Fault Scarp Morphology Research Articles

Constraining Paleoseismicity of the Wulashan Piedmont Fault on the Northern Margin of the Ordos Block From Fault Scarp Morphology

Fault scarps preserve important information about past earthquakes on a fault, and thus can be applied to investigate the fault slip histories and rupture patterns. In this study, the morphology of fault scarps was used to constrain the paleoseismicity of the Wulashan Piedmont Fault located on the northern margin of the Ordos Block based on high-resolution LiDAR topography. We constructed the vertical displacement distribution of the fault through measuring the heights of a large number of scarp profiles extracted on different geomorphic surfaces along the fault. Through statistical analysis of the dense collection of vertical displacement dataset, a total of seven paleoseismic events were identified which followed a characteristic slip pattern with an average slip of ∼1.0 m. We further detected slope breaks in the fault scarp morphology to quantify the number of paleoearthquakes that occurred on the scarps, and discriminated at least five individual surface-breaking events. Both the number and slip of paleoearthquakes recognized from the morphology of fault scarps were in good agreement with previous paleoseismic trenching records. Based on the empirical scaling relationship between moment magnitude and rupture parameters, a moment magnitude ofMw6.7–7.5 was determined for the paleoearthquakes occurred on the fault. With the fault slip rate derived by previous studies, we estimated an average recurrence interval of 1.3–1.8 kyr for the paleoseismic events, which is very close to the elapsed time since the most recent earthquake, indicating a high potential seismic hazard on the Wulashan Piedmont Fault.

Investigating paleoseismicity using fault scarp morphology of the Dushanzi Reverse Fault in the northern Tian Shan, China

This study uses the morphology of fault scarps to study the paleoseismic history of the Dushanzi Reverse Fault (DRF) in the northern Tian Shan region, China. Using high-resolution LiDAR DEMs to create a large number of scarp profiles, we detected slope breaks in the fault scarp morphology to quantify the number of paleoearthquakes that occurred on the scarps and obtained the along-strike scarp height distribution to determine the cumulative and single-event vertical displacements. Our results revealed four surface-rupturing events with similar co-seismic vertical displacements of ~1.4 m via the statistical analysis of displacement datasets and slope breaks, which were in agreement with the findings from paleoseismic trenching. We then assessed the paleomagnitudes using the empirical scaling relationships between moment magnitude and rupture parameters and the recurrence interval for surface-rupturing events using the formation ages of the displaced terrace surfaces. The results indicate that the possible magnitudes range from Mw 6.5 to 6.7, even reaching Mw ~7.2 in the case of a multi-fault rupture of the entire anticline-fault belt, with an average recurrence interval of ~2.4–3.0 ka. This study also demonstrates the importance of using high-resolution topographic data in paleoseismic investigations, which enables the determination of fine-scale tectono-geomorphic interpretations and detailed offset measurements over large regions along the fault.

Late Quaternary tectonic activity and shortening rate of the Beiluntai fault zone in the South Tianshan, Xinjiang, NW China

The Beiluntai fault is a Holocene active fault and is the boundary fault between the South Tianshan and Tarim Basin. Since the Late Quaternary, the continuous activities of the Beiluntai fault have caused dislocation, deformation, and fold uplift of multi-phase alluvial geomorphologic surfaces. Using a high-precision differential global positioning system, we measured the fault scarp morphology of multi-phase geomorphologic surfaces in the Akeaiken and Zhuanchang sections of the Beiluntai fault. Via large-scale active fault mapping, we found that the Akeaiken section is dominated by thrusting, while the Zhuanchang section is dominated by fold uplift. We obtained the ages of the different phases of the geomorphologic surfaces (Fan4, Fan3b, Fan3c, and Fan2) using the optically stimulated luminescence dating method. We found that since the formation of the Fan4 geomorphologic surface, the crustal shortening rate (approximately 2.4 mm/a) of the Akeaiken section has remained basically constant. However, the crustal shortening rate of the Zhuanchang section in the south–north direction (1.43–1.81 mm/a) during the Late Quaternary was significantly less than that in the Akeaiken section, suggesting that the crustal shortening rate in the south–north direction of the Beiluntai fault zone decreases from west to east. A comprehensive comparison of the South Tianshan piedmont thrust-and-fold belt system also showed that the crustal shortening rate decreases from west to east.

