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The increasing wealthy of high-resolution topography allows for remotely measuring and analysing offset features and their associated surface slip distributions at a very high resolution and along a significant length of a fault, hence providing important insights into many aspects of the fault behaviour. The West Helanshan Fault is a Holocene active fault located at the junction of the Tibetan Plateau, Alashan, and Ordos blocks. Despite its special tectonic location, it has rarely been studied before. In this study, a 2-m-resolution DEM of the West Helanshan Fault was built from the high-resolution (0.5 m) WorldView-3 stereo satellite imagery based on the photogrammetry method, and a total of 181 strike-slip offsets and 201 vertical displacements were acquired along different segments of the fault. By statistical analysis of the offset observations, we conclude that at least six large paleoearthquakes have ruptured the fault, producing a minimum rupture length of ∼50 km, and the paleoearthquakes have followed a characteristic slip pattern with a coseismic strike slip of ∼3 m and a vertical slip of ∼1 m, corresponding to a geologic moment magnitude of 7.1–7.5. [ABSTRACT FROM AUTHOR]

Normal faulting drives extensional growth folding of the Earth's upper crust during continental extension, yet we know little of how fold geometry relates to the structural segmentation of the underlying fault. We use field data from the Hadahid Fault System, Suez Rift, Egypt, to investigate the geometry and kinematics of a large (30 km long, up to 2.5 km displacement), exceptionally well-exposed normal fault system and to test and develop models for extensional growth folding. The Hadahid Fault System comprises eight up to 5 km long segments that are defined by unbreached or breached monoclines. These segments are soft-linked, hard-linked, or defined by a more subtle along-strike transition in overall structural style. High overlap : separation (O:S) ratios between its segments suggest the Hadahid Fault System comprises a single, now hard-linked structure at depth. We demonstrate that a progressive loss of at-surface displacement along-strike of the Hadahid Fault System results in surface-breaking faults and breached monoclines being replaced by unbreached monoclines developed above blind faults. However, shorter along-strike length-scale variations in structural style also occur, with unbreached monoclines developed between breached monoclines. The origin of this variability is unclear, but it might reflect local variations in host rock material properties that drive short length-scale variations in fault propagation-to-slip ratio, and thus the timing and location of fold breaching. We show that folding is a key expression of the strain that accumulates in areas of continental extension, arguing that tectono-sedimentary models for rift development should capture the related structural complexity. [ABSTRACT FROM AUTHOR]

Normal faulting drives extensional growth folding of the Earth's upper crust during continental extension, yet we know little of how fold geometry relates to the structural segmentation of the underlying fault. We use field data from the Hadahid Fault System, Suez Rift, Egypt to investigate the geometry and kinematics of a large (30 km long, up to 2.5 km displacement), exceptionally well-exposed normal fault system to test and develop models for extensional growth folding. The Hadahid Fault System comprises eight, up to 5 km long segments that are defined by unbreached, breached, or partly breached monoclines. These segments are soft- or hard-link, or defined by a more subtle transition in overall structural style. High overlap : separation (O : S) ratios between its segments suggest the Hadahid Fault System comprises a single, now hard-linked structure at-depth. We demonstrate that a progressive loss of displacement along strike of the Hadahid Fault System results in surface-breaking faults and breached monoclines being replaced by unbreached monoclines developed above blind faults. However, shorter along-strike length-scale variations in structural style also occur, with unbreached monoclines developed between breached monoclines. The origin of this variability is unclear, but might reflect local variations in host rock material properties that drive short length-scale variations in fault propagation-to-slip ratio, and thus the timing and location of fold breaching. We show that folding is a key expression of the strain that accumulates in areas of continental extension, and argue that tectono-sedimentary models for rift development should capture the related structural complexity. [ABSTRACT FROM AUTHOR]

