean of site PDFs (including white outline)

0.004 0.002 0 2.0

Explanation Rice Creek PDF and mean Kaysville PDF and mean East Ogden PDF and mean Garner Canyon PDF and mean

All earthquake times reported as mean ± 2σ (cal yr B.P.) 3.0

4.0

5.0

6.0

7.0

8.0

9.0

Thousands of calendar years (ka)

Figure 6. Comparison between the mean and product-PDF methods for determining segment PDFs. Site PDFs (solid and dashed lines) from OxCal models discussed in text (see Data and Resources section). Light-gray-shaded PDF is the simple mean of site PDFs; dark-grayshaded PDF is the product of site PDFs. The color version of this figure is available only in the electronic edition. by multiplying the probabilities of the site PDFs to generate a segment PDF for each earthquake (Fig. 6). The basis for this choice is the fact that some paleoseismic sites better constrain the timing of an earthquake than others and that, for two independent events, the probability of both events occurring within the same time bin is the product of their probabilities at that time bin. For example, if we have three overlapping site PDFs—A1, B1, and C1—that represent the same earthquake, E1, on the segment, then for each common time bin, we multiplied the A1, B1, and C1 probabilities to

generate a segment-earthquake probability for that time bin. The product PDF method refines the segment PDFs (e.g., produces smaller uncertainties for E1) because it emphasizes the overlap in the site PDFs and thus, is heavily influenced by the narrowest (and thus, best-constrained) site PDFs. This method is similar to maximum likelihood estimation, where the likelihood of a set of parameters is calculated given an observed outcome, or in our case, the likelihood of a set of site PDF parameters (e.g., mean and standard deviation) given the outcome that the site PDFs correlate to a single


constrained site PDFs, which reflect the best-available data from more recent paleoseismic studies. Earthquake Correlation and Timing Our analysis indicates that five surface-faulting earthquakes (Table 2, Fig. 7; E5 to E1) have occurred on the WS since the mid-Holocene (∼6 ka). We report the earthquake times as the mean 2σ and the mode (fifth–ninety-fifth percentile range) (Table 2), which is more suitable for asymmetrically skewed earthquake PDFs. Earthquake E5 occurred at 5:9 0:5 ka, based on the correlation of the oldest, but moderately well-constrained, earthquakes identified at Rice Creek (RC5) and Kaysville (K4). E5 likely occurred before deposition of the alluvial-fan sediments exposed at Garner Canyon and East Ogden. Because E5 has a positively skewed time distribution (Fig. 7), the mode (fifth–ninety-fifth) of 5.6 (5.6–6.4) ka is a better representation of the earthquake time than the mean is. Earthquake E4 occurred at 4:5 0:3 ka, based on overlapping site PDFs from the East Ogden, Garner Canyon, and Rice Creek sites (Fig. 5b) on the northern part of the segment that have mean times of 4.0–4.6 ka. McCalpin et al. (1994) did not find evidence of E4 at Kaysville; however, their Kaysville trench did not extend completely across the 22-m-high scarp and thus, they may not have exposed evidence of this event. Thus, until additional data show that E4 did not rupture the southern WS, we conservatively consider it a full rupture of the segment. Earthquakes E3 to E1 are based on correlating the three youngest events at each trench site. E3 occurred at 3:1 0:3 ka. Although this earthquake was identified at all four sites, the PDFs from Rice Creek, Garner Canyon, and East Ogden are of the best quality and have the greatest overlap (Fig. 5b). Earthquake E2 is based on site PDFs that have disparate peak probabilities, and