Investigating multiple fault rupture at the Salar del Carmen segment of the Atacama Fault System (northern Chile): Fault scarp morphology and knickpoint analysis

This study presents a new geomorphological approach to investigate the past activity and potential seismic hazard of upper crustal faults at the Salar del Carmen segment of the Atacama Fault System in the northern Chile forearc. Our contribution is based on the analysis of a large set of topographic profiles and allows extrapolating fault analysis from a few selected locations to distances of kilometers along strike of the fault. We detected subtle changes in the fault scarp geometry which may represent the number of paleoearthquakes experienced by the structure and extracted the cumulative and last incremental displacement along strike of the investigated scarps. We also tested the potential of knickpoints in channels crossing the fault scarps as markers for repeated fault rupture and proxies for seismic displacement. The number of paleoearthquakes derived from our analysis is 2–3, well in agreement with recent paleoseismological investigations, which suggest 2–3 earthquakes (Mw = 6.5–6.7) at the studied segments. Knickpoints record the number of events for about 55% of the analyzed profile pairs. Only few knickpoints represent the full seismic displacement, while most retain only a fraction of the displacement. The along-strike displacement distributions suggest fault growth from the center toward the tips and linkage of individual ruptures. Our approach also improves the estimation of paleomagnitudes in case of multiple fault rupture by allowing to quantify the last increment of displacement separately. Paleomagnitudes calculated from total segment length and the last increment of displacement (Mw = 6.5–7.1) are in agreement with paleoseismological results.

Geophysical and Geomorphological Analysis of the Teton Fault, Wyoming

This investigation applied geophysical and geomorphological analyses of the Teton Fault to assess its geometry, history, and influences on landscape evolution. This project builds on results from a preceding geophysical study completed one year ago (Thackray et al. 2014), a recent study of fault scarp morphology (Thackray and Staley, in review), and years of previous studies of the fault by many practitioners.

Holocene scarp on the Sawtooth fault, central Idaho, USA, documented through lidar topographic analysis

High-resolution lidar data reveal a prominent latest Pleistocene–Holocene scarp on the Sawtooth fault (central Idaho, United States). The fault scarp marks 55–65 km of the range front, and may comprise two segments. The scarp is 4–9 m high in latest Pleistocene glacial landforms (11–14 ka) and 2–3 m high in Holocene alluvial landforms, implying 2–3 postglacial rupture events. Patterns of fault scarp continuity, coupled with existing gravity data, suggest that active faulting may have migrated northward during Pleistocene time. Detailed comparisons of raw lidar digital elevation models (DEMs), bare-earth lidar DEMs, and field surveys indicate that the bare-earth lidar data document the fault scarp morphology accurately and allow for detailed fault analysis where field evaluation is difficult. The documentation of Holocene motion on the Sawtooth fault demonstrates that ENE-directed extension extends across central Idaho, and that the fault contributes to seismic hazards.

Spatial patterns of geomorphic surface features and fault morphology based on diffusion equation modeling of the Kumroch Fault Kamchatka Peninsula, Russia

