Increase In Slip Rate Research Articles

An investigation into the effect of slip rate on the traction coefficient behaviour with a laboratory replication of a locomotive wheel rolling/sliding along a railway track

This paper investigates the effect of slip rate on the traction coefficient of the wheel/rail interface using a twin-disc machine under the replicated tribological and locomotive operating conditions. The traction coefficient decreases with increasing slip rate. The interface materials of the tested samples get less time to accumulate strain hardening, resulting in a lower increase in surface hardness under a faster slip rate. The running-in period is shorter under a higher slip rate, indicating that the surface micro-geometry of the interface materials reaches a steady state faster. The wear and traction coefficient outcomes of this research could be helpful in improving locomotive traction control systems, and in increasing the safety and maintenance efficiency of railway operations.

Modeling frictional precursory phenomena using a wear-based rate- and state-dependent friction model in the laboratory

We examine numerical models that employ the rate-and-state frictional (RSF) framework to investigate earthquake sequences using laboratory driven descriptions of heterogeneous frictional properties. Using previously obtained experimental measurement of roughness, we observed that wear produced a bimodal Gaussian distribution of surface heights, which we hypothesized produced spatial heterogeneity of the critical slip distance Dc. In this numerical study, the fault surface was binarized into discrete smooth or rough sections, producing a barcode style version of frictional heterogeneity. The fault was predominantly rough except for two dominant asperities (A1 and A2) representative of larger polished sections. We simulated the resistive effect of increasing the fracture energy (toughness) of the rough barriers while maintaining constant properties of the embedded brittle/smooth asperities. Our numerical simulations generated burst-like seismic events and aseismic transients throughout the interseismic phase. At the late interseismic phase, bursts of seismicity (foreshocks) interacted with the accelerating preslip region at the transition to the preseismic (nucleation) phase. At lower levels of toughness heterogeneity, the slip rate increase was roughly inversely proportional to the time-to-failure tf for larger events. As fault toughness was increased, the dominant asperities initiated nucleation and thus force deviations of the fault from the smooth 1/tf acceleration observed for the homogeneous case, producing a rate-dependent cascade response. The calculations were validated by comparing two independently measured metrics from the experiments: (1) The expansion rate of slow ruptures during the interseismic and preslip phase and (2) the scalar seismic moment and source dimensions. While our study does not address the scaling problem, these results help to understand laboratory experiments that investigate transition to the preseismic (nucleation) phase during complex earthquake sequences.

Analysis of Tire‐Pavement Contact Morphology Characteristics during the Virtual Track Train Maneuvering

The tire‐pavement contact morphology during the virtual track train maneuvering has an essential function in assessing vehicle performance and pavement damage. The longitudinal tread tire and viscoelastic asphalt pavement were developed based on the finite element method. Then, the tire‐pavement contact model was developed according to the Euler–Lagrange algorithm and validated. Contact area and stress distribution under uniform, braking, and cornering conditions are explored with the dynamic tire force of the virtual track train as the external load. Also, the effect factors were analyzed. The results indicate that vertical stress distribution is symmetrical along the length and width directions, and contact area is rectangular under uniform condition. Vertical and longitudinal stresses have similar distributions under braking condition, with a wide front and a narrow back along the length direction. Under cornering condition, the closer to the inside of the cornering, the larger the contact area. Vertical stress is similar to the lateral stress distribution, and the larger stress value is concentrated on the inner side of each tread. Load, tire pressure, slip rate, and slip angle have significant effects on contact morphology, while velocity has little effect. Contact area increases with the increase of load and slip rate and decreases with the increase of tire pressure and slip angle. Vertical stress increases gradually with the increase of load. Higher tire pressure makes stress distribution more concentrated. The increase in slip rate increases vertical and longitudinal stresses and gradually moves in the running direction. Vertical and lateral stresses increase with the slip angle and are mainly distributed on the inside of the turn. This research can be used to evaluate and analyze the response and damage of the virtual track train to asphalt pavement under uniform, braking, and cornering conditions. It can be used as a reference for the design and paving of asphalt pavement.

