Fault Offsets Research Articles

Different expressions of the crustal structure across the Dongsha Rise along the northeastern margin of the South China Sea

This study presents two long-cable, deep-penetration, multi-channel seismic reflection images and associated gravity modeling to delineate structures and magmatic features that occur along the margins of the eastern and western Dongsha Rise in the northeast South China Sea (SCS). These data show approximately symmetrical sags imaged on the shelf, but the structures begin to vary in multiple ways along the eastern and western Dongsha Rise after crossing the continental shelf. At the slope break, an anticline with two thrust faults and a Mesozoic unconformity are imaged on the eastern Dongsha Rise, whereas negative flower structures are detected on the western Dongsha Rise. Most large-scale normal faults sole into a mid- or lower-crustal detachment below the lower slope on the eastern Dongsha Rise whereas some uplifts with small fault offsets were imaged at the lower slope and seaward-dipping tilted fault blocks were imaged between the lower slope and oceanic basin on the western Dongsha Rise. Magmatism, expressed by different types, such as intrusions, volcanoes and sills, was active since at least 5.5 Ma; and magmatic deposits appear to be well-developed on the eastern Dongsha Rise of the northeastern SCS margin.Gravity modeling showed that the crust below the lower slope/oceanic basin in the eastern Dongsha Rise is hyper-extended continental crust underplated by a high velocity layer with a maximum thickness of 9.4 km below the slope break, rather than thickened oceanic crust. The continent-ocean transition zone (COT) has a broader extent on the eastern Dongsha Rise compared to the western Dongsha Rise. Pure-shear deformation and lower crustal flow have led to the formation of the crustal structural contrasts and the wide northeastern continental margin of the SCS.

“3D_Fault_Offsets,” a Matlab Code to Automatically Measure Lateral and Vertical Fault Offsets in Topographic Data: Application to San Andreas, Owens Valley, and Hope Faults

AbstractMeasuring fault offsets preserved at the ground surface is of primary importance to recover earthquake and long‐term slip distributions and understand fault mechanics. The recent explosion of high‐resolution topographic data, such as Lidar and photogrammetric digital elevation models, offers an unprecedented opportunity to measure dense collections of fault offsets. We have developed a new Matlab code, 3D_Fault_Offsets, to automate these measurements. In topographic data, 3D_Fault_Offsets mathematically identifies and represents nine of the most prominent geometric characteristics of common sublinear markers along faults (especially strike slip) in 3‐D, such as the streambed (minimum elevation), top, free face and base of channel banks or scarps (minimum Laplacian, maximum gradient, and maximum Laplacian), and ridges (maximum elevation). By calculating best fit lines through the nine point clouds on either side of the fault, the code computes the lateral and vertical offsets between the piercing points of these lines onto the fault plane, providing nine lateral and nine vertical offset measures per marker. Through a Monte Carlo approach, the code calculates the total uncertainty on each offset. It then provides tools to statistically analyze the dense collection of measures and to reconstruct the prefaulted marker geometry in the horizontal and vertical planes. We applied 3D_Fault_Offsets to remeasure previously published offsets across 88 markers on the San Andreas, Owens Valley, and Hope faults. We obtained 5,454 lateral and vertical offset measures. These automatic measures compare well to prior ones, field and remote, while their rich record provides new insights on the preservation of fault displacements in the morphology.

Integrated study on the topographic and shallow subsurface expression of the Grote Brogel Fault at the boundary of the Roer Valley Graben, Belgium

