Late Quaternary Faulting Research Articles

Little‐studied arc‐backarc system in the spotlight

A research cruise has documented changes in rift tectonics, volcanism, and hydrothermalism along the least studied and most enigmatic sector of a crustal complex in the southwest Pacific Ocean. Results from the longitudinal transect are expected to provide insight into processes involving the Kermadec arc‐Havre backarc (KAHB) system, a continuum from oceanic spreading to continental rifting at a convergent plate boundary KAHB forms the central sector of an active, 2000‐km arc‐backarc complex between Tonga and New Zealand (Figure 1).The expedition also engaged in the first comprehensive survey of submarine vents in the Taupo Volcanic Zone (TVZ) at the south end of the KAHB system. Identified in the off‐shore segment of TVZ were three major hydrothermal vent areas associated with late Quaternary fault structures. Data from the expedition and from other recent research in the same area addressed questions concerning the type of hydrothermal venting, magmatic heterogeneity along and across KAHB, the style of backarc rifting, and tectonic and magmatic consequences of anomalous terranes colliding with the subduction margin.

Late Wisconsin to Holocene deformation in the New Madrid seismic zone

AbstractThe New Madrid earthquakes of 1811-1812 produced regional deformation in northwestern Tennessee, southeastern Missouri, and northeastern Arkansas. Landforms such as the Lake County uplift, Reelfoot Lake, and the Sunklands of northeast Arkansas either formed or were reactivated during these great earthquakes. Two hundred and fifty three cores were used to construct a structure-contour map of the unconformity between the Eocene strata and the overlying Quaternary Mississippi River alluvium in the New Madrid seismic zone. This map reveals relief that mirrors the surface deformation and provides additional data on the distribution of regional deformation. We interpret the structure-contour map as a representation of the Late Wisconsin to the present strain field of the New Madrid seismic zone. This map provides constraints for future kinematic analyses of late Quaternary New Madrid faulting and allows forecasting of future coseismic deformation.

Recognition of active strike-slip faulting from high-resolution marine seismic reflection profiles: Eastern Marlborough fault system, New Zealand

Major strike-slip faults within the eastern part of the Marlborough fault system of New Zealand extend offshore beneath the continental shelf, into the southern end of the Hikurangi margin. Six major submarine faults, each tens of kilometers in length and exhibiting the three-dimensional structural characteristics typical of strike-slip deformation zones, are mapped in detail using closely spaced high-resolution seismic-reflection profiles. Each fault displaces late Quaternary sediments that have been interpreted within the framework of the sequence stratigraphic model and the established history of glacio-eustatic sea-level cyclicity. Some of the faults are associated with strike-slip microearthquake focal mechanisms. The faults are within a complex structural high that underlies much of the outer continental shelf, and within the Flaxbourne basin between the structural high and the coast. The plan-view pattern of faulting recorded in the Quaternary sediments reflects only the major surface traces that have propagated upsection as contemporaneous sedimentation has blanketed most of the evolving structures. Regionally, the late Quaternary faults compose two groups that bound blocks with rhomboid surface areas on a scale of tens of square kilometers to hundreds of square kilometers. One group strikes typically between 023° and 057°, i.e., 22°–56° from the plate motion vector (079°), and includes dextral, oblique-slip thrust faults as well as inherited, steeply dipping, pre-Pliocene strike-slip faults that bound thick sedimentary basins. The second group comprises possibly young (<1 Ma), steeply dipping strike-slip faults that commonly exhibit normal-slip separation, and strike between 067° and 085°, subparallel to the azimuth of the instantaneous Pacific-Australian plate motion vector.