Spatial Slip Rate Distribution Along the SE Xianshuihe Fault, Eastern Tibet, and Earthquake Hazard Assessment

AbstractThe Xianshuihe (XSH) fault in eastern Tibet is one of the most active faults in China, with the next large earthquake most likely to occur along its SE part, where the fault splits into three parallel branches: Yalahe, Selaha and Zheduotang (ZDT). Precisely quantifying their slip rates at various timescales is essential to evaluate regional earthquake hazard. Here, we expand our previous work on the Selaha fault to the nearby ZDT and Moxi (MX) faults, and add observations on the Yalahe fault and on the newly discovered Mugecuo South fault zone. Using tectonic‐geomorphology approaches with10Be dating, we had previously determined average late Quaternary slip rates of 9.75 ± 0.15 and 4.4 ± 0.5 mm/yr along the NW and SE Selaha fault, respectively. Using the same methods here, we determine a slip rate of 3.4–4.8 mm/yr on the ZDT fault and of 9.6–13.4 mm/yr on the MX fault. This is consistent with the southeastward slip rate increase we had proposed along the XSH fault system from 6‐8 mm/yr (Ganzi fault) to ∼10 mm/yr (Selaha fault), and >9.6 mm/yr (MX fault). We propose a new model for the SE XSH fault, where the large‐scale Mugecuo pull‐apart basin lies within an even larger scale compressive uplift zone in a restraining bend of the XSH fault, where the highest peak in eastern Tibet is located (Gongga Shan, 7,556 m). Our slip rate determination helps to constrain a relatively high regional Mw ∼ 7 earthquake hazard at present on the SE XSH fault.

Cyclic Water Injection Potentially Mitigates Seismic Risks by Promoting Slow and Stable Slip of a Natural Fracture in Granite

Induced seismicity associated with fluid injection has raised serious concerns for the safety and efficiency of geo-energy systems. Cyclic injection has recently been proposed as an alternative injection scheme to reduce the large magnitude injection-induced seismicity. However, the influence of cyclic injection on the activation of natural fractures in granite and the resulting seismic risk is not yet clear. This study investigates the injection-induced activation of a critically stressed natural fracture in a granite core sample, particularly focusing on the comparison between monotonic and cyclic water injection under pressure-controlled and volume-controlled conditions. Experimental results show that the acceleration and deceleration of fracture slip are modulated by the shear stress imbalance between the fixed shear stress and the evolving frictional strength of the fracture. Fracture slip affects the fluid pressure distribution on the fracture, which in turn regulates the frictional strength of the fracture. At a small total shear displacement (i.e., ~ 0.9 mm in this study), cyclic injection with a restricted peak injection pressure results in aseismic fracture slip at much smaller peak slip rates compared to that during the monotonic injection. On the one hand, the more uniform reduction in effective normal stress caused by cyclic injection encourages slow and stable fracture slip, characterized by the smaller peak slip rates. On the other hand, the flowback of injected fluid or suspension of injection could prevent the occurrence of fast-accelerated fracture slip during cyclic injection. However, the fracture can become unstable when it has experienced a considerable amount of total shear displacement (larger than ~ 0.9 mm in this study), and likely gained a significantly enhanced permeability. Continued injection after the unstable shut-in stage, signified by an unusual increase in slip rate and an accelerated drop in injection pressure, could result in rapid and unstable fracture slip.

Evolution of aseismic slip rate along plate boundary faults before and after megathrust earthquakes

Similar earthquakes that occur in approximately the same location have the potential to reveal the spatio-temporal changes in aseismic slip along plate boundaries. Here we identify similar earthquakes with moderate magnitudes that occurred worldwide between 1989 and 2016 by using seismograms recorded by the Japanese dense seismic network. The slip rate along the plate boundaries estimated from similar earthquakes increased rapidly following M > 8 megathrust ruptures and then gradually decayed over periods of ~10 years, which correlates with after-slip progressing around the source areas. More than 30 years after large megathrust earthquakes, the slip rate begins to show a gradual increase. This gradual increase in slip rate after the decay may be due to an increase in stress levels that accumulate during tectonic loading. The spatio-temporal characteristics of inter-plate aseismic slip can be used to provide a valuable framework for understanding the long-term evolution of slip-rate during megathrust earthquake cycles.