Remote sensing data sets are widely used for evaluation of surface manifestations in active tectonic areas. This study utilizes ASTER GDEM and Landsat ETM+data sets with Google Earth images draped over terrain models. This study evaluates the surrounding surface geomorphology of the study area with these data sets and the morphology of the Kumroch Fault using diffusion modeling to estimate diffusion constants (κ), as well as to estimate slip-rates by means of real ground data measured across fault scarps by Kozhurin et al. (2006). The study area is located southeast of the Bezymianny Volcano within the Central Kamchatka Depression. It is part of the Kumroch Fault Zone. The area contains numerous geomorphic features indicative of active deformation, such as fault scarps, ponds, offset streams, and glacier moraines.The ASTER GDEM and Landsat ETM+data together with Google Earth imagery were used to create accurate geomorphic maps of specific sites within the study area along the western flank of the East Kamchatka Range. Previous studies have estimated the time elapsed since slip initiation of the faulted surface on the Kumroch Fault and the slip-rate by dividing the offset of moraines by the age of the ruptured surface. Slip-rates along the Kumroch Fault vary from 2.0mm/yr–2.5mm/yr, determined from previous 14C studies with time frames of 11.5ka to 14ka. Other slip-rates for the Kumroch Fault were determined from previous studies through tephra and stratigraphic chronology. These produced rates of 0.6mm/yr, with time frames of 3.2ka–10.5ka. Models of the evolution of fault scarp morphology provide the amount of time elapsed since slip initiated on a faults surface. They may therefore provide more accurate estimates of slip-rate than those calculated by dividing scarp offset by the age of the ruptured surface. Profile modeling of scarps collected by Kozhurin et al. (2006) show that the scarps were formed by several events distributed over time. They were evaluated using a constant slip-rate (CSR) solution which yields a value A/κ (1/2 slip rate/diffusivity). Time elapsed since slip initiated on the fault is determined by establishing a value for κ and measuring total scarp offset. For this study, CSR nonlinear modeling assisted in estimates of κ which along the Kumroch Fault range from 8m2/ka to 14m2/ka. Slip-rates have been estimated to range from 0.6mm/yr to 1.0mm/yr since 3.4ka–3.7ka. This method provides a quick and inexpensive way to gather data for a regional tectonic study and establish estimated rates of tectonic activity.

Morphologic dating of fault scarps using airborne laser swath mapping (ALSM) data

Models of fault scarp morphology have been previously used to infer the relative age of different fault scarps in a fault zone using labor‐intensive ground surveying. We present a method for automatically extracting scarp morphologic ages within high‐resolution digital topography. Scarp degradation is modeled as a diffusive mass transport process in the across‐scarp direction. The second derivative of the modeled degraded fault scarp was normalized to yield the best‐fitting (in a least‐squared sense) scarp height at each point, and the signal‐to‐noise ratio identified those areas containing scarp‐like topography. We applied this method to three areas along the San Andreas Fault and found correspondence between the mapped geometry of the fault and that extracted by our analysis. This suggests that the spatial distribution of scarp ages may be revealed by such an analysis, allowing the recent temporal development of a fault zone to be imaged along its length.

Ποσοτική ανάλυση - μορφοτεκτονικοί δείκτες για τις περιοχές των ρηγμάτων Στρατωνίου - Βαρβάρας και Γοματίου - μεγάλης Παναγιάς (Ανατολική Χαλκιδική)

The study area is located at the NE part of Chalkidiki peninsula. Geologicaly it belongs at Servomacedonian mass. The faults of Stratoni - Barbara and Gomati- M. Panagia have an important role at the surface morphology of the area. These are typically active faults and they are the most important neotectonic structures at Eastern Chaikidiki. For the study of surface morphology a quantitative morphotectonic analysis has been done with the contribution of Geographical Information Systems GIS. The morphological indicators we have studied are: hypsometric curve and hypsometric integral, drainage basin asymmetry, mountain - front sinuosity, stream length - gradient index, transverse topographic symmetry, radio of valley - floor width to valley height and fault scarp morphology. This morphotectonic analysis has confirmed the field observations. Major role in this research had the use of satellite pictures and 3D models (Digital Elevation Model - D.E.M.).