Rapid deformation rates along the Wasatch Fault Zone, Utah, from first GPS measurements with implications for earthquake hazard

Anomalously high rates of crustal deformation have been measured at the Basin‐Range transition to the Rocky Mountains along the Wasatch fault zone, Utah, by repeated Global Positioning System (GPS) measurements. Four GPS field campaigns (1992–1995) and comparisons with older (1962–1991) geodetic data have revealed east‐west extensional strain at a rate of 0.05 ± 0.02 µstrain/yr corresponding to a 2.7 ±1.3 mm/yr rate of horizontal displacement across a 55‐km wide area. This rate is more than 20% of the total ∼12 mm/yr extension rate across of the ∼800‐km wide Basin and Range province. It is also two to three times larger than the average Late Quaternary fault slip rate on the Wasatch fault and tens of times larger than the displacement rates inferred from the cumulative seismic moments of historic earthquakes. While we do not yet know the source of this unexpected contemporary deformation, possible mechanisms include homogeneous crustal extension, loading of the Wasatch and adjacent faults, and pressure solution creep. If the Wasatch fault is being loaded by this high strain rate, it increases the expected peak ground acceleration significantly from standard values. These new findings demonstrate the importance of GPS in earthquake hazard assessment.

The crustal fault structure responsible for the 1703 earthquake sequence of central Italy

An active crustal shear zone has been recognized in the central Apennines (Italy) by detailed investigations of the surface fault trace pattern of the axial sectors of the mountain belt. Our data suggest that a 16–18 km thick seismogenic layer rests above the ductile lower part of this shear zone and is affected by an interconnecting Late-Quaternary fault system which includes three main transcurrent (left-lateral) fault sets trending roughly north-south. Several lines of evidence indicate that the central fault set is the seismogenic structure responsible for generating the 1703 seismic sequence of central Italy. Fault slip data, morphotectonic and paleoseismological observations, together with fractal statistics, suggest that coseismic deformation within the epicentral area of the 1703 earthquake sequence is partitioned among a few linked faults, and that the ratio of vertical vs. horizontal slip rates within the active central fault set is about 1:3. Slip rate estimates also show that recurrence times for 1703 type earthquakes are in the range of a millennium. The seismogenic fault structure of the 1703 earthquake sequence may be viewed as a multiple rupture zone with an immature fractal geometry evolving within a crustal volume which deforms in response to transcurrent motion. The main results of this work support our view that the seismogenic potential of this sector of the Apennines can be evaluated by integrating, into a fractal model, the effects due to higher and lower-rank structural components of the Late Quaternary fault system of central Italy.

Late Quaternary faulting and neotectonics, South Victoria Land, Antarctica

Three Quaternary fault traces in Hidden and Garwood Valleys, South Victoria Land, provide direct evidence for neotectonic activity in the Transantarctic Mountain rift shoulder of the West Antarctic Rift system. The faults trend NE-SW and all show a large component of sinistral strike-slip displacement. The late Quaternary faults lie parallel to Palaeozoic brittle structures, suggesting that inherited anistropy has governed younger fault orientations. Sinistral displacement on NE-SW-striking Quaternary faults appears to be inconsistent with the predicted Cenozoic NW-SE dextral oblique extensional regime in South Victoria Land but can be accommodated by minor clockwise rotation of crustal blocks.

Strike-slip fault system in Amargosa Valley and Yucca Mountain, Nevada

The Amargosa Desert fault system, part of a 250-km-long system of dextral strike-slip faults east of the Death Valley region, is a throughgoing, long-lived, NW-striking, dextral strike-slip shear system that lies beneath Crater Flat and Yucca Mountain. Evidence for the fault system includes gravity, seismic, structural, stratigraphic, and paleomagnetic data, the distribution of springs and basaltic volcanic centers, and patterns of late Quaternary surface faulting. Where the Amargosa Desert fault system passes beneath Miocene ash-flow tuffs of the Southwestern Nevada Volcanic Field, we infer that the fault system changes into a series of detachments near the basement/cover contact and is expressed as a system of rotational normal faults in the ash-flow tuffs. Displaced geologic features indicate that total dextral displacement on the Amargosa Desert fault system exceeds 30 km. Vertical-axis rotations in ash-flow tuffs suggest that about 25 km of dextral displacement may have occurred since about 12.75 Ma. This fault system has important implications for the proposed high-level nuclear waste repository at Yucca Mountain, since it has experienced Quaternary and possibly Holocene displacements. Both normal faults and basaltic volcanic cones near the proposed repository may be controlled by the evolution of this regional strike-slip fault system.