The Grote Brogel Fault (GBF) is a major WNW-ESE striking normal fault in Belgium that diverges westward from the NW-SE striking western border fault system of the Roer Valley Graben. The GBF delimits the topographically higher Campine Block from the subsiding Roer Valley Graben, and is expressed in the Digital Terrain Model (DTM) by relief gradients or scarps. By integrating DTM, Electrical Resistivity Tomography (ERT), Cone Penetration Test (CPT) and borehole data, we studied the Quaternary activity of the GBF and its effects on local hydrogeology. In the shallow subsurface (<50m) underneath these scarps, fault splays of the GBF were interpreted on newly acquired ERT profiles at two investigation sites: one on the eastern section and the other on the western section, near the limit of the visible surface trace of the fault. Borehole and CPT data enabled stratigraphic interpretations of the ERT profiles and thereby allowed measuring vertical fault offsets at the base of Pleistocene fluvial deposits of up to 12m. Groundwater measurements in the boreholes and CPTs indicate that the GBF acts as a hydrologic boundary that prevents groundwater flow from the elevated footwall towards the hangingwall, resulting in hydraulic head differences of up to 12.7m. For the two investigation sites, the hydraulic head changes correlate with the relief gradient, which in turn correlates with the Quaternary vertical offset of the GBF. ERT profiles at the eastern site also revealed a local soft-linked stepover in the shallow subsurface, which affects groundwater levels in the different fault blocks, and illustrates the complex small-scale geometry of the GBF.

Fault reactivation mechanisms and dynamic rupture modelling of depletion-induced seismic events in a Rotliegend gas reservoir

AbstractUnderstanding the mechanisms and key parameters controlling depletion-induced seismicity is key for seismic hazard analyses and the design of mitigation measures. In this paper a methodology is presented to model in 2D the static stress development on faults offsetting depleting reservoir compartments, reactivation of the fault, nucleation of seismic instability, and the subsequent fully dynamic rupture including seismic fault rupture and near-field wave propagation. Slip-dependent reduction of the fault's strength (cohesion and friction) was used to model the development of the instability and seismic rupture. The inclusion of the dynamic calculation allows for a closer comparison to field observables such as borehole seismic data compared to traditional static geomechanical models. We applied this model procedure to a fault and stratigraphy typical for the Groningen field, and compared the results for an offset fault to a fault without offset. A non-zero offset on the fault strongly influenced the stress distribution along the fault due to stress concentrations in the near-fault area close to the top of the hanging wall and the base of the footwall. The heterogeneous stress distribution not only controlled where nucleation occurred within the reservoir interval, but also influenced the subsequent propagation of seismic rupture with low stresses inhibiting the propagation of slip. In a reservoir without offset the stress distribution was relatively uniform throughout the reservoir depth interval. Reactivation occurred at a much larger pressure decrease, but the subsequent seismic event was much larger due to the more uniform state of stress within the reservoir. In both cases the models predicted a unidirectional downward-propagating rupture, with the largest wave amplitudes being radiated downwards into the hanging wall. This study showed how a realistic seismic event could be successfully modelled, including the depletion-induced stressing, nucleation, dynamic propagation, and wave propagation. The influence of fault offset on the depletion-induced stress is significant; the heterogeneous stress development along offset faults not only strongly controls the timing and location of a seismic slip, but also influences the subsequent rupture size of the dynamic event.

Highly Variable Latest Pleistocene‐Holocene Incremental Slip Rates on the Awatere Fault at Saxton River, South Island, New Zealand, Revealed by Lidar Mapping and Luminescence Dating

AbstractGeomorphic mapping using high‐resolution lidar imagery and luminescence dating reveal highly variable incremental Holocene‐latest Pleistocene slip rates at the well‐known Saxton River site along the Awatere fault, a dextral strike‐slip fault in the Marlborough Fault System, South Island, New Zealand. Using lidar and field observations, we measured seven fault offsets recorded by fluvial terraces and bedrock markers. Improved dating of the offsets is provided by post‐IR‐IRSL225 luminescence ages. Incremental slip rates varied from &lt;2 mm/yr to &gt;15 mm/yr over intervals of thousands of years and tens of meters of slip, demonstrating order‐of‐magnitude temporal variations in rate at a single site. These observations have basic implications for earthquake fault behavior, lithospheric mechanics, discrepancies between geodetic and geologic slip rates, and probabilistic seismic hazard assessment.