Weber segment earthquakes (cal yr B.P. ± 2σ) E1 E2 E3 E4 E5

0.2

Probability density

earthquake. This method is commonly applied to parameter modeling in seismic-hazard analyses (e.g., Nishenko and Buland, 1987; Davies et al., 1989; Ellsworth, et al., 1999); for example, calculating a joint probability distribution for a set of independent interevent recurrence intervals by taking the product of their individual recurrence distributions (Sykes and Menke, 2006). We considered other methods of combining site PDFs, including using a simple mean of the site-PDF probabilities (Biasi and Weldon, 2009) or a weighted mean based on the shapes of the individual site PDFs. Using the previous example of earthquake E1 based on site PDFs A1, B1, and C1, the simple mean is the bin-by-bin mean of the normalized probabilities for A1, B1, and C1. The weighted mean applies a scaling factor based on the shape of the PDF (product overlap calculation of Biasi and Weldon, 2009), where the narrowest PDFs receive full weight (scale factor of 1) and broader PDFs receive less weight. We found that the simple mean produced segment PDFs having very long tails and large uncertainties caused by those site PDFs that are very broad and poorly constrained (e.g., E2 time on Fig. 6). The weighted mean method helps reduce these tails; however, the weighting scheme is subjective and somewhat arbitrary (e.g., should the broadest PDFs receive zero weight or partial weight?). Thus, we prefer the product method in which all site-PDF data are used, but the final segment PDFs are more strongly influenced by the narrowest, best-defined PDFs and the overlap in the site data. The product method has the potential to overconstrain an earthquake time, and thus, is best suited to earthquakes constrained by broad, overlapping site PDFs. For example, where site PDFs corresponding to the same earthquake have poor PDF overlap, the product method will yield a very narrow time distribution for the earthquake. Poor PDF overlap can arise from (1) an inaccurate OxCal model (e.g., site PDFs constrained by ages on burrowed or detrital material), (2) a very narrowly defined site PDF, or (3) a possible misinterpretation of the site-PDF correlation. For the WS, we have confidence in our OxCal models, which include our analysis of potential age outliers, have good model agreement indices (above 60%), and generally show consistent earthquaketiming results among the four trench sites (Fig. 4). In general, the site PDFs that are very narrowly constrained (e.g., RC1) are related to numerous, overlapping limiting ages on AMSdated charcoal, and thus, we prefer these narrow time ranges over less well-constrained site PDFs (e.g., GC1) based on relatively few bulk-soil ages with unknown MRTs. In only one case (RC2 and GC2 versus EO2 and K2) do we consider the possibility of a misinterpreted site-PDF correlation; an alternative correlation and more conservative approach is discussed subsequently. Thus, although the product method could, in some cases, potentially overconstrain an earthquake time, we prefer this approach (with exception) on the basis of (1) our careful evaluation of the OxCal source data, (2) the generally broad, overlapping site PDFs that constrain WS earthquakes, and (3) our preference for using the best-

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560 ± 70 1137 ± 641 3090 ± 280 4470 ± 300 5890 ± 500

0.15

0.1 E1

E3

0.05

E4 E5

E2

0 0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

Earthquake timing (ka)

Figure 7. Revised chronology of surface-faulting earthquakes on the WS based on the product of site PDFs for earthquakes E1, E3, E4, and E5, and the simple mean of the site PDFs for earthquake E2 (see text for discussion). Earthquake times reported as mean 2σ (see Table 2 for mode and fifth–ninety-fifth percentile ranges): horizontal bars show 2σ ranges.