Paleoseismology

About 20 years ago, when I was a newly graduated student of geology, neotectonics in the Mediterranean area was just emerging as a popular argument and a special project was begun in Italy. At that time, nobody on the old continent dared to conceive that the clear evidence of late Quaternary faulting could be directly linked to earthquakes. In fact, earthquakes were considered to be deep, hidden processes that essentially occurred in the crystalline basement beneath the sedimentary cover, such as in the fold and thrust belt of the Apennines, therefore accessible only to geophysicists. Only secondary earthquake effects were thought to leave evidence above the basement.

Earthquake magnitude potential of the Intermountain Seismic Belt, USA, from surface-parameter scaling of late Quaternary faults

AbstractTwo of the largest historical earthquakes documented for the United States occurred within the past four decades in a zone of anomalous seismicity in the western interior of the country known as the Intermountain Seismic Belt (ISB). Paleoseismology has revealed the presence of major active faults throughout this region, some of them extending through densely populated area. This study presents mean-value estimates of the maximum earthquake magnitudes that can be expected from 65 of the most prominent late Quaternary fault segments of the ISB using surface rupture parameters. Linear, least-squares regressions of magnitude on surface rupture length, L, maximum surface displacement, D, and DL, respectively, were computed for an updated worldwide set of historical, normal-slip earthquakes. These were compared with similar regressions from other publications, and it was found that a DL-based formula restricted to normal-faulting mechanisms has a relatively high correlation, low standard deviation, and tends to yield magnitudes between those given by D and L alone. This last property is useful for applications to the ISB prehistoric data, where the record of historical, ground-rupturing earthquakes is too sparse to estimate the proportions in which D and L are likely to contribute to seismic energy release of a fault. Application of regressions to published paleoseismic displacements and estimated earthquake segment lengths for the ISB faults shows that D consistently scales a larger magnitude than the corresponding single-segment L by an average of 0.2 to 0.3 magnitude unit, after accounting for bias from the regressions. Considerations of uncertainties in paleoseismic displacements suggest that they cannot fully explain the discrepancy. Since it is known that large earthquakes often encompass multiple segments of a fault, these observations indicate that longer rupture lengths should be used to estimate maximum magnitudes for ISB faults. A dual-segment rupture scenario produces better agreement between D- and L-based magnitudes and is supported by historical and paleoseismic earthquake data from the region. Magnitudes scaled by the DL-based regression for dual-segment rupture range from Ms 6.8 to 7.5 (σm = ± 0.20), and the largest are associated with the central segments of the Wasatch fault and the Swan/Grand Valley, Teton, Madison, and East Bear Lake faults.

Global climate change and active tectonics: effective tools for teaching and research

Abstract Multidisciplinary student teams foster innovative research projects that can discern the locations, times, and styles of late Quaternary faulting. Field studies in the area of the 1992 Landers magnitude 7.3 earthquake in southeastern California provide excellent opportunities for teaching and research in climatic and tectonic geomorphology. Graduate-student work begins with descriptions of type sections for mappable units of Quaternary alluvium — climate-change induced aggradation events. These are stair-stepped flights of alluvial fan and terrace surfaces. Topographic profiles along faulted stream channels and fans provide estimates of vertical tectonic deformation, and insight into the responses of fluvial systems to wrench-fault tectonics. Time lines in this tectonically deforming landscape are the result of global climate changes that caused synchronous pulses of alluviation in the Mojave Desert at about 125, 55, and 10 (12 and 8) ka. These dated alluvial surfaces record tectonic deformation resulting from cumulative Late Quaternary surface ruptures. Tectonic geomorphologic analyses use topographic, pedogenic, stratigraphic, and structural data, but do not require expensive and time-consuming trenching projects. Ages of faulted and unfaulted alluvium constrain the dates of two pre-1992 earthquakes. The 1992 Landers event is not characteristic of earlier faulting because it ruptured through adjacent segments of the Emerson fault that previously ruptured in separate events; one at 3 −1 +2 ka, and the other before 8 ka. Styles of tectonic deformation and locations of active faults, however may not have changed during the past 125 ka.