Unraveling long-term volcano flank instability at Pacaya Volcano, Guatemala, using satellite geodesy

Edifice collapse represents one of the most dangerous volcanic hazards threatening communities and infrastructure near volcanoes, having caused over 20,000 casualties in the past 400 years. Despite its prevalence across volcanic settings, flank instability has mostly been considered at ocean island volcanoes. In Guatemala, all but one volcano with elevation greater than 2000 m have undergone edifice failure. Pacaya is one of these Guatemalan volcanoes, which experienced at least one past episode of flank collapse and where recent transient flank motion has been identified. Using Interferometric Synthetic Aperture Radar (InSAR) time-series analysis, we reveal, for the first time, long-term displacement of the southwest flank of Pacaya between 2010 and 2014, including the interval 2011–2013 where the volcano remained relatively quiescent. Subsequent geodetic data inversions and static stress change analysis suggest that the observed flank motion could be accommodated by slip on a southwest-dipping detachment fault, with an observed increase in slip rate attributed to magma intrusion associated with a major eruption in 2014. This study highlights that long-term volcanic flank creep is likely more widespread than previously recognized and that magma-faulting interactions, as well as the existence of structural weaknesses within volcanic edifices, are vital considerations in the assessment of likelihood and nature of flank collapse at volcanoes worldwide.

Slip-rate-dependent friction as a universal mechanism for slow slip events

A growing body of observations worldwide has documented fault slip transients that radiate little or no seismic energy. The mechanisms that govern these slow slip events (SSEs) and their wide range of depths, slip rates, durations, stress drops and recurrence intervals remain poorly known. Here we show that slow slip can be explained by a transition from rate-weakening frictional sliding at low slip rates towards rate-neutral or rate-strengthening behaviour at higher slip rates, as has been observed experimentally. We use numerical simulations to illustrate that this rate-dependent transition quantitatively explains the experimental data for natural fault rocks representative of materials in the source regions of SSEs. With a standard constant-parameter rate-and-state friction law, SSEs arise only near the threshold for slip instability. The inclusion of velocity-dependent friction parameters substantially broadens the range of conditions for slow slip occurrence, and produces a wide range of event characteristics, which include stress drop, duration and recurrence, as observed in nature. Upscaled numerical simulations that incorporate parameters consistent with laboratory measurements can reproduce geodetic observations of repeating SSEs on tectonic faults. We conclude that slip-rate-dependent friction explains the ubiquitous occurrence of SSEs in a broad range of geological environments. A transition from rate-weakening to rate-strengthening frictional behaviour with increasing slip rate could explain the observed diversity of slow slip events on faults, according to numerical simulations.

Formation and re-orientation of the Suerkuli Basin within the Altyn Tagh in northeastern Tibetan Plateau since late Miocene

The Altyn Tagh Fault (ATF) had formed in Eocene time, however, the temporal evolution of slip along the fault remains elusive, despite its important role in governing growth of the Tibetan Plateau (TP) and evaluating models of continental lithosphere deformation. Here we perform a high-resolution magnetostratigraphic study on Neogene sediments from the Suerkuli Basin, a small basin located between two active strands of the ATF. Results show that both depositional environment and the sedimentation rate changed at about 5.4 and 6 Ma, respectively. Lithofacies shift from conglomerates and sandstones with parallel and cross beddings to mudstones with horizontal bedding, suggesting a sedimentary fluvial to lacustrine depositional environment shift. Even aridification increased over northern China, from the Tarim Basin to the Qaidam Basin, during late Miocene to early Pliocene times, lacustrine deposition had started in the Suerkuli Basin still. We argue that an increase in slip rate along the ATF triggered concurrently forming of the intermontane basin. The inferred timing of increased slip-rate on the ATF coincides with a variation in movement of the Indian Plate relative to Eurasian Plate since Late Miocene-Early Pliocene time. Our work suggests that a re-orientation of crustal shortening and deformation in the northern TP began since late Miocene and supports the idea that the modern boundary of the TP orogen had rejuvenated to adjust deformation in plate boundary conditions.