Dating rupture events on alluvial fault scarps using cosmogenic nuclides and scarp morphology

Scarp morphology evolution has commonly been used to estimate the timing of fault-scarp rupture events. However, rates of scarp degradation depend strongly on the geomorphic diffusivity, a parameter that is difficult to constrain independently. This difficulty may lead to large uncertainties in the estimated ages of rupture events. In this study, we have coupled the accumulation of the cosmogenic nuclide 36Cl to a model for scarp morphology in order to constrain the value of the geomorphic diffusivity and thus determine a more accurate rupture chronology than a rupture history based solely on scarp morphology. We measured depth profiles of 36Cl accumulated in situ within alluvial sediments beneath the surface of the Socorro Canyon fault scarp in central New Mexico. The material analyzed consisted of ∼150 individual gravel clasts from each depth interval, amalgamated into a single sample. The alluvium was sampled in three vertical profiles ∼4 m deep. The first profile was ∼1.5 m downslope of the fault plane on the hanging wall, the second ∼1.5 m upslope of the fault plane on the footwall, and the third ∼27 m upslope of the fault plane on the footwall. The third profile, which served as a control, and which soil geomorphic evidence indicated was beneath a stable surface, showed a simple exponential 36Cl profile from which a depositional age of 122±18 ka was calculated. The second profile (on the footwall) showed a 36Cl deficit relative to the control profile, indicating net erosion. The first profile (on the hanging wall) showed a 36Cl excess relative to the control profile, indicating net deposition. Stratigraphic evidence in the hanging wall indicated two rupture events prior to the Late Holocene. We modeled the accumulation of 36Cl in the vicinity of the scarp, simulating erosional redistribution using a diffusion equation for scarp morphology. The model accounted for redistribution of 36Cl along with the sediment and the effects of the changing fault-scarp morphology on the 36Cl production. By matching both the observed hanging-wall and footwall 36Cl profiles to profiles calculated by the model, as well as the observed topographic profile, we obtained ages of 92+16−13 and 28+18−23 ka for the two ruptures. The results of our study indicate that cosmogenic nuclides can be useful in constraining rupture chronologies of fault scarps in alluvium.

Morphological dating of cumulative reverse fault scarps: examples from the Gurvan Bogd fault system, Mongolia

We relate reverse fault scarp morphology formed by several earthquake dislocations to the average deformation rate, using a morphological dating model based on a diffusion analogue of erosion. Our scarp degradation model includes diffusive erosion during the interseismic period, the gravitational collapse of the coseismic fault scarp just after formation, and the variation of the surface rupture location. Interactions between thrusting and geomorphic processes acting on scarp morphology are analysed along the Gurvan Bogd Range in Mongolia. Four main processes acting on scarp morphology were distinguished: 1) gravitational collapse of the frontal scarp, resetting the diffusive scarp if fault offsets are big and faulting is localized; 2) progressive erosion of the fault scarp during the interseismic period; 3) folding associated with the frontal thrust and backthrusts; 4) competing alluvial deposition on mountain piedmont slopes and abrasion of the fault scarp by wash processes. The growth of cumulative reverse fault scarps is suppressed when they are located in the outwash of major drainage basins. They can grow higher in distance from major catchment discharges. The modelling suggests that the morphology of the scarp and its apparent degradation stage, depend on the parameters controlling the amount of frontal collapse; the magnitude of coseismic offsets, the dip of the fault near the surface and the step distance between faults. Folding associated with thrusting creates a convexity on the upper part of the scarp and increases its height. The comparison of different scarp profiles suggests that folding leads to an overestimate of the morphological age. We estimate a diffusion coefficient at 3.3 ± 1.7 m2 ka−1. Morphological ages calculated with our model confirm that slip rate along reverse faults of the Gurvan Bogd range has not been constant over the last 100 ka.