Nonlinear characteristics of paleoseismicity in China

Detailed studies of historical seismicity and paleoseismological data for major late Quaternary faults in China reveal spatial and temporal patterns in paleoseismic activity associated with these faults. Analysis of space‐time distributions of late Quaternary faults and consideration of new models for deformation of the Asian continent provide a basis for proposing a division of China and surrounding areas into five active tectonic provinces. Slip rates for active faults are highest in the Himalayan collision zone between the Indian and Eurasian plates and gradually decrease away from the collision zone into the adjacent active tectonic provinces. Paleoearthquake recurrence intervals in the active tectonic provinces of China reflect seismogenic properties that are closely related to the position of these provinces relative to the converging Indian and Eurasian plates. Average recurrence intervals are &lt;2000 years to as little as 310–900 years for active faults in the Qinghai‐Tibetan active tectonic province, whereas recurrence intervals are 2000–4500 years in the Xinjiang active tectonic province and longer than 2000 years to as much as 7250 years in the North China active tectonic province. Although average recurrence intervals are closely related to the slip rates of late Quaternary faults, no simple linear relationship exists between recurrence interval and slip rate. Further, analysis of paleoseismicity on major active faults shows that there are temporal variations in recurrence intervals. The case studies also indicate that for some provinces, faults, or fault segments there are three paleoseismic patterns: linear, logarithmic, and stepped. In the logarithmic and stepped patterns, the earthquake temporal sequence is characterized by active periods (earthquake clusters with relatively short recurrence intervals) and quiet periods (lack of clustering and relatively long recurrence intervals).

Cretaceous and Cenozoic geology east of the Tinui Fault Complex in northeastern Wairarapa, New Zealand

Abstract The study area, which has Mataikona in the southeast, lies within the east coast ranges, northeast Wairarapa. The deformed strata are chiefly bathyal and Cretaceous in age, whereas Cenozoic strata have a minor distribution. Deposition of the thick stratigraphic succession was almost continuous, although there were short periods of nondeposition chiefly in the Late Cretaceous, Oligocene, and early Miocene. The oldest strata, the indurated Pahaoa Group, are found in the north of the study area. The Springhill Formation of the Makatote Stream Subgroup which follows is indurated at its base. It is overlain unconformably by turbidite of the Wig Subgroup (comprising the Kipihana and Te Mai Formations). Conformable on the Wig Subgroup is the Tinui Group (composed of the silica‐rich Whangai and the organic‐rich Waipawa Formations). Cenozoic strata are represented by the Mangatu Group (Wanstead, Weber, and Whakatati Formations) and the Owahanga Group (Coast Road, Greenhollows, Takiritini, Tanawa, and Marainanga Road Conglomerate Formations) and sarsen boulders of Late Southland Series limestone. The mapped area is cut by eight major NNE‐trending faults which were active during the Neogene. The Tinui Fault Complex is the most important fault because it separates Miocene strata in the west from Cretaceous strata in the east. The faults vary from vertical to inclined to the WNW. Thrust sheets, west over east, are developed northeast of Te Mai Station (near Pukeamuku Trig) and near Glenwhai Station, and a klippe is present near Wig Trig in the east. Sinistral faulting probably occurred along the Tinui Fault Complex during the Late Cretaceous when the Pukeamuka pull‐apart basin, which contains >800 m of Whangai strata, formed. Dextral faulting along the Tinui Fault Complex occurred intermittently during Miocene (and may have occurred during the late Eocene) but faulting, on the basis of scarce Late Quaternary fault traces, is now less frequent.

Late Quaternary geomorphology and soils in Crater Flat, Yucca Mountain area, southern Nevada

Crater Flat is an alluvium-filled structural basin on the west side of Yucca Mountain, Nevada, which is under consideration for a high-level nuclear waste repository. North-trending, late Quaternary faults offset alluvium in Crater Flat both along the canyons of the western flanks of Yucca Mountain and out on the piedmont slope. We believe the initial lack of young offsets at Yucca Mountain was in part due to unrecognized late Quaternary stratigraphy. We hypothesize that alluviation in the Yucca Mountain region was more active during the late Quaternary than previously thought. Several techniques were tried to test this hypothesis. Results are compared with previous soils and surface-exposure dating studies, and correlated to stratigraphy of other late Quaternary units in the southern Nevada, Death Valley, and Mojave Desert areas, and provide new stratigraphic data relevant to understanding climatic-alluvial processes in the Basin and Range Province during the late Quaternary. 76 refs., 7 figs., 6 tabs.