Segmentation and increasing activity in the Neogene-Quaternary Teruel Basin rift (Spain) revealed by morphotectonic approach

The NNW-SSE trending Teruel Basin rift is the largest Late Miocene-Quaternary extensional intracontinental structure located within the central-eastern Iberian Chain (Spain). The structural and morphotectonic study carried out in the central-northern part of this half graben basin (north of Teruel city) has allowed us to analyse rift segmentation, deformation partitioning and rift evolution. Results are based on vertical displacement calculations (fault throw and bending) of the main border and intrabasin fault zones. We use two geomorfological-stratigraphical markers, the Intramiocene Erosion Surface (IES; 11.2 Ma) and the Fundamental Erosion Surface (FES; 3.5 Ma). While the first marker reveals rift initiation under an E-W extension, the late marker records vertical displacements associated to a second, Late Pliocene–Quaternary rifting stage characterized by a nearly multidirectional extension regime with prevailing ENE-WSW trending ó3. Despite the along-axis rift segmentation into three structural domains (northern, central and southern) and the distribution of deformation among border and intrabasin faults in the central and southern domains, a consistent average slip rate (post-IES) of 0.09 mm/a has been calculated on distinct transects across the basin, suggesting a homogeneous crustal-scale extension process in the region. The results also reveal that slip rates during the Late Pliocene-Quaternary (0.12–0.16 mm/a) are higher than the Late Miocene-Early Pliocene (0.05–0.07 mm/a). Slip rate increase is caused by (i) a westward propagation of deformation from the Valencia Through, and (ii) a change in the regional stress field, both enhanced by crustal doming affecting central-eastern Iberia, as well as progressive fault linkage. Throw vs. distance graphs suggest that the main faults are in a transient stage towards coalescence, less advanced within the southern domain. Regional Late Pliocene-Quaternary uplift, concomitant with increasing slip rates in the Teruel Basin rift, has caused the basin to rise, so that synrift sedimentation only took place in rapidly subsiding residual basins until the region became exorheic and the basin was incised by the present-day fluvial network.

Localized Slip and Associated Fluidized Structures Record Seismic Slip in Clay‐Rich Fault Gouge

AbstractFault rocks can weaken dramatically with increasing slip rate, which results in localization of slip and earthquakes. Exhumed fault zones and fault rocks deformed at seismic rates in the laboratory both show that deformation can become extremely localized to zones less than or equal to millimeters thick. However, localization can occur during aseismic slip, so evidence of localization cannot necessarily be interpreted as having occurred coseismically. Dynamic weakening that occurs during earthquakes is the result of processes that are unique to seismic slip rates, and previous results from carbonates show that these processes produce unique microstructures. We evaluate whether coseismic deformation at low normal stress produces unique structures within the localized slip zones and adjacent gouge that develop in clay‐rich gouge from the Central Deforming Zone of the San Andreas fault. We measured the thickness and orientations of localized slip zones and their internal lamina, particle orientations, and particle size distributions of gouge sheared from 0.35 to 1.3 m/s velocity, up to 25‐m displacement, and 1‐MPa normal stress, under water‐wet and room‐dry conditions. We find that the thicknesses of localized slip zones and their internal laminae are consistent with numerical formulations for thermal pressurization in wet gouge and both thermal decomposition and cataclastic deformation in dry gouge. Localized zones form coincident with a zone of fluidized gouge that accommodates at most 10% of the shear strain. We conclude that the combined occurrence of foliated localized shear zones with a zone of fluidized gouge may provide a record of seismicity in clay‐rich gouges.

Terrestrial cosmogenic surface exposure dating of moraines at Lake Tahoe in the Sierra Nevada of California and slip rate estimate for the West Tahoe Fault