Geology of the Yap Trench: new observations from a transect near 10°N from manned submersible Jiaolong

ABSTRACTThis paper describes the petrology and geochemistry of rocks from the Yap Trench acquired by three dives of the Jiaolong research submarine. Combining the geophysical data and submersible observations, this paper describes the geomorphology, shallow structures, and sedimentology of the Yap Trench and further discusses the tectonics and activities of this region. Two obvious slope breaks are found on the landward slope, and horsts and grabens with small fault offsets are observed in the ocean-ward slope of the trench. Peridotites sampled from the Yap Trench inner wall are highly depleted subduction-related mantle residues. Volcanic rocks in the northern segment of the trench have subduction-related characteristics that Yap fore-arc rocks underwent metasomatism during Cenozoic subduction. The rocks with remarkable lithologic difference from lithospheric mantle and upper crust sampled in the break slopes suggest that the slope break area may represent a lithologic boundary or transition zone. The landward slope of the Yap Trench was removed by subduction erosion as a result of collision with the Caroline Ridge. The bending of the down-going plate caused normal faults, horsts, and grabens with little or no sediments indicating that the Caroline Ridge is subducting beneath the Yap arc along the trench even though the convergence rate is very slow.

Morphology of a large paleo-lake: Analysis of compaction in the Miocene-Quaternary Pannonian Basin

Abstract Lake-floor morphologies may be significantly different from seafloor topographies of other basins, typically observed in passive or active continental margins. The bathymetry of large paleo-lakes is often overwritten by subsequent tectonic evolution, burial beneath thick overburden and inherent compaction effects. We study the evolution of such an initial underfilled, balance fill and finally overfilled large paleo-lake basin by the interpretation of 2D and 3D seismic data set corroborated with calibrating wells in the example of the Neogene Pannonian Basin of Central Europe. Lake Pannon persisted for about 7–8 Myr and was progressively filled by clastic material sourced by the surrounding mountain chains and transported by large rivers, such as the paleo-Danube and paleo-Tisza. We combined sedimentological observations with a backstripping methodology facilitated by well lithology and porosity data to gradually remove the sediment overburden. This approach has resulted in a morphological reconstruction of the former depositional surfaces with special focus on the prograding shelf-margin slopes. Our calculations show that the water depth of the lake was more than 1000 m in the deepest sub-basins of the Great Hungarian Plain of the Pannonian Basin. The significant compaction associated with lateral variations of Neogene sediment thicknesses has created non-tectonic normal fault offsets and folds. These features have important effects on fluid migration and hydrocarbon trapping. We furthermore compare the geometries and effects of such non-tectonic features with the activity of larger offset sinistral strike-slip zones using 3D seismic attributes.

From coseismic offsets to fault-block mountains

In the Basin and Range extensional province of the western United States, coseismic offsets, under the influence of gravity, display predominantly subsidence of the basin side (fault hanging wall), with comparatively little or no uplift of the mountainside (fault footwall). A few decades later, geodetic measurements [GPS and interferometric synthetic aperture radar (InSAR)] show broad (∼100 km) aseismic uplift symmetrically spanning the fault zone. Finally, after millions of years and hundreds of fault offsets, the mountain blocks display large uplift and tilting over a breadth of only about 10 km. These sparse but robust observations pose a problem in that the coesismic uplifts of the footwall are small and inadequate to raise the mountain blocks. To address this paradox we develop finite-element models subjected to extensional and gravitational forces to study time-varying deformation associated with normal faulting. Stretching the model under gravity demonstrates that asymmetric slip via collapse of the hanging wall is a natural consequence of coseismic deformation. Focused flow in the upper mantle imposed by deformation of the lower crust localizes uplift, which is predicted to take place within one to two decades after each large earthquake. Thus, the best-preserved topographic signature of earthquakes is expected to occur early in the postseismic period.

Controls on fracture network characteristics of the middle member of the Bakken Formation, Elm Coulee field, Williston Basin, United States

Although most of the production from the Bakken Formation in the Elm Coulee field of the Williston Basin is interpreted to come from matrix permeability, plots of reservoir performance indicators, such as estimated ultimate recovery (EUR), show areas of anomalously high production believed to be caused by secondary (fracture) permeability. Recent studies have alluded to basement tectonics controlling the structure of the overlying sedimentary cover and thus the reservoir performance in the Elm Coulee field, but the exact controlling mechanism has not been demonstrated. This paper describes an antiformal structure overlying a zone of deformation in a seismic data set. The zone of deformation consists of small vertical offset faults displaying evidence of wrench faulting and involves the Bakken petroleum system and deeper units. An analog model is used to guide interpolation and extrapolation of the faults when they are traced through the data cube. With the help of analog models, it is established that the antiformal structure and the associated deformation zone evolved over a stepover in a northwest-southeast–oriented left-lateral–strike-slip system in the rigid basement. This strike-slip system may be secondary to the regional left-lateral Brockton-Froid fault zone, which lies to the north of the study area. The fracture network in the Elm Coulee field is likely to be enhanced around such antiformal structures, providing a plausible explanation for the locations of the EUR sweet spots. Thus, it is proposed that locating such deformation zones forms a potential new exploration trend in this field.