Interaction between normal faults and fractures and fault scarp morphology

Fault slip and geomorphic surface processes create and modify bedrock normal fault scarps. Our field studies and numerical models show that mechanical interaction between near‐surface fractures and active faults may diminish scarp slopes and broaden deformation near the surface. In our models, increasing fracture density and depth reduces and complicates the scarp's topographic expression. Fracture depth and density, and orientation of the structures control the location and magnitude of zones of positive and negative shear displacements along the fractures. Our field studies of the interaction of active graben bounding normal faults and fractures in Northern Arizona illustrate the mechanical analysis. Reduction in scarp slopes and broadening of the deformation field may cause incorrect interpretations of shallow bedrock scarp morphologies as old, rather than as the result of slip along fractures around the fault.

Fault slip rates and initiation age based on diffusion equation modeling: Wasatch Fault Zone and eastern Great Basin

Models of the evolution of fault scarp morphology provide time elapsed since slip initiated on a faulted surface and may therefore provide more accurate estimates of slip rate than the rate calculated by dividing scarp offset by the age of the ruptured surface. To accomplish this task, linear and nonlinear models of sediment transport are calibrated from the morphology of Lake Bonneville shoreline scarps and fault scarps formed by multiple, surface‐rupturing earthquakes along the Wasatch Fault Zone (WFZ). Profile modeling of scarps formed by several events distributed through time is done using a constant slip rate (CSR) solution and yields a value ofA/κ (1/2 slip rate/diffusivity). Time elapsed since slip initiated on a fault is determined by establishing a value for κ and measuring total scarp offset. CSR nonlinear modeling (κ0= 2.8 ± 1.1 m2/kyr, WFZ) of faults along the west slope of the Oquirrh Mountains indicates a slip rate of ∼0.1 mm/yr since 50 to 65 ka, which is corroborated by cosmogenic dating (10Be/26Al age = 75 ka). The slip rate along the west flank of the Stansbury Mountains varies from 0.04 to 0.2 mm/yr for time frames of 10 to >100 ka, with the most recent rupture on the northern portion of the fault zone ∼10 ka. Scarp analysis of the southern end of the Nephi segment, WFZ, suggests either temporal clustering or variable slip rate as indicated by differences in the short‐term (1.3 mm/yr for 4.3 ka) versus long‐term (0.4 mm/yr for 70 ka) slip rates.

Soils as a tool for estimating ages of Quaternary fault scarps in a hyperarid environment — the southern Arava valley, the Dead Sea Rift, Israel

On the alluvial fan of Nahal Shehoret in the extremely arid region of the Negev desert, Israel, Reg soils developed on stable alluvial surfaces in colluvium along terrace risers and in colluvial units on a complex fault scarp were compared. The degree of soil profile development provided an age estimate of the surface that was faulted, a maximum age estimate of the scarp or terrace riser, and an estimate of the maximum recurrence intervals of faulting events. IRSL dates of the tectonically displaced alluvial surfaces and colluvial units at the same site enabled us to re-evaluate the use of Reg soils for relative dating. The major faulting event started at 34.8 ± 4.3 ka. It displaced a late Pleistocene surface of the Nahal Shehoret alluvial fan dated at 56 ± 10.8 ka. Renewed fluvial activity and deposition of alluvium on the downfaulted block had almost terminated by 13.6 ± 2.3 ka. The scarp was formed during the latest Pleistocene and was followed by smaller faulting events which had only minor effects on the fault scarp morphology. The similarity of age estimates from IRSL dating and the degree of soil development in this environment indicates that soils can be used to date surfaces and colluvial units.

Structural analysis and fault segment boundary identification along the Hurricane fault in southwestern Utah