Two sets of Pleistocene moraines (Tioga and Tahoe) are preserved at Cascade Lake along the west side of Lake Tahoe. The 10Be terrestrial cosmogenic nuclide surface exposure ages for two younger Tioga moraines yield an average age of 22.3±1.2ka, coincident with the global Last Glacial Maximum. The ages suggest that the Tioga glaciation may have reached its maximum several thousand years earlier in the Lake Tahoe basin than to the south along the east flank of the Sierra Nevada. The oldest 10Be age (120±8ka) determined for an additional suite of 10 boulders exhibiting significant scatter in 10Be ages is interpreted to be the minimum age of formation for older Tahoe moraines in the Tahoe basin, suggesting they were deposited during marine oxygen isotope stage 6. The moraines at Cascade Lake are displaced by the West Tahoe Fault that strikes northward for 45km along the western edge of the Lake Tahoe basin. Vertical displacements of the crests of the Tahoe and Tioga moraines are 59±10 and 32±12m respectively. Averaged over the time since the formation of the Tahoe and Tioga moraines, the average vertical separation rates are <0.5±0.1 and ~1.4±0.7mm/y respectively. The measured vertical separation across the broad graben on the Tioga moraine may be accentuated by its deposition on a preexisting scarp and, in this regard, the increase in slip rate since the Tioga glaciation may be apparent rather than real. The fault slip rate and accompanying horizontal rate of extension averaged over the time since the formation of the older Tahoe moraines are respectively 0.6±0.2 and 0.3±0.2mm/y. The slip rate averaged over the time since emplacement of the Tahoe moraine is in general accord with prior geologic studies reporting slip rate estimates elsewhere along the fault, and the horizontal extension rate is at the lower end of extension rates estimated by others with geodesy across the Tahoe basin.

Influence of weakening minerals on ensemble strength and slip stability of faults

AbstractWe explore the impact of phyllosilicate (weak but velocity strengthening) in a majority tectosilicate (strong but velocity weakening) matrix on bulk shear strength and slip stability of faults. Numerical simple shear experiments using a distinct element model (DEM) are conducted on both uniform mixtures of quartz and talc analogs and on textured mixtures consisting of a talc layer embedded in a quartz matrix. The mechanical response of particles is represented by a linear elastic contact model with a slip‐weakening constitutive relation representing the essence of rate‐state friction. The weight percentage of the talc in the uniform mixtures and the relative thickness of the talc layer in the textured mixtures are varied to investigate the transitional behavior of shear strength and slip stability. Specifically, for uniform mixtures, ~50% reduction on bulk shear strength is observed with 25% talc present, and a dominant influence of talc occurs at 50%; for textured mixtures, a noticeable weakening effect is shown at a relative layer thickness of 1 particle, ~50% shear strength reduction is observed with 3‐particles, and a dominant influence occurs at 5 particles. In terms of slip stability, a transition from velocity weakening to velocity strengthening is observed with 10% to 25% talc present in the uniform mixtures or with 3 particles to 5 particles in the textured mixtures. In addition, further analysis suggests that quartz has a high tendency toward dilation, potentially promoting permeability; while talc dilates with increased slip rate but compacts rapidly when slip rate is reduced, potentially destroying permeability. The simulation results match well with previous laboratory observations.

Apparent Late Quaternary Fault‐Slip Rate Increase in the Southern Lower Rhine Graben, Central Europe

In regions of low strain, long earthquake recurrence intervals (104–106 yrs) and erosive processes limit preservation of Quaternary markers suitable for distinguishing whether faults slip at uniform or secularly varying rates. The Lower Rhine graben in the border region of Germany, The Netherlands, and Belgium provides a unique opportunity to explore Quaternary slip‐rate variations in a region of low strain using the basal (2.29±0.29 Ma) and surface (700±80 ka) contacts of the regionally extensive main terrace (“Hauptterrasse”), deposited by the Rhine and Maas Rivers. These surfaces are vertically offset 3–140 m and 0–68 m, respectively, across individual fault strands within a distributed network of northwest‐trending, slow‐slipping ( 80% of the sites record an apparent increase in slip rate for the more recent interval from 700 ka to present, which corresponds to a period of increased uplift of the nearby Rhenish Massif and regional volcanism. However, the apparent increase in slip rate could result, in part, from erosion of the footwall surface below the main terrace, leading to an apparent displacement that is smaller than the total vertical offset since the start of the Quaternary. Prior work focused on characterization of these faults as seismic sources in the Lower Rhine graben has preferentially relied on the average fault‐slip rate constrained using the base of the main terrace. We suggest that average fault‐slip rates calculated using the ∼700 ka main terrace surface are subjected to fewer uncertainties and sample a time interval that is more relevant for seismic‐hazard analysis. [Electronic Supplement:][1] Table of main terrace surface and basal contact offset measurements, figure describing an unrealistic faulting scenario, and a KML file with the fault‐trace mapping spanning the Lower Rhine graben. [1]: http://www.bssaonline.org/lookup/suppl/doi:10.1785/0120160197/-/DC1