Shallow reflection seismic evidence of tectonic activity in the Cheb Basin, NW Bohemia

With a high-resolution seismic survey, we targeted the shallow structural features of the Cheb Basin, Northwest Bohemia. The aim of this study was to evaluate these features’ relation to seismic activity below the Cheb Basin with the first reflection seismic image of parts of the basin. We recorded a densely sampled, 3200 m long seismic survey stretching from Kopanina towards SE, into the basin. From the recordings of 170 shots at 192 geophones with 384-m total spread, we obtained a reflection seismic image down to the basement at 300-m depth, supplemented by a shallow tomographic velocity model for the uppermost 40 m. Strong lateral velocity contrasts indicate Quaternary river terraces, into which the Plesna river incises. We observe reverse faults in the lower basin sediments, which we interpret as signs of a push-up structure related to the N-S oriented Pocatky-Plesna Zone (PPZ). However, we do not observe any vertical fault offsets in the younger sedimentary layers, which suggests that any normal or reverse faulting must be older than 20 Ma. The image agrees well with previous lithostratigraphic borehole data and previous sedimentological and tectonic models. Our explanation for the formation of the escarpment at the eastern bank of the Plesna valley, previously interpreted as outcrop of the PPZ, relies solely on incision of the Plesna river into weak sediments.

Source model for the Copahue volcano magma plumbing system constrained by InSAR surface deformation observations

AbstractCopahue volcano straddling the edge of the Agrio‐Caviahue caldera along the Chile‐Argentina border in the southern Andes has been in unrest since inflation began in late 2011. We constrain Copahue's source models with satellite and airborne interferometric synthetic aperture radar (InSAR) deformation observations. InSAR time series from descending track RADARSAT‐2 and COSMO‐SkyMed data span the entire inflation period from 2011 to 2016, with their initially high rates of 12 and 15 cm/yr, respectively, slowing only slightly despite ongoing small eruptions through 2016. InSAR ascending and descending track time series for the 2013–2016 time period constrain a two‐source compound dislocation model, with a rate of volume increase of 13 × 106 m3/yr. They consist of a shallow, near‐vertical, elongated source centered at 2.5 km beneath the summit and a deeper, shallowly plunging source centered at 7 km depth connecting the shallow source to the deeper caldera. The deeper source is located directly beneath the volcano tectonic seismicity with the lower bounds of the seismicity parallel to the plunge of the deep source. InSAR time series also show normal fault offsets on the NE flank Copahue faults. Coulomb stress change calculations for right‐lateral strike slip (RLSS), thrust, and normal receiver faults show positive values in the north caldera for both RLSS and normal faults, suggesting that northward trending seismicity and Copahue fault motion within the caldera are caused by the modeled sources. Together, the InSAR‐constrained source model and the seismicity suggest a deep conduit or transfer zone where magma moves from the central caldera to Copahue's upper edifice.

Late Pleistocene slip rate of the northern Qilian Shan frontal thrust, western Hexi Corridor, China

AbstractWe derive a slip rate for a frontal thrust in the western Hexi Corridor along the northern Qilian Shan by combining topographic profiling and 10Be exposure dating. The active Yumen‐Beidahe thrust fault offsets late Pleistocene alluvial‐fan deposits, and a prominent north‐facing scarp is well preserved. To quantify the slip rate, we surveyed the uplifted terraces and sampled quartz‐rich pebbles on terrace surfaces and river channels to determine surface exposure ages and pre‐depositional inheritance. The minimum vertical slip rate of the fault is 0.73 ± 0.09 mm a−1. This represents a horizontal shortening rate of 1.26 ± 0.31 mm a−1 for a fault dip of 30 ± 5°. This estimated slip rate supports the inference made from previous geological and GPS constraints that NNE‐directed shortening across the western Qilian Shan and the Hexi Corridor is distributed on several active faults with a total shortening rate of 4–10 mm a−1.