2776


thus poor PDF overlap (Figs. 5 and 6). The product result for E2 is 1:2 0:1 ka; however, we discuss alternate E2 models subsequently. Earthquake E1 ruptured all four sites and is well constrained to 0:6 0:1 ka. Site PDFs contributing to E1 overlap with similar mean times (Fig. 5b; mean PDF overlap of 0.45), but have timing uncertainties that range from 0:1 ka (Rice Creek) to 0:4 ka (Garner Canyon). Site PDFs for E2 are consistently older on the northernmost WS (RC2 at ∼1:2 ka and GC2 at ∼1:5 ka) compared to those from study sites to the south (EO2 and K2 both at ∼0:9 ka) (Fig. 4). As a result, poor PDF overlap (< ∼0:4) results for comparisons of RC2 or GC2 with EO2 or K2 (Fig. 5b), and site PDFs for K2 and EO2 have very high overlap of ∼0:8—the greatest overlap of any two WS site PDFs (Fig. 5b). Thus, we consider two alternate models for E2: (1) site PDFs RC2, GC2, EO2, and GC2 correlate, but potentially suffer from inaccurate OxCal models; and (2) the northernmost WS ruptured in a partial rupture of the northernmost WS (Rice Creek and Garner Canyon) at ∼1:2–1:5 ka (Fig. 4; E2a) compared to a separate and younger earthquake limited to the WS south of Garner Canyon (East Ogden and Kaysville sites) at ∼0:9 ka (Fig. 4; E2b). In model 1, the simple-mean result for E2 of 1:1 0:6 ka adequately captures the timing uncertainty and should be used. In model 2, combining the RC2 and GC2 PDFs would define the older, northern WS rupture, and combining the EO2 and K2 PDFs would define the younger, central and southern WS rupture. We prefer model 1 on the basis of additional evidence for WS earthquake E2 found at the Pearsons Canon trench site (DuRoss et al., 2010) on the southernmost Brigham City segment (BCS) (Fig. 1; 7 km north of the WS–BCS boundary). DuRoss et al. (2010) reported a very well-constrained earthquake at 1:2 0:1 ka (PC1), which was not observed in trenches on the central BCS (Personius, 1991; McCalpin and Forman, 2002; DuRoss et al., 2010). Based on the BCS and WS earthquake-timing data, the fault displacement in PC1, and the offset of presumed late-Holocene geomorphic surfaces on the southernmost BCS, DuRoss et al. (2010) concluded that the 1.2-ka PC1 earthquake represents surface faulting that crossed the WS–BCS boundary in WS earthquake E2. Because the PC1 time falls in the ∼1:1–1:3-ka overlap zone between RC2/GC2 and EO2/K2 (PC1 overlaps with all site PDFs for E2), we prefer model 1, in which E2 occurred as a single rupture of the WS at 1:1 0:6 ka (simple-mean method), which continued north to the southern BCS. Because the E2 simple-mean PDF (Fig. 7) is slightly negatively skewed by the moderately well-constrained RC2 PDF (Fig. 6), the E2 modal time is slightly older at 1.3 ka (0.7–1.7) (Table 2). Although a question remains about whether E2 ruptured the Kaysville site (K2), which has important implications for the E2 rupture length and estimated magnitude, K2 fully overlaps with the other site PDFs composing E2 (RC2, GC2, and EO2), and thus, including or excluding K2 does not affect the mean time of E2. Similar to E4, we conservatively consider E2 a full-segment rupture.

Earthquake Recurrence To calculate recurrence intervals, we used a Monte Carlo simulation to randomly sample the segment PDFs. For each scaled segment PDF, we multiplied the probability for each 5-yr time bin by 10,000 to compile a dataset of earthquake times based on the area under the PDF curve. Using earthquake E3 as an example (Fig. 7), the peak probability is about 0.06 at 3.0 ka, and tails of the PDF extend to 2.8 and 3.6 ka, both having probabilities of about 0.001. Multiplying the probabilities for these time bins by 10,000 produces 600 entries for 3.0 ka (10; 000 × 0:06) and 10 entries each for 2.8 and 3.6 ka. This process is completed for each 5-yr time bin, and the values are compiled in a single dataset that can then be randomly sampled. If the dataset is sampled enough times (e.g., 10,000 times), then the resulting histogram of the sampled time values is nearly identical to the original segment PDF (minor differences result from the random sampling procedure). Thus, to determine recurrence intervals, we ran 10,000 simulations, with each simulation randomly sampling earthquake times for earthquakes E5 to E1 based on the original segment PDFs. The time intervals between these event times were then used to calculate recurrence. In each simulation, we calculated (1) the recurrence time between pairs of events (individual-event recurrence; e.g., E5–E4), (2) the mean recurrence based on the total elapsed time between the oldest and youngest events divided by the number of closed intervals between events (closed mean recurrence; E5–E1 divided by 4), and (3) the open recurrence (to the past and present) based on the total elapsed time from the maximum age constraint on the oldest event (E5) to the present (2010) divided by the number of earthquakes that occurred in that period (open mean recurrence; E5 maximum age– present divided by 5). These recurrence estimates were then filtered to eliminate recurrence values less than 195 165 yr (2σ) based on the elapsed times since (1) the Borah Peak, Idaho, earthquake rupture (∼30 yr), which is now forming colluvial wedges (Haller and Crone, 2004); and (2) the most recent earthquake on the WFZ (Nephi segment, < 360 yr). We consider this elapsed time range of 30–360 yr to be a reasonable minimum estimate of the time required to degrade a fault-scarp free face and begin to develop a colluvial wedge in a semiarid environment—our main source of evidence for separate surface-faulting earthquakes. We converted the recurrence values from all simulations into probability plots (Fig. 8) that include the mean and 2σ values reported here. Individual-event recurrence intervals (Fig. 8a) highlight the apparent aperiodicity of the WS earthquake history. The broadly constrained recurrence distributions reflect the relatively large 2σ timing uncertainties for earthquakes E5 to E2 (Fig. 7) and suggest that the possible range of recurrence between events is 80 yr (low-probability approximate minimum time between E2 and E1) to 2640 yr (low-probability approximate maximum time between E3 and E2). However, we prefer the mean interevent recurrence times, which range