Submarine hydrothermal activity within the offshore Taupo Volcanic Zone, Bay of Plenty continental shelf, New Zealand

The onshore Taupo Volcanic Zone (New Zealand) is a region of vigorous, and relatively well studied, geothermalism and hydrothermalism distributed at 20 known sites. Within the offshore extension of the Taupo Volcanic Zone, new data acquired in 1988 and 1989, and a review of survey data since 1966 has delineated three main areas of submarine hydrothermal activity (SHA) on the Bay of Plenty continental shelf. Of these, the most intense and extensive area of SHA is associated with the active late Quaternary faulting of the offshore Whakatane Graben. Within this latter area, there are four major hydrothermal vent sites; the previously reported Calypso vent site, and three others newly located. Three distinct plume forms are recognisable within the water-column, and are related to the state of buoyancy of the hydrothermal fluid, although the character of the plume can change in the vertical since the overlying water mass is density stratified. Some SHA has proved ephemeral over a 22 year period which is here related to a cycle of conduit opening driven by secular extension, and conduit collapse and closure caused by seismic shocks. Mathematical modelling (phase-plane analysis) of the hydrothermal plumes provides a method of analyzing plume forms from known initial parameters.

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.

Characterizing potential offshore seismic sources using high‐resolution geophysical and seafloor sampling programs: An example from Cape Egmont fault zone, Taranaki shelf, New Zealand

Standard marine geological techniques (high‐resolution seismic reflection methods, piston cores) are used to investigate the neotectonics and paleoseismology of an active part of an offshore fault system, the Cape Egmont fault zone, Taranaki continental shelf, New Zealand. The main active structure within the zone, the Cape Egmont fault, is a reactivated Pliocene to Recent normal to oblique‐slip fault that is associated with shallow crustal seismicity and exhibits a 53‐km long, 1‐to 5‐m high, low‐angle drape fold or “scarp” at the seafloor. Four, possibly five, fault segments varying in length from 7–51 km are delineated using geometric, structural, and stratigraphic information. Empirical estimates of maximum credible earthquake magnitudes, based upon seismic moment calculations, surface rupture lengths, and vertical surface offsets, suggest that the prehistoric earthquake(s) that formed the present seafloor surface trace probably had a magnitude of at least MW 6.4(MW, moment magnitude) and could have been as large as MS 7.3–7.4(MS, surface wave magnitude). Tentative maximum recurrence intervals for such events, inferred for each segment, range from 8000–13,000 years, with lower magnitude events having correspondingly shorter recurrence intervals of 500–900 years. Without careful study of offshore late Quaternary faults and fault zones, particularly where they are contiguous with areas of active onshore deformation, existing regional seismic hazard predictions may be inaccurate.

Late Quaternary normal faulting of the Hat Creek Basalt, northern California

The Hat Creek fault is a major, young, north-striking, normal fault along the western boundary of extensional Basin and Range deformation in the Lessen region of northeastern California. Volcanic rocks of Quaternary and late Pliocene age are displaced a total of >500 m down to the west along west-facing, en echelon scarps now retreated to ∼35° slopes. Fresh, young scarps as much as 30 m high cut the Hat Creek Basalt (erupted between 15 and ∼40 ka) a few tens of meters west of the retreated scarps. Prior to the late 1980s, these young scarps were interpreted as lava slump scarps formed as the Hat Creek Basalt ponded against the older fault scarps and then drained away to the northwest. Numerous pieces of geologic evidence, however, show that the young scarps formed after the Hat Creek Basalt solidified and cooled and are true fault features formed by the youngest displacements of the Hat Creek fault. Structural details are remarkably well preserved along the series of left-stepping scarps cutting the Hat Creek Basalt. Near the central parts of individual segments, the fault is displayed as a single, vertical scarp. Near the ends of the segments, the scarp decreases in height and becomes a monodinal flexure on which the recent dip separation has been taken up by small-scale offset along columnar cooling joints in the basalt These monodinal flexures commonly rotate into east-west monoclines that join adjacent north-south segments. Displacement of outwash gravel overlying the Hat Creek Basalt shows that vertical separation on the Hat Creek fault has averaged ∼1.3 mm yr −1 for the past 15,000 yr. The Hat Creek fault thus represents a potential earthquake hazard, despite the low level and diffuse nature of modern seismidty in the region.