Geophysical expression of buried range-front embayment structure: Great Sand Dunes National Park, Rio Grande rift, Colorado

Great Sand Dunes National Park and Preserve (GRSA, Colorado) lies along the eastern margin of the San Luis Basin and the tectonically active Sangre de Cristo fault system that are part of the northern Rio Grande rift. GRSA lies within a prominent embayment in the range front where two separate sections of the Sangre de Cristo fault system intersect. Fault scarps are observed along both intersecting fault zones within older basin-fill alluvium, but have been obscured by the actively migrating dunefield. The dune sand is also strongly magnetic, locally limiting the usefulness of aeromagnetic methods for mapping concealed structure. This study uses airborne geophysical methods, primarily airborne gravity gradient data, along with constraints from geologic mapping and limited subsurface data and groundwater modeling, to interpret the subsurface basin geometry and range-front structure of the embayment. Using forward modeling of the gravity gradient data and locations of faults inferred from gravity gradient and aeromagnetic lineaments, several previously unrecognized tectonic elements are interpreted adjacent to the range front. Some of the largest rift-­related fault offsets are demonstrated to be basinward of the normal fault zones mapped at the surface along the range front of the Sangre de Cristo Mountains, along faults concealed under the dunefield and subparallel to the two fault sections. A fault-bounded structural bench, likely composed of Proterozoic rocks, underlies most of the high dunefield at depths of 500 m to 1 km. The bench is truncated on its southwest margin by a northwest-trending, southwest-dipping normal fault. A northeast-trending, northwest-dipping normal fault with ∼600 m of estimated relief lies under the southern margin of the dunefield and bounds a structurally higher bench of Proterozoic rocks concealed at <400 m depth near the range front. The northwest- and northeast-trending geophysical lineaments generally correspond well with the trends of faults mapped at the surface, and with both pre- and syn-rift structures in the Sangre de Cristo Mountains. Aeromagnetic anomalies are explained by variations in the magnetization of pre-rift rocks, and the strongly magnetic dune sand.

The role of bed-parallel slip in the development of complex normal fault zones

Normal faults exposed in Kardia lignite mine, Ptolemais Basin, NW Greece formed at the same time as bed-parallel slip-surfaces, so that while the normal faults grew they were intermittently offset by bed-parallel slip. Following offset by a bed-parallel slip-surface, further fault growth is accommodated by reactivation on one or both of the offset fault segments. Where one fault is reactivated the site of bed-parallel slip is a bypassed asperity. Where both faults are reactivated, they propagate past each other to form a volume between overlapping fault segments that displays many of the characteristics of relay zones, including elevated strains and transfer of displacement between segments. Unlike conventional relay zones, however, these structures contain either a repeated or a missing section of stratigraphy which has a thickness equal to the throw of the fault at the time of the bed-parallel slip event, and the displacement profiles along the relay-bounding fault segments have discrete steps at their intersections with bed-parallel slip-surfaces. With further increase in displacement, the overlapping fault segments connect to form a fault-bound lens. Conventional relay zones form during initial fault propagation, but with coeval bed-parallel slip, relay-like structures can form later in the growth of a fault. Geometrical restoration of cross-sections through selected faults shows that repeated bed-parallel slip events during fault growth can lead to complex internal fault zone structure that masks its origin. Bed-parallel slip, in this case, is attributed to flexural-slip arising from hanging-wall rollover associated with a basin-bounding fault outside the study area.