Rhythmic seismicity associated with spine extrusion is a well-documented phenomenon at a number of dome-forming volcanic systems. At Unzen volcano, Japan, a 4-year dome-forming eruption concluded with the emplacement of a spine from October 1994 to February 1995, offering a valuable opportunity to further investigate seismogenic processes at dome-forming volcanoes. Using continuous data recorded at a seismic station located close to the dome, this study explores trends in the seismic activity during the extrusion of the spine. We identify a total of 12 208 volcano-seismic events in the period between October 1994 and February 1995. Hourly event counts indicate cyclic activity with periods of ~ 40 to ~ 100 h, attributed to pulsatory ascent defined by strain localisation and faulting at the conduit margins. Waveform correlation revealed two strong clusters (a.k.a. multiplets, families) which are attributed to fracturing along the margins of the shallow, ascending spine. Further analysis indicates variable seismic velocities during the spine extrusion as well as migration of the cluster sources along the spine margins. Our interpretation of the results from seismic data analyses is supported by previously published field and experimental observations, suggesting that the spine was extruded along an inclined conduit with brittle and ductile deformation occurring along the margins. We infer that changes in stress conditions acting on the upper and lower spine margins led to deepening and shallowing of the faulting sources, respectively. We demonstrate that the combination of geophysical, field and experimental evidence can help improve physical models of shallow conduit processes. [ABSTRACT FROM AUTHOR]

Over the last decade tsunami propagation models have been used extensively for both tsunami forecasting and hazard and risk assessment. However, the effect of uncertainty in the earthquake source parameters on the results of the tsunami model has not always been examined in great detail. Here we have undertaken a systematic study of the uncertainty in the maximum wave height of a tsunami (hmax) as a function of the uncertainty in the rupture parameters of the earthquake that generates it (specifically the strike, dip, rake, depth and magnitude). We have shown that even for the simple case of a tsunami propagating over flat bathymetry, the Coefficient of Variation (CoV) and skewness of the distribution of hmax was a complex function of the choice of rupture parameter, distance and azimuth. The relationships became even more complex as the bathymetry used became more realistic. This has major potential implications for both how warning centres operate in the future and how the uncertainty in parameters describing the source should be incorporated into future probabilistic tsunami hazard assessments. [ABSTRACT FROM AUTHOR]

Despite slow slip events have been observed in a variety of tectonic environments worldwide, the underlying physical processes are still poorly understood. They are a manifestation of the complexity of the spectrum of fault slip behavior. Interpreting observations of slow slip events is crucial for understanding the causative processes and the associated slip time histories. The latter are also poorly constrained by observational evidence since the slip velocity function is usually imposed and assumed a-priori during kinematic and dynamic modeling of earthquake ruptures.In this study, we analyze slow-slip events obtained during laboratory experiments at the stability boundary (k~kc) by matching the critical fault stiffness (kc) with the machine stiffness (k). The experiments were performed in the double direct shear configuration equipped with LVDT (linear variable differential transducers) mounted on the sample assembly to accurately solve details of sample compaction/dilation and to measure the fault slip. To discern the role of shear localization during fault weakening we simulated fault gouges using quartz powders and a mixture of anhydrite and dolomite, which have a strong rheological contrast. For both lithologies, slow-slip events have typical friction drop of ~0.01 and duration between 6s and 12s. During each event we observe that peak slip velocity is attained before reaching the minimum shear stress, and most of the slip velocity evolution is associated with the dynamic weakening phase. We document that the fault zone fabric controls the slip velocity function as well as the details of micro-mechanical deformation (i.e. dilation/compaction). For quartz gouge, shear deformation localizes along continuous and sharp shear planes (~1µm thick) resulting in a smooth slip velocity function (similar to a Gaussian) where a clear pre-seismic slip is easily detectable. Fault gouge compaction is observed during the whole experiment, with the larger compaction rate occurring before peak slip velocity is attained, likely due to cataclasis and grain size reduction in the bulk. In the anhydrite and dolomite mixture, shear localizes along thick boundary shear planes interconnected by a P-foliation, resulting in a more distributed deformation, showing a Yoffe slip velocity having a short acceleration phase (~1s) with no pre-seismic slip and a long deceleration. Fault dilation occurs simultaneously with slip acceleration likely due to slip partitioning within the entire the experimental fault.The inferred breakdown work (seismological fracture energy) is very similar for both lithologies, but for quartz we observe a relevant portion of mechanical work absorbed before the main slip acceleration phase, probably due to the compaction within the bulk volume.Our results confirm that the slip rate function contains the key dynamical information to characterize the evolution of dynamic traction. The retrieved differences in the slip velocity functions and mechanical work absorbed in dynamic weakening depend on the strain partitioning within the fault volume. Our results suggest that slip time histories depend on fault zone structure and strain partitioning in the fault core. [ABSTRACT FROM AUTHOR]