Seismotectonics of the 1987–1988 Lakeside, Utah, Earthquakes

Abstract From September 1987 through March 1988, an earthquake sequence which induded shocks of ML 4.8 and 4.7 on September 25 and October 26, respectively, and a total of 8 moderate-sized events of ML ≥ 3.8, occurred in NW Utah beneath a desert basin west of the Great Salt Lake (center of activity: 41° 12.0′ N, 113° 10.5′ W). Wood-Anderson seismograms indicate nearly identical magnitudes for the two largest earthquakes but a factor of two to five larger seismic moment for the first. Significant aspects of the 1987–1988 sequence induded: foreshock activity, proximity (epicentral distance, Δ, of 7 to 12 km) to a major pumping facility completed in early 1987 to lower the level of the Great Salt Lake, an unambiguous strike-slip focal mechanism for the ML 4.8 mainshock, and the lack of a clear association with late Quaternary surface faults. Despite constraints on accessibility to the epicentral area, the stations of the regional seismic network (Δ ≥ 60 km) were supplemented with local stations—initially four portable seismographs and later up to four telemetered stations (2 ≤ Δ ≤ 27 km) that operated continuously from October 7, 1987, through March 1988. Well-located aftershock foci form a 6-by-6-km zone between 6 and 12 km depth which is steeply dipping and trends SSE, parallel to the right-lateral nodal plane of the mainshock focal mechanism. Despite coincidental timing and proximity of the earthquakes to major pumping activity at the surface, the case for induced seismicity related to the pumping is weak.

Neotectonics and large-scale geomorphology of Canada

Canada includes active convergent and strike-slip plate boundaries, several major mountain systems, two passive continental margins, and a stable craton. Neotectonic activity, as indicated by earthquake occurrence, is highest along the west coast and lowest in the interior of the country. Correlations between tectonics and physiography are strongest in the west. Here, the landscape bears a strong imprint of convergent and strike-slip plate regimes. Late Mesozoic and early Cenozoic tectonic events established the setting in which the present physiography of western Canada developed, but the landscape acquired its present form much more recently, in Pliocene and Quaternary time. In contrast, the neotectonic imprint in eastern and northern Canada is enigmatic, and although major concentrations of earthquakes in many areas are associated with reactivated, early Phanerozoic structures, there has been only limited late Quaternary faulting. The vast Canadian craton, despite its very low seismicity, is deforming isostatically at a moderate rate due to melting of the Laurentide Ice Sheet thousands of years ago.

An integrated model for seismogenesis in the intermountain seismic belt

Abstract The idea that all late Quaternary surface faults in the Intermountain Seismic Belt (ISB) are steeply dipping and penetrate from the surface to depths of 12 to 15 km may be an over-simplification of the actual tectonic-seismogenic process in the ISB/eastern Basin and Range transition zone. Studies of late Quaternary faulting in southwestern Wyoming and northcentral Utah, 125 km east of the Wasatch fault zone, indicate evolving tectonic/structural relationships exist with time and location. The Hebgen Lake and Borah Peak earthquakes represent a geologically mature stage of tectonic/seismogenic development manifested by fault-bounded mountain blocks and recurrent late Quaternary fault movements. Normally reactivated thrust faults and listric normal faults represent an early to intermediate stage of deformation in preexisting thrust-faulted terrains. The degree to which preexisting structures unrelated to the modern stress field affect tectonic development is complex, and a variety of structural relations may exist with time and location. An incipient seismogenic fault in the early stages of extension may exist only as a subdecollement, “blind” structure. As extension continues, “blind” seismogenic faults propagate upward eventually rupturing the surface as 45° to 60° planar structures. Normally reactivated thrust faults and listric normal faults, characteristic of early to intermediate extension, are tectonically beheaded and become inactive with propagation of planar seismogenic faults to the surface. The seismic potential of normally reactivated thrust faults and listric normal faults is open to debate. Whether the seismic potential of low-angle faults can be assessed from analysis of surface rupture parameters alone is a vitally important problem in seismic hazard assessment.