Current sensor fault diagnosis and adaptive fault-tolerant control of PMSM drive system based on differential algebraic method

As the key sensor of Electric Vehicle Permanent Magnet Synchronous Machine (PMSM) drive system, the current sensor usually occurs gain and offset fault, and the fault will directly affect the performance and stability of PMSM drive system. Therefore, the inverter nonlinearity dead-time compensation method is first investigated allowing for parasitic capacitor effect, and the proposed method are compared with the traditional dead-time compensation method, moreover, the technical advantages are all validated via experimental results, the fault diagnosis accuracy problem of current sensor caused by the inverter nonlinearity induced offset voltage between inverter reference and output voltage are cleverly solved. Then, the differential algebraic based fault diagnosis method and adaptive fault-tolerant control strategy are presented. An integrated solution of PMSM drive system is innovatively proposed integrated the proposed dead-time compensation method with proposed differential algebraic based fault diagnosis method and adaptive fault-tolerant control strategy together. At last, system simulation and dSPACE based experimental test are implemented to validate the feasibility and effectiveness of proposed integrated solution of PMSM drive system and the conclusions are shown.

Localized slip controlled by dehydration embrittlement of partly serpentinized dunites, Leka Ophiolite Complex, Norway

Dehydration of partly or completely serpentinized ultramafic rocks can increase the pore fluid pressure and induce brittle failure, a process referred to as dehydration embrittlement. However the extents of strain localization and unstable frictional sliding during deserpentinization are still under debate. In the layered ultramafic sections of the Leka Ophiolite Complex in the Central Norwegian Caledonides, prograde metamorphism of serpentinite veins led to local fluid production and to the growth of Mg-rich and coarse-grained olivine with abundant magnetite inclusions and δ18O values 1.0–1.5‰ below the host rock. Embrittlement associated with the dehydration caused faulting along highly localized (<10 μm-wide) slip planes near the centers of the original serpentinite veins and pulverization of wall rock olivine. These features along with an earthquake-like size distribution of fault offsets suggest unstable frictional sliding rather than slower creep. Structural heterogeneities in the form of serpentinite veins clearly have first-order controls on strain localization and frictional sliding during dehydration. As most of the oceanic lithosphere is incompletely serpentinized, heterogeneities represented by a non-uniform distribution of serpentinite are common and may increase the likelihood that dehydration embrittlement triggers earthquakes.

Slip rate estimates and slip gradient for the Alpine Fault at Calf Paddock, Maruia River, New Zealand

ABSTRACTThe dextral–reverse Alpine Fault offsets alluvial terraces of the Maruia River at Calf Paddock. RTK‐GPS surveying of the faulted terraces yields detailed measurements of fault slip across terraces T1–T5. Pits were excavated to log the stratigraphy, date the deposits and to thereby constrain fault slip rates for this site. The mean of the four largest dextral offsets across terrace T2 is 12.0 ± 1.3 m. The corresponding mean vertical offset is 1.6 ± 0.6 m. A single charcoal date with an age of 1095–1275 cal yr BP is used to derive minimum dextral and reverse slip rates of 10 ± 2 and 1.3 ± 0.5 mm/yr, respectively. These rates are comparable with others observed nearby at Little and Lost Mary creeks and from previous studies at Calf Paddock. These new late Holocene slip rate estimates highlight the northeastward decrease in strike-slip and dip-slip rate along the northern section of the Alpine Fault as motion is partitioned onto faults of the Marlborough Fault System. The lack of aseismic creep observed across ‘Evison’s wall’ highlights that slip is achieved at Calf Paddock during metre-scale co-seismic displacements.

Episodic activity of a dormant fault in tectonically stable Europe: The Rauw fault (NE Belgium)

Our knowledge about large earthquakes in stable continental regions comes from studies of faults that generated historical surface rupturing earthquakes or were identified by their recent imprint in the morphology. Here, we evaluate the co-seismic character and movement history of the Rauw fault in Belgium, which lacks geomorphological expression and historical/present seismicity. This 55-km-long normal fault, with known Neogene and possibly Early Pleistocene activity, is the largest offset fault west of the active Roer Valley Graben. Its trace was identified in the shallow subsurface based on high resolution geophysics. All the layers within the Late Pliocene Mol Formation (3.6 to 2.59Ma) are displaced 7m vertically, without growth faulting, but deeper deposits show increasing offset. A paleoseismic trench study revealed cryoturbated, but unfaulted, late glacial coversands overlying faulted layers of Mol Formation. In-between those deposits, the fault tip was eroded, along with evidence for individual displacement events. Fragmented clay gouge observed in a micromorphology sample of the main fault evidences co-seismic faulting, as opposed to fault creep. Based on optical and electron spin resonance dating and trench stratigraphy, the 7m combined displacement is bracketed to have occurred between 2.59Ma and 45ka. The regional presence of the Sterksel Formation alluvial terrace deposits, limited to the hanging wall of the Rauw fault, indicates a deflection of the Meuse/Rhine confluence (1.0 to 0.5Ma) by the fault's activity, suggesting that most of the offset occurred prior to/at this time interval. In the trench, Sterksel Formation is eroded but reworked gravel testifies for its former presence. Hence, the Rauw fault appears as typical of plate interior context, with an episodic seismic activity concentrated between 1.0 and 0.5Ma or at least between 2.59Ma to 45ka, possibly related to activity variations in the adjacent, continuously active Roer Valley Graben.