Long normal fault zones are common in extending regions and can be divided into segments with different geometries and faulting histories based on surface observations. The boundaries between fault segments are important because they may be the sites of significant strain, may impede rupture propagation, and may greatly influence the locations of earthquakes. Because large geometric bends exist at some fault segment boundaries throughout the history of the boundary, we suggest that such bends can last throughout much of the life of the fault. We use fault segmentation concepts to define a segment boundary and parts of two fault segments along the active Hurricane normal fault, southwestern Utah U.S.A. We use fault geometry, slip direction, shortening structures in the hanging wall and footwall, and fault scarp morphology to identify the fault segments and segment boundary. The fault strikes N13 °W south of the boundary and N21 °E north of the boundary. Stratigraphic separation of geochemically identical Quaternary basalt in the footwall and the hanging wall suggests that the slip vector of Quaternary displacement trends between N70 °W and S18 °W, and slickenlines, earthquake rakes, and dip analysis on syndeformational basalt further constrains the slip direction to N75–85 °W. Relative motion on the northern segment, termed here the Ash Creek segment, is purely dip-slip, and on the southern (Anderson Junction) segment is dominantly dip-slip with a small dextral slip component. Fault scarps along the Ash Creek segment compared with offset gravels lacking scarps along the Anderson Junction segment suggest that the two segments have different Late Quaternary faulting histories, which is indicative of discrete fault segments. The Ash Creek segment is about 24 km long and the Anderson Junction segment is 19–45 km long, although the non-adjacent segment terminations remain poorly defined. The segment boundary zone consists of: (1) a small-scale hanging wall anticline that trends roughly normal to the fault; (2) a small-offset footwall thrust fault; and (3) space-filling imbricate normal faults in the hanging wall. Secondary structures suggest the segment boundary is a non-conservative boundary.

Late Pleistocene-Holocene sediments on the Spanish continental shelves: Model for very high resolution sequence stratigraphy

Sequence stratigraphy analysis of high resolution seismic profiles (Geopulse, Uniboom and 3.5 kHz) of late Pleistocene-Holocene sediments has been carried out on five sectors of the Spanish continental margin. Four types of depositional settings are distinguished in these sectors: (1) low subsident ramps (Alborán Margin-Cádiz Gulf); (2) high subsident (2m/kyr) ramps (Alicante-Valencia); (3) “Ria”-type morphology on the Atlantic passive margin (Ria de Muros); and (4) fault-scarp morphology systems with subsidence (Balearic Margin). A Type 1 sequence is interpreted in all these sectors, being composed of lowstand systems tract, transgressive systems tract and highstand systems tract. This conforms to the basic concepts of sequence stratigraphy and each systems tract correlates with a particular part of the last eustatic hemicycle. Characteristic shelf features such as terraces, terraces with beach deposits and progradational sediment wedges evidence a complex stacking of lesser sub-sequences in all the systems tracts, which must be related to very-short period sea-level stillstands and fall. We propose a very high resolution sequence stratigraphy model in which the last sea-level hemicycle is punctuated by: “P” cycles (4500 years), which give rise to the neo-glacial events; “h” cycles (2200-950 years), and “c” cycles (500-50 years). These cycles interact with each other, thus establishing the placing of the high and low sea levels. This attention to detail: (1) explains sedimentary evolution on both the shelf and upper slope during late Pleistocene-Holocene time; (2) illustrates some departures from the classical sequence stratigraphy model; and (3) also demonstrates that the late Pleistocene-Holocene eustatic curve is not one simple transgression but is modulated by three differing-period cycle groups below the Milankovitch band. Our model is delimited by fluctuating sea level during Pleistocene-Holocene times. Such features should be identifiable on any continental margin. However, localized features occur due to subsidence and continental shelf morphology which determ ine the location and depth of sedimentary bodies generated in each eustatic cycle.

Geometry and kinematics of active normal faults, South Oquirrh Mountains, Utah: implication for fault growth