Holocene slip rate variability along the Pernicana fault system (Mt. Etna, Italy): Evidence from offset lava flows

The eastern flank of the Mount Etna stratovolcano is affected by extension and is slowly sliding eastward into the Ionian Sea. The Pernicana fault system forms the border of the northern part of this sliding area. It consists of three E-W−oriented fault sectors that are seismically active and characterized by earthquakes up to 4.7 in magnitude (M) capable of producing ground rupture and damage located mainly along the western and central sectors, and by continuous creep on the eastern sector. A new topographic study of the central sector of the Pernicana fault system shows an overall bell-shaped profile, with maximum scarp height of 35 m in the center of the sector, and two local minima that are probably due to the complex morphological relation between fault scarp and lava flows. We determined the ages of lava flows cut by the Pernicana fault system at 12 sites using cosmogenic 3 He and 40 Ar/ 39 Ar techniques in order to determine the recent slip history of the fault. From the displacement-age relations, we estimate an average throw rate of ∼2.5 mm/yr over the last 15 k.y. The slip rate appears to have accelerated during the last 3.5 k.y., with displacement rates of up to ∼15 mm/yr, whereas between 3.5 and 15 ka, the throw rate averaged ∼1 mm/yr. This increase in slip rate resulted in significant changes in seismicity rates, for instance, decreasing the mean recurrence time of M ≥ 4.7 earthquakes from ∼200 to ∼20 yr. Based on empirical relationships, we attribute the variation in seismic activity on the Pernicana fault system to factors intrinsic to the system that are likely related to changes in the volcanic system. These internal factors could be fault interdependencies (such as those across the Taupo Rift, New Zealand) or they could represent interactions among magmatic, tectonic, and gravitational processes (e.g., Kīlauea volcano, Hawaii). Given their effect on earthquake recurrence intervals, these interactions need to be fully assessed in seismic hazard evaluations.

Normal fault growth and linkage in the Gediz (Alaşehir) Graben, Western Turkey, revealed by transient river long‐profiles and slope‐break knickpoints

AbstractThe Gediz (Alaşehir) Graben is located in the highly tectonically active and seismogenic region of Western Turkey. The rivers upstream of the normal fault‐bounded graben each contain a non‐lithologic knickpoint, including those that drain through inferred fault segment boundaries. Knickpoint heights measured vertically from the fault scale with footwall relief and documented fault throw (vertical displacement). Consequently, we deduce these knickpoints were initiated by an increase in slip rate on the basin‐bounding fault, driven by linkage of the three main fault segments of the high‐angle graben bounding fault array. Fault interaction theory and ratios of channel steepness suggest that the slip rate enhancement factor on linkage was a factor of 3. We combine this information with geomorphic and structural constraints to estimate that linkage took place between 0.6 Ma and 1 Ma. Calculated pre‐ and post‐linkage throw rates are 0.6 and 2 mm/yr respectively. Maximum knickpoint retreat rates upstream of the faults range from 4.5 to 28 mm/yr, faster than for similar catchments upstream of normal faults in the Central Apennines and the Hatay Graben of Turkey, and implying a fluvial landscape response time of 1.6 to 2.7 Myr. We explore the relative controls of drainage area and precipitation on these retreat rates, and conclude that while climate variation and fault throw rate partially explain the variations seen, lithology remains a potentially important but poorly characterised variable. Copyright © 2016 John Wiley &amp; Sons, Ltd.