Transform margins are first-order structural features that represent a total of 16% of the cumulative length of continental margins accommodating plate motion. Yet, their evolution, genetic relation to oceanic spreading, and general structural character still present a challenging research field. In this work, we investigate the evolution of rift-transform intersections using a combined analogue and numerical modelling approach. Through a series of analogue experiments, we investigate the effect of an imposed rotation in the rift extension direction, a component in rift-transform interaction that has not been studied previously. In the model, we use a two layer ductile-brittle configuration to simulate the crustal rheology. We initiate rifting in an orthogonal direction and then proceed to gradually rotate the plate vector to a 7o angle. This angle is comparable to the amount of rotation seen in natural examples. Rifting then continues with the new plate motion vector. The experimental configuration we use allows the study of transpressional and transtensional rotation of the moving plate simultaneously. Results show that: a) a transtensional shift in the plate direction produces en-echelon oblique slip faults (alongside a principal displacement zone) which accommodate the horizontal displacement until the new plate motion vector is stabilized and b) a transpressional shift produces buckle folding near the rift-transform intersection and widespread transpression further away from the rift. Using the open source geodynamics code ASPECT, we present a suite of 3D numerical models of lithospheric rifting in the presence of plate vector rotation. We test both the timing and the angle of the vector rotation to investigate the effect these have on deformation patterns along a rift-transform intersection. Our numerical simulations complement the analogue models while exploring the rheological and mechanical parameter space that guides our preliminary results.Finally, we then compare our observations from the combined modelling approach with seismic reflection images from a range of margins around the world, including the Gulf of California partitioned oblique margin and the Tanzania Coastal Basin. [ABSTRACT FROM AUTHOR]

The geometry of fault zones (e.g. fault surface roughness, fault rock distribution, network of secondary faults and fractures) is one of the main factors controlling earthquake nucleation, rupture speed and length of main shocks, foreshock and aftershock evolution, ground motion and seismic radiation pattern. Despite the pivotal role that fault geometry bears on earthquake mechanics, little is known about the precise architecture of fault zones at seismogenic depths. In fact, most exhumed faults, especially those capable of producing moderate to large earthquakes (faults > 20 km in length) are discontinuously and poorly exposed at the surface because of vegetation cover and/or weathering.Here we present a detailed, though preliminary, structural geology survey of a large exceptionally exposed, exhumed seismogenic fault in the Atacama Desert (Chile), the Bolfin Fault Zone (BFZ). The BFZ is a sub-vertical NNW-striking, left-lateral strike-slip fault belonging to the ~1000 km long Atacama Fault System. The BFZ is hosted in amphibolites, granodiorites and quartzdiorites of the Coastal Cordillera and records Mesozoic (sinistral) and Cenozoic (normal) displacements.We integrated Unmanned Aerial Vehicle surveys with detailed mapping of the network of ductile shear zones and brittle faults of the BFZ to produce high-resolution maps of the fault zone structure over four representative transects along fault strike. We conducted mineralogical, microstructural and geochemical investigations of the fault zone rocks.From this field and microstructural study, four main deformation events were recognized, based on their crosscutting relationships observed in the field. An early deformation event (D0) consists of NW- to N- striking ductile (amphibolitic to granulitic facies) shear zones. The first brittle deformation event (D1, BFZ sensu-strictu) consists of NW- to NE-striking sinistral strike-slip faults associated with greenschists to sub-greenschists (albite + chlorite + epidote + actinolite) foliated cataclasites and pseudotachylytes. The mineral assemblage and Al content in chlorite constrain deformation conditions at 6-8 km depth and 280-350°C ambient temperature. The presence of pseudotachylytes, formed in a fluid-rich environment, as attested by the abundance of amygdules in the matrix (only associated with the D1 event) documents the ancient seismicity of the fault zone. A N-, NW- and NE-striking fault set consisting of dextral transtensional faults represents the intermediate brittle deformation event (D2) and is marked by hydrothermal epidote (+ quartz + calcite) precipitation. Lastly, two conjugate fault sets, a NNE-SSW to NE-SW strike dextral-normal and a ESE-WNW strike sinistral-normal transtensional faults filled with calcite (+ palygorskite ± gypsum ± halite), are interpreted as the youngest (Miocene to post-Moicene) deformation event (D3). The occurrence of palygorskite constrains D3 deformation conditions of < 150°C to ambient temperature.Given the exceptional exposure and the relatively easy accessibility compared to other fault zones cropping out in desert areas, the Bolfin Fault Zone provides a unique opportunity to (1) examine the structure and the evolution of seismogenic fault zones in the Earth's continental crust and, (2) produce 3-D fault models for earthquake rupture numerical simulations. [ABSTRACT FROM AUTHOR]