The NNW-striking South Oquirrh Mountains normal fault zone consists of four individual faults. The individual faults are arranged in a right-stepping pattern in the north and a left-stepping pattern in the south, forming a convex-shaped fault zone in map view with the apex towards the hanging wall. Late Quaternary fault activity is characterized by 2.0–4.5 m high discontinuous fault scarps developed in both late Quaternary alluvium and bedrock. The fault scarps in bedrock contain evidence of two large rupture events. The last large earthquake occurred prior to the highstand of the Bonneville lake cycle (15 ka), based on cross-cutting relationship between the Bonneville shoreline and the fault scarp, and on comparisons of fault scarp morphology. Cumulative displacement patterns inferred from range crest elevations, Bouguer gravity data in the adjacent basin and rotation of the subsidiary faults adjacent to the West Mercur fault are similar to the pattern of displacements measured across late Quaternary fault scarps; that is, the maximum displacement is near the apex of the convex-shaped fault zone, where the West Mercur fault is located, and then tapers off towards both ends of the fault zone. Fault traces that range from a few meters to tens of kilometers long are characterized by two dominant orientations (strike N11°W and N43°W): the two orientations are separated from each other by a statistically constant angle of 33° ± 3°. Slip directions concentrate on a trend of S70°W for both groups of faults, perpendicular to the average strike of the fault zone. These data indicate that the geometry of the fault surfaces is non-cylindrical and may be composed of self-similar structural ridges and troughs of variable wavelength and amplitude that are elongated parallel to slip direction.The relationships between fault geometry, displacement and geomorphology in the South Oquirrh Mountains fault zone suggests a growth model of normal faults in which apex points on the convex-shaped fault sections mark the nucleation points of primary faults. Secondary faults developed at the ends of the primary fault step into the footwall block as the fault zone grows laterally. Intersecting regions between two laterally growing fault zones are concave-shaped in map view.

Segmentation and the coseismic behavior of Basin and Range normal faults: examples from east-central Idaho and southwestern Montana, U.S.A.

The range-front normal faults of the Lost River and Lemhi Ranges, and the Beaverhead and Tendoy Mountains in east-central Idaho and southwestern Montana have well-preserved fault scarps on Quaternary deposits along much of their lengths. Fault-scarp morphology, the age of deposits displaced by the faults, and the morphology of the range fronts provide a basis for dividing the faults into segments that are typically 20–25 km long. The Lost River, Lemhi and Beaverhead fault zones are 141–151 km long, and each has six segments. The 60-km-long Red Rock fault (the range-front fault of the Tendoy Mountains) has two central segments that have been active in late Quaternary time; these two segments span the central 27 km of the fault. We recognize four characteristics that help to identify segment boundaries: (1) major en échelon offsets or pronounced gaps in the continuity of fault scarps; (2) distinct, persistent, along-strike changes in fault-scarp morphology that indicate different ages of faulting; (3) major salients in the range front; and (4) transverse bedrock ridges where the cumulative throw is low compared to other places along the fault zone. Only features whose size is measured on the scale of kilometers are regarded as significant enough to represent a segment boundary that could inhibit or halt a propagating rupture. The ability to identify segments of faults that are likely to behave as independent structural entities will improve seismic-hazard assessment. However, one should not assume that the barriers at segment boundaries will completely stop all propagating ruptures. The topographic expression of mountain ranges is evidence that, at times during their history, all barriers fail. Some barriers apparently create ‘leaky’ segment boundaries that impede propagating ruptures but do not completely prevent faulting on adjacent segments.

Surface faulting accompanying the Borah Peak earthquake and segmentation of the lost river fault, central Idaho