Integration of Paleoseismic Data to Develop Earthquake Chronology: Wasatch Fault Zone, Utah from ∼0:7 kyr (E2–E1) to ∼1:9 kyr (E3–E2) and include the more consistent ∼1:4-kyr recurrence for E5–E4 and E4–E3. Our preferred mid-Holocene recurrence interval for the WS is 1:3 0:6 kyr, based on the closed mean recurrence interval for five earthquakes on the WS since about 5.9 ka (1:3 0:1 kyr) and interevent recurrence intervals ranging from 0:7 0:6 kyr (E2–E1) to 1:9 0:7 kyr (E3–E2) (Fig. 8). The small uncertainty in the closed mean recurrence interval (0:1 kyr) stems from our modeling method: in each simulation, a mean recurrence value is calculated (E5 time minus E1 time divided by four intervals); thus, the closed mean recurrence PDF shown in Figure 8b is for the population of mean values calculated over 10,000 model simulations. As a result, the uncertainty in the closed mean recurrence is small (0:1 kyr) because it does not account for variability in the inter-event recurrence intervals. In view of this, and the limited WS earthquake record (only four closed intervals), the

(a)

0.1

E5–E4 1420 ± 590 yr (2σ)

0 2σ

Probability density

0.1

E4–E3 1380 ± 410 yr

0 0.1

E3–E2 1950 ± 690 yr

0 0.1 E2–E1 650 ± 570 yr

0 0

0.5

1.0

1.5

2.0

2.5

3.0

Inter-event recurrence interval (yr)

(b)

0.2

Closed mean recurrence (E5–E1 / 4) 1330 ± 130 yr (2σ)

Probability density

0.1

0 2σ

0.2 Open mean recurrence (E5 maximum age–present / 5) 1420 ± 270 yr

0.1

0.5

1.0

1.5

resulting mean recurrence PDF may not be complete. Thus, we report a more conservative 2σ uncertainty of 0:6 kyr, based on the minimum and maximum mean individual-event recurrence estimates (Fig. 8a; 0.7–1.9 kyr). We calculated an open mean recurrence interval using the elapsed time from the maximum age constraint on E5 (7:1 1:4 ka from Rice Creek; see the Data and Resources section) to the present (2010) divided by five earthquakes. The open mean recurrence is 1.4 kyr (Fig. 8b): however, we favor the closed mean recurrence (1.3 kyr), which is based on known earthquakes and intervals, rather than including the elapsed times from E5 to the past and E1 to the present, which are minimum recurrence intervals. Comparison of Mean and Product PDF Methods WS earthquake PDFs generated using the product PDF method have smaller timing uncertainties, but similar mean times compared to earthquake PDFs produced by taking the mean of the site PDFs (mean method) (Fig. 6). Earthquaketiming uncertainties based on the product PDFs are about 20%–40% of the uncertainties produced in the mean method, which reflects the trimming of the broadest, least-wellconstrained site PDFs (e.g., GC1 that contributes to E1; Fig. 6) in the product method. For example, the product method refines the earthquake E1 timing uncertainty from 300 (mean) to 70 yr. This refinement in the E1 timing uncertainty is important to modeling time-dependent earthquake probabilities and hazard for the WS. In contrast, there is good agreement in the mean earthquake times between the two methods, which vary by zero (E1) to 160 yr (E4). Because of these similar mean times, the recurrence intervals computed in both methods are nearly identical: the closed mean recurrence for E5–E1 is 1330 130 yr using the product method (Fig. 8) and 1330 290 yr using the mean method. Using the simplemean method for E2 does not effect the mean recurrence calculation. Mean interevent recurrence intervals—which we used to define an estimated 2σ uncertainty about the 1.3-kyr mean—are also similar. Using a simple-mean, rather than product result for E1 and E3, the E2–E1 and E3–E2 mean interevent recurrence intervals do not change by more than 100 yr.