Late Quaternary sackungen in the highest mountains of the Carpathians

Sackungen represents a common mode of deep-seated rock-slope failures in alpine landscapes, but proof of their temporal and causal relationship to extrinsic factors such as climatic changes, glacier retreat or seismic activity remains elusive. Based on the terrestrial cosmogenic nuclide (TCN) dating of 18 sackung scarps supported by one radiocarbon-dated scarp, we reconstructed the post-glacial chronology of sackungen in the Tatra Mts. (central Europe, Slovakia and Poland), the highest part of the Carpathians. The obtained ages (∼15.7–4.3 ka) indicate that sackungen post-date the regional LGM and some of them originated soon after the glacier withdrawal from adjacent valleys. Furthermore, systematic decrease of scarp ages with their increasing altitude suggests a direct link between sackung origin and post-LGM glacier thinning. However, substantial lag (>5 ka) of some sackungen in respect to glacier retreat implies complex relationships between sackung onset and deglaciation where retreat of glaciers acted predominantly as a preparatory, not a triggering factor during the genesis of these slope deformations. They originated either as a consequence of stress relaxation within the rock mass lasting several ka or alternatively could be triggered by climatic processes or seismicity. Indeed, a significant part of sackung activity took place during predominantly warmer and more humid periods, with some dates coinciding with the Bølling–Allerød chronozone, but especially with the onset of the Holocene and the Holocene Climatic Optimum. Earthquake triggering is less probable, as the Tatra Mts. lack significant modern and historic seismic activity and there is no geomorphic evidence of fault offsets on the Late Quaternary landforms. In concert with other recent studies, we propose that large rock slope failures in high mountains seldom react immediately to glacier withdrawal, but could display temporal delay lasting up to several millennia.

Constraining recent fault offsets with statistical and geometrical methods: Example from the Jasneuf Fault (Western Alps, France)

We propose two new approaches regarding recent fault offset measurements by studying the seismogenic potential of the Jasneuf Fault.The NE trending right-lateral strike-slip Jasneuf Fault is the southwestern extension of the Belledonne fault system in the Vercors Massif. This fault, which is located in an intraplate domain, does not show strongly associated seismicity but displays morphological anomalies that are related to recent faulting (right-lateral offset of Late Cenozoic cliffs, recent talwegs, and post Günz scarps).The two methods that we use to quantify fault slip are as follows. 1 - Recursive measurements of stream offsets are conducted. This analysis reveals that a stream network can display characteristic distances between streams that can be mistakenly interpreted as long-term cumulative fault offsets. A comparison of the apparent stream offset values and stream spacing values is necessary to identify the true offsets. 2 - We propose a new method that enables us to determine the fault offset and kinematics by using recursive measurements of topographic apparent offsets to counter the lack of morphologic features that are used to determine piercing points or lines. This method enables us to define each possible slip vector for numerous artificial piercing points along a fault. The slip vector that is shared by these piercing points is the true slip vector.By applying these methodologies, we determine that the Jasneuf Fault has accumulated slip since the Messinian, which corresponds to an average slip rate of 0.13±0.03mmyear−1. The extension of the fault is poorly constrained. Nevertheless, if we do not consider the potential aseismic (creep slip) component of the faulting, we calculate that this fault could generate Mw 5.7 earthquakes every ~500years according to Wells and Coppersmith's scaling laws and by assuming that the faulting is limited to the sedimentary cover and the Vercors Plateau.