On the morning of 28 October 1983, the Ms 7.3 Borah Peak earthquake struck central Idaho and formed a Y-shaped zone of surface faults that is divided into a southern, a western, and a northern section. The total length of the surface faults is 36.4 ± 3.1 km, and the maximum net throw is 2.5 to 2.7 m. The near-surface net slip direction, determined from the rakes of striations in colluvium, averaged 0.17 m of sinistral slip for 1.00 m of dip slip . The 20.8-km-long southern section is the main zone of surface faulting and coincides with the Thousand Springs segment of the Lost River fault. It has the largest amount of net throw, most complex rupture patterns, and best evidence of sinistral slip. The surface faults include zones of ground breakage as much as 140 m wide, en echelon scarps with synthetic and antithetic displacements, and individual scarps that are nearly 5 m high . The 14.2-km-long western section diverges away from the Lost River fault near the northern end of the southern section. The net throw on this section is generally less than 0.5 m but locally is as much as 1.6 m. The new ruptures are poorly developed across the crest and north flank of the Willow Creek hills; they are mostly downhill-facing, arcuate scars, perhaps incipient landslides, that may overlie a deeper zone of tectonic movement . The northern section, at least 7.9 km long, is on the Warm Spring segment of the Lost River fault and has a maximum net throw of about 1 m. The pattern of surface faulting on this section is simple compared to the other sections. A 4.7-km-long gap in 1983 surface faults separates the northern and southern sections but contains an older scarp of late Pleistocene age . Geologic, seismologic, and geodetic data from the earthquake suggest that barriers confined the primary coseismic rupture to the Thousand Springs segment of the fault. The rupture propagated unilaterally to the northwest from a hypocenter near the southeastern end of the segment. The southeastern boundary of the segment is marked by an abrupt bend in the range front, a 4-km-long gap in late Quaternary scarps, and transverse faults of Eocene age that intersect the Lost River fault . The northwestern boundary of the Thousand Springs segment is at the junction of the Willow Creek hills and the Lost River fault. Here, the southern and western sections of surface faults diverge and there is a gap in the 1983 scarps. During the first few weeks after the main shock, the large-magnitude and large stress-drop aftershocks clustered near this barrier. Later, aftershocks were mainly northwest of the barrier on the Warm Spring and Challis segments, and showed that strain adjustments eventually affected the entire northern part of the Lost River fault. Fault-scarp morphology and the bedrock geology suggest that the boundary between the Thousand Springs and Warm Spring segments has probably ruptured less frequently and had less net slip during much of the late Cenozoic than the interior of the adjacent segments. The 1983 faulting shows that although segment boundaries can stop or deflect primary ruptures, secondary surface faulting can occur on adjacent segments of the main fault. A late Pleistocene scarp in the 1983 gap suggests that infrequent earthquakes, perhaps larger than the 1983 event, might break through a segment boundary and thus release strain on two adjacent segments .

Recurrence of moderate to large magnitude earthquakes produced by surface faulting on the Wasatch fault zone, Utah

No historical earthquake associated with surface faulting have occurred along the Wasatch fault zone during at least the past 133 yr. Exploratory trenching, analysis of fault scarp morphology and scarp-derived colluvial deposits, and detailed mapping at two locations along the fault zone indicated there have been repeated moderate to large magnitude earthquakes ( M 6 1 2 to 7 1 2 ) associated with surface faulting during the late Pleistocene and Holocene. These detailed geological studies provide new data on the style of deformation associated with normal-slip faulting, amount of cumulative displacement, amount of displacement per event, and the recurrence intervals of surface faulting earthquakes. At the Kaysville site, 30 km north of Salt Lake City, at least three surface faulting events have produced 10 to 11 m of cumulative net vertical tectonic displacement since the middle Holocene; estimates of the net tectonic displacement for individual faulting events range from 1.7 to 3.7 m. The two most recent events occurred within the past 1,580 ± 150 yr and interval between these events was from 500 to 1,000 yr. Along this segment of the fault, the average recurrence interval is probably close to 1,000 yr. The middle to late Holocene slip rate is 1.8 (+1.0; −0.6) nm/yr. At the Hobble Creek site, 46 km south of Salt Lake City, six or seven surface faulting events have produced 11.5 to 13.5 m of cumulative net vertical tectonic displacement during the past 12,000 to 13,000 yr; the average net tectonic displacement per event is 0.8 to 2.8 m. The average Holocene recurrence interval at this location is 1,500 to 2,600 yr and the slip rate is 1.0 ± 0.1 mm/yr. If the recurrence intervals on these segments of the Wasatch fault zone are typical of the other segments of the zone, the recurrence interval of moderate to large magnitude earthquakes for the entire Wasatch fault zone may be 50 to 430 yr.