Comparison with Previous Analyses and Interpretation of Paleoseismic Data

0 0

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2.0

2.5

3.0

Mean recurrence interval (yr)

Figure 8. Recurrence-interval PDFs for earthquakes on the WS, based on Monte Carlo modeling of segment PDFs (see text for explanation), showing (a) individual-event recurrence (e.g., E5–E4) and (b) mean recurrence (e.g., E5–E1). Closed mean recurrence is based on the E5–E1 elapsed time divided by four closed intervals; open mean recurrence is based on the elapsed time from the maximum constraint on E5 to the present (2010) divided by the number of earthquakes (5) that occurred in this time period. Recurrence uncertainties reported as the mean 2σ: Horizontal bars show 2σ ranges.

We present a simple earthquake correlation, in which five earthquakes (E5–E1) ruptured all or most of the WS. This scheme differs from that proposed by Nelson et al. (2006), who only correlated two late-Holocene earthquakes among the three trench sites: East Ogden earthquakes EO3 and EO2 with Kaysville events K3 and K2, and Garner Canyon earthquakes GC2 and GC1 (Table 1). Nelson et al. (2006) also suggested the possibility that East Ogden earthquake EO4 (2.8–4.8 ka) is the third event at Kaysville (K4 in Table 1) of McCalpin et al. (1994) based on their possible time range of 3.9–7.9 ka. However, McCalpin et al. (1994)

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and McCalpin and Nishenko (1996) did not correlate these earthquakes, inferring that Kaysville event K4 (using their 5.7–6.1-ka preferred time) likely predates the faulted alluvial-fan deposits at East Ogden and that East Ogden earthquake EO4 (2.8–4.8 ka) did not rupture the Kaysville site. At the Rice Creek site, evidence for earthquakes at ∼4:6 ka (RC4) and ∼5:5 ka (RC5) favors the interpretation of McCalpin et al. (1994). Our OxCal models conclusively show that the most recent surface-rupturing earthquake on the WS, E1, occurred at all four trench sites, with a mean time of 0.6 ka. In contrast, Nelson et al. (2006) interpreted their youngest East Ogden earthquake at ∼0:5 ka to be younger than the most recent events at Garner Canyon (∼0:6 ka) and Kaysville (∼0:6 ka). Based on this interpretation, they proposed a scenario of a partial rupture along the northern part of the segment. However, DuRoss et al. (2009) found evidence for the 0.5-ka event at Rice Creek, which is north of the Garner Canyon site (Fig. 2). Thus it seems likely that the 0.6-ka earthquake at Garner Canyon, which is located between East Ogden and Rice Creek, correlates with the 0.5-ka Rice Creek and East Ogden events. The slightly older age of 0.6 ka may be related to the MRT applied to the radiocarbon ages for the Garner Canyon bulk-soil 14 C samples. Similarly, we correlate Kaysville earthquake K1—revised to 0.6 ka based on our OxCal model—with the 0.5-ka northern WS earthquake, rather than define it as a separate partial rupture of the southern WS, or correlate it with East Ogden earthquake EO2 as suggested by McCalpin et al. (1994). Our revised earthquake-timing estimates are very similar to the consensus values of the Utah Quaternary Fault Parameters Working Group (UQFPWG) (Lund, 2005), which were based on the summary of McCalpin and Nishenko (1996). The mean times of earthquakes from our analysis differ from the preferred times of UQFPWG by about 1–2 kyr. This difference is related to our OxCal modeling and the site PDFs, and to the fact that we integrated the site PDFs into segment PDFs. Our methodology yields 2σ uncertainties that are significantly less (about 20%–70%) than the estimated 2σ uncertainties of the UQFPWG (Table 2): the UQFPWG estimates were qualitatively modified from those reported by McCalpin and Nishenko (1996) based on the limiting 14 C and luminescence ages. In contrast, our method of objectively determining earthquake times and uncertainties by propagating uncertainties through each modeling step is reproducible and thus preferable to the qualitative approach used earlier by the UQWFPG. Our 1.3-kyr closed mean recurrence interval for E5–E1 compares favorably with previously published estimates. Nelson et al. (2006) reported a preferred recurrence of 1.5– 1.6 kyr, based on the two intervals between their earthquakes 4, 3, and 2, and DuRoss et al. (2009) calculated a mean Holocene recurrence interval of 1.5 kyr based on five earthquakes between about 6.5 and 0.5 ka (Table 1). The shorter recurrence estimate from our study stems from the revised times for earthquakes E5 and E1, and the positively skewed

