Width Of Damage Zone Research Articles

Low resistivity and permeability in actively deforming shear zones on the San Andreas Fault at SAFOD

AbstractThe San Andreas Fault Observatory at Depth (SAFOD) scientific drill hole near Parkfield, California, crosses the San Andreas Fault at a depth of 2.7 km. Downhole measurements and analysis of core retrieved from Phase 3 drilling reveal two narrow, actively deforming zones of smectite‐clay gouge within a roughly 200 m wide fault damage zone of sandstones, siltstones, and mudstones. Here we report electrical resistivity and permeability measurements on core samples from all of these structural units at effective confining pressures up to 120 MPa. Electrical resistivity (~10 Ω‐m) and permeability (10−21 to 10−22 m2) in the actively deforming zones were 1 to 2 orders of magnitude lower than the surrounding damage zone material, consistent with broader‐scale observations from the downhole resistivity and seismic velocity logs. The higher porosity of the clay gouge, 2 to 8 times greater than that in the damage zone rocks, along with surface conduction were the principal factors contributing to the observed low resistivities. The high percentage of fine‐grained clay in the deforming zones also greatly reduced permeability to values low enough to create a barrier to fluid flow across the fault. Together, resistivity and permeability data can be used to assess the hydrogeologic characteristics of the fault, key to understanding fault structure and strength. The low resistivities and strength measurements of the SAFOD core are consistent with observations of low resistivity clays that are often found in the principal slip zones of other active faults making resistivity logs a valuable tool for identifying these zones.

Evolution and progressive geomorphic manifestation of surface faulting: A comparison of the Wairau and Awatere faults, South Island, New Zealand

Field mapping and lidar analysis of surface faulting patterns expressed in flights of geologically similar fluvial terraces at the well-