Geophysical observations illustrate that a range of fault slip styles accommodate displacement, ranging from steady creep at plate boundary displacement rates through to episodic earthquake slip at speeds of ~ 1 m/s. Slow slip events (SSEs) represent an intermediate fault slip rate that is faster than plate rate, but too slow to generate observable seismic waves. SSEs are typically observed in or near the transition zone between interseismically locked and fully creeping fault segments. Because SSEs have been observed both up- and downdip of interseismically locked zones, they occur at temperatures ranging from ≤ 100˚C to > 500˚C, and are therefore not controlled by a single, thermally activated mechanism. The physical mechanism behind SSEs, however, remains unclear.Geological observations show that faults can accommodate slip by modes ranging from slip localised on one or more discrete failure planes through to shearing flow distributed within a tabular zone of finite thickness, indicating a large range of possible strain rates in natural faults. Crosscutting relations between brittle and ductile structures demonstrate that strain rate can change both progressively and cyclically within a single fault zone. Cyclic strain rate variations can arise from fluctuations in effective stress, but here we discuss how strain rate transients may also occur in response to internal deformation within a polyphase shear zone.Both geological observations and numerical models have now shown that a range of slip styles may arise from rheological heterogeneity in space. Such heterogeneity is almost ubiquitous in natural faults, where it may be represented by different minerals in a fault gouge, by anastomosing phyllosilicate networks around competent clasts in melanges, and variably metamorphosed and dehydrated rocks within subducting basalts, to name a few examples. In such examples, spatial variations in strain rate are inevitable, and temporal variations may follow. If fault slip style is related to fault zone heterogeneity, then there must be certain conditions that are required to produce SSEs, that can be achieved at a large range of temperatures, but yet are not achieved in fault segments that are fully creeping or locked. To explore these conditions, we use the finite element numerical code 'Underworld' to analyse the behaviour of two-phase shear zones. We suggest there is a minimum viscosity ratio, above which two things happen: 1) stresses are amplified in the stronger component, potentially leading to failure in strong lenses within a flowing, weaker matrix; and 2) strong lenses interact, and episodically form 'force chains' that lead to jamming of the shear zone. The required viscosity contrast (strong/weak) depends on the volume fraction of the weak phase but is typically on the order of 100 – 1000; we suggest that among much evidence for heterogeneity, it is where this level of contrast occurs that slow slip is most likely to arise. [ABSTRACT FROM AUTHOR]