shape of the E5 PDF (a younger E5 time is more probable) (Table 2). Our estimated 2σ range of 0.7–1.9 kyr is smaller than ranges of 0–3 kyr and 0.5–3.0 kyr reported by Nelson et al. (2006) and DuRoss et al. (2009), respectively, because we use the minimum and maximum values of the mean individual-event recurrence intervals, rather than the minimum and maximum range of possible recurrence. Similarly, the UQFPWG (Lund, 2005) reported a preferred Holocene recurrence interval of 1.4 kyr (0.5–2.4 kyr estimated 2σ range) using five earthquakes between 6.1 and 0.5 ka. If we exclude E4, based on the southern-segment-earthquake data, then the mean recurrence time increases to approximately 1.8 kyr, which is closer to the McCalpin et al. (1994) estimate of 2.7 kyr. However, we favor the occurrence of earthquake E4 based on our analysis of data from all the sites and questions about the completeness of the Kaysville earthquake record. Accordingly, we suggest the recurrence information shown in Figure 8 best reflects the late- to mid-Holocene earthquake history of the entire WS.

Discussion and Conclusions The objective analysis of paleoseismic data is an important first step in a comprehensive regional earthquake-hazard analysis. However, for the WFZ, trenching and dating methods have evolved over three decades and the evaluation of paleoseismic data is complicated by the difficulty in correlating and integrating variable-quality earthquake-timing data. Thus, we present a method in which we (1) use OxCal to objectively model earthquake PDFs for each site (site PDFs), (2) qualitatively correlate site PDFs between trench sites taking into account the overlap in the PDFs, (3) use the product PDF method to integrate these data into an earthquake chronology for the segment (with the exception of E2, where a simple-mean method is used), and (4) calculate earthquake recurrence using the segment chronology in a Monte Carlo model. We prefer the product PDF method because it includes all of the site PDF data rather than excluding or subjectively weighting the least well constrained data, and focuses on the overlap in the site PDFs, thus giving more weight to the narrowest, bestconstrained PDFs from sites that establish the best limits on the time of an earthquake. However, the product method is best suited to paleoseismic datasets in which the OxCal models (and resulting PDFs) are supported by geologic observation and judgment, there is reasonable confidence in the correlation of site PDFs to form a segment or fault rupture chronology, and earthquakes are constrained by overlapping site PDFs. Where site PDFs contributing to an earthquake have poor overlap, or the correlation of site PDFs is uncertain, the mean of the site PDFs may more accurately model the uncertainty in the earthquake time. Applied to the WS, our product PDF method helps resolve questions regarding the timing, extent, and recurrence of surface-faulting earthquakes. Paleoseismic data from four sites on the WS show that five earthquakes occurred at 5:9 0:5 ka (5.6-ka mode) (E5), 4:5 0:3 ka (E4),