Evolution of the Brass texture in an Al-Cu-Mg alloy during hot rolling

Texture and microstructure development of Al-4Cu-1.6Mg alloy during hot rolling was examined by using XRD, EBSD and TEM. The results showed that starting with a random texture during the early stages of rolling with reduction lower than 58.9% at blooming temperature of 430 °C, the materials developed a typical α-fiber texture in the center layer as deformation reduction reached 75.7%. And then the α-fiber textures in the center layer rotated into Brass component mainly through the activity of {111}<110> slip system as the reduction reached 96.3%. Different from the center textures, the main texture in the surface layer was r-Cube with 96.3% reduction. The increase in rolling temperature was beneficial for the enhanced texture intensity of Brass component in the center layer. The analysis of substructure energy density indicated that Brass subgrain had a lower substructural energy density than other oriented subgrains, which together with increased slip rate at elevated temperature, contributed to the development of center Brass texture.

Transient water adsorption on newly formed fault gouge and its relation to frictional heating

AbstractDetermination of the amount of frictional heating on faults during slips provides insight into the mechanics of faulting. Fault slips cause frictional heating as well as production of gouge materials. Newly formed gouge is mechanochemically stimulated to gain a transient water adsorption ability. We showed that the synthetic gouge from friction tests on rocks at a 1 mm/s slip rate without frictional heating adsorbed a large amount of water, comparable to that adsorbed by clay minerals. The amount of adsorbed water decreased significantly with increasing slip rate to 100 mm/s, where the fault temperature increased above 400°C. We confirmed there was no additional adsorption on the heated gouges after termination of the slip. We conclude that the adsorption state can be considered a new indicator of a temperature rise of &lt; 400°C by frictional heating associated with recent, near‐surface faulting.

Scale dependence of rock friction at high work rate.

Determination of the frictional properties of rocks is crucial for an understanding of earthquake mechanics, because most earthquakes are caused by frictional sliding along faults. Prior studies using rotary shear apparatus revealed a marked decrease in frictional strength, which can cause a large stress drop and strong shaking, with increasing slip rate and increasing work rate. (The mechanical work rate per unit area equals the product of the shear stress and the slip rate.) However, those important findings were obtained in experiments using rock specimens with dimensions of only several centimetres, which are much smaller than the dimensions of a natural fault (of the order of 1,000 metres). Here we use a large-scale biaxial friction apparatus with metre-sized rock specimens to investigate scale-dependent rock friction. The experiments show that rock friction in metre-sized rock specimens starts to decrease at a work rate that is one order of magnitude smaller than that in centimetre-sized rock specimens. Mechanical, visual and material observations suggest that slip-evolved stress heterogeneity on the fault accounts for the difference. On the basis of these observations, we propose that stress-concentrated areas exist in which frictional slip produces more wear materials (gouge) than in areas outside, resulting in further stress concentrations at these areas. Shear stress on the fault is primarily sustained by stress-concentrated areas that undergo a high work rate, so those areas should weaken rapidly and cause the macroscopic frictional strength to decrease abruptly. To verify this idea, we conducted numerical simulations assuming that local friction follows the frictional properties observed on centimetre-sized rock specimens. The simulations reproduced the macroscopic frictional properties observed on the metre-sized rock specimens. Given that localized stress concentrations commonly occur naturally, our results suggest that a natural fault may lose its strength faster than would be expected from the properties estimated from centimetre-sized rock samples.

Autonomous Greenhouse Sprayer Navigation Using Automatic Tracking Algorithm

Abstract. This article presents an accurate driving algorithm for autonomous greenhouse sprayer navigation. Due to the geological complexity of ground inside the greenhouse, the sprayer requires a stable driving algorithm under non-uniformal road conditions. We designed a body frame with an optimal size for the narrow aisle in a greenhouse. The driving algorithm was carried by an automatic turning algorithm under no-rail circumstance to maximize the utilization of a greenhouse space. Also, in order to segregate the turning algorithm and the straight path navigation, a photo sensor was additionally installed on the vehicle to detect the beginning of a turning area. The driving algorithm consisted of two kinds of autonomous procedures, which are the guidance rail tracking and the turning without any rail. The experimental results proved a driving function with an optimal turning radius, and a stable tracking performance on an unpaved clay road. It confirmed an increased slip rate of the vehicle’s wheel on the off-road situations, under the influence of a reduced rail width and an increased driving speed. Despite the severe driving conditions, the autonomous sprayer’s driving motion proved a stable tracking performance.