The Victoria plate in the East African Rift System (EARS) is one of the largest continentalmicroplates on Earth. Victoria is encompassed by the partly overlapping eastern and westernEARS branches that follow inherited weaknesses of Proterozoic mobile belts. Multiple linesof evidence unequivocally show that Victoria rotates counterclockwise with respect to Nubia,in striking contrast to its neighboring plates. However, the cause of this distinctive rotationhas remained speculative so far. Using 3D upper-mantle scale numerical models of extension, we investigate the role ofpreexisting strength heterogeneities in the rotation of a continental microplate. We find thatthe amount of rotation is primarily controlled by the distribution of (i) stronger zones thattransmit the drag of the major plates along the edges of the block and (ii) weaker regions thatfacilitate the rotation. For right-lateral step-overs, this 'edge-driven&#x2019; mechanism (suggestedby Schouten et al. 1993 for oceanic microplates) produces a counterclockwise rotation of themicroplate, inducing a clockwise shift of the local extension direction along the overlappingrift branches. The counterclockwise rotation of Victoria, its rotation pole and its angular velocity arebest reproduced in our models if we include preexisting weaknesses that follow thedistribution of the mobile belts surrounding the Victoria plate as well as the stronger regionsof the Tanzania craton and Turkana depression. Our models show that under regional,north-to-south decreasing E-W extension this rotation results in local extensiondirections that strike more WNW-ESE. Together with the oblique orientation of thepreexisting weaknesses, this leads to predominantly normal faulting oblique to theregional and local extension direction. In the most oblique (45∘) section of thewestern branch, the Tanganyika-Rukwa-Malawi segment, this can produce transientstrike-slip faulting, although comparison to stress observations suggests a local stressreorientation, possibly due to smaller-scale inherited mechanical anisotropy (e.g. Morley2010).Schouten, H., Klitgord, K. D. and Gallo, D. G. (1993), Edge-driven microplate kinematics.Journal of Geophysical Research 98, B4, 6689–6701.Morley, C. K. (2010), Stress re-orientation along zones of weak fabrics in rifts: Anexplanation for pure extension in 'oblique&#x2019; rift segments? Earth and Planetary Science Letters297, 3–4, 667–673. [ABSTRACT FROM AUTHOR]

Surface rupturing earthquakes occur in a wealth of plate tectonic settings and pose important hazard to nearby cities and infrastructure. Nevertheless mechanical models to describe the slip behavior and displacement accumulation pattern of the shallow fault zone are contradictory. Recent observations suggest that, despite low confining pressure and phyllosilicate rich fault gouges at shallow crustal levels, faults can exhibit velocity weakening behavior all the way to the surface. We use continuous time series from the IPOC Creepmeter Array (www.ipoc-network.org/observatory/creepmeter/creepmeter-at-ipoc/) in N-Chile, monitoring 4 segments of the Atacama Fault System, to investigate the slip mode of the different fault segments.This instrumental dataset covers a time span of 8 years at 11 monitoring sites and comprises both triggered shallow slip events and creep signals. The recorded shallow slip events range from μm to mm scale in fault parallel displacement. Slip velocities for these events range between 1 -2000 mm/year therefore representing slow slip events. In fact, the current sampling rate of 2 samples/min does not allow for the detection of seismic velocities for the size of the so far recorded slip events.Decomposing the time series at the different monitored fault segments we are able to differentiate between slow creeping fault segments not accumulating displacement by triggered shallow slip events and others which are accumulating displacement solely by triggered shallow slip events. Comparing these observations with the fault gouge thickness at the monitored sites, we find a strong correlation of segments exhibiting 10 cm thick fault gouge at the monitoring site and creeping fault behavior. Whereas these fault segments, accumulating displacement only via triggered shallow slip events, do not have a fault gouge developed. Instead those fault contacts are developed in a strongly fragmented basement/basement contact zone or show a submillimeter thin sharp contact in alluvial fan sediment. These first results demonstrate the ability to extract fault frictional behavior from time series collected with the IPOC Creep Observatory, that match fault gouge properties studied in the field directly at the monitoring sites. [ABSTRACT FROM AUTHOR]