Integration of Paleoseismic Data to Develop Earthquake Chronology: Wasatch Fault Zone, Utah 3:1 0:3 ka (E3), 1:1 0:6 ka (E2), and 0:6 0:1 ka (E1). We found that product-PDF mean times are similar to those generated using the simple mean of the site PDFs but have smaller associated uncertainties. For example, we refined the uncertainty in the E1 time from 560 290 cal yr B:P: (mean) to 560 70 cal yr B:P. We interpret these five earthquakes as rupturing most of the WS (the 33 km between the Rice Creek and Kaysville sites; Fig. 2) and probably the entire segment. Thus, the mid-Holocene-to-present closed mean recurrence interval for the WS is 1.3 kyr (0.7–1.9 kyr estimated 2σ range). However, there is uncertainty in the rupture length of E4, which was not identified at the Kaysville site, and E2, which we interpret to have ruptured the Kaysville site based on our synthesis of previous data. Importantly, our results conclusively show that the youngest earthquakes at each of the four sites correlate into a single segment-wide earthquake (E1) at 0.6 ka: previously, the timing and extent of E1 was poorly understood. Our revised mean earthquake times are similar to previously published values, especially those of the UQFPWG (Lund, 2005) but have smaller uncertainties than the values determined from the more subjective working-group consensus process. In addition, we present a correlation of site earthquakes that reflects our use of OxCal to objectively model site earthquake times, and new data from the study at Rice Creek. Although our selection of a preferred correlation of site PDFs introduces a subjective component to our analysis, our correlation is supported by site PDFs having mean overlap values per earthquake of 0.40–0.60, large per-event displacements, and prominent segment boundaries. However, we recognize that the segment boundaries between the WS and adjacent Brigham City and Salt Lake City segments may fail from time to time (e.g., WS E2) in noncharacteristic, multisegment ruptures (Chang and Smith, 2002; DuRoss, 2008). Similar analyses of paleoseismic data for these adjacent segments will allow for the development and quantitative testing of multiplesegment rupture scenarios on the central WFZ. Our product PDF method is an important tool that objectively evaluates and combines paleoseismic-site data of varying age and quality to yield a reproducible segment-wide measure of earthquake timing and recurrence. Our analysis of paleoseismic data for the WS demonstrates the utility of the method; we have refined the timing of five Holocene surface-faulting earthquakes on the segment and reduced uncertainties in their rupture extents. These data advance our understanding of the overall earthquake behavior and segmentation of the WFZ, and are important to the improvement of earthquake-probability assessments for the region such as the time-dependent earthquake forecast being developed for the central WFZ by the WGUEP (http://geology.utah.gov/ghp/ workgroups/wguep.htm).

Data and Resources The paleoseismic data for the WS that we discuss here are available from published sources listed in the references.

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Ⓔ The OxCal models for the individual trench sites, summaries of our OxCal modeling results, and WS earthquaketiming PDFs are included in the electronic supplement to this article.

Acknowledgments The analyses described in this paper were motivated by our paleoseismic study at the Rice Creek site, supported by the U.S. Geological Survey National Earthquake Hazards Reduction Program (award No. 07HQGR0093) and the Utah Geological Survey. We thank the members of the WGUEP for early discussions of this work and Glenn Biasi (University of Nevada, Reno) for help with random sampling of PDFs and for providing an informal review of our product-PDF methods. We also thank Christopher Bronk Ramsey (University of Oxford, United Kingdom) for helpful discussions regarding OxCal analyses. Constructive reviews by Ryan Gold (USGS), Michael Hylland (Utah Geological Survey), and two anonymous reviewers improved this manuscript.

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Utah Geological Survey 1594 West North Temple, P.O. Box 146100 Salt Lake City, Utah 84114 [email protected] (C.B.D., W.R.L.)

U.S. Geological Survey MS 966, P.O. Box 25046 Denver, Colorado 80225 (S.F.P., A.J.C.)

URS Corporation 1333 Broadway, Suite 800 Oakland, California 94612 (S.S.O.) Manuscript received 7 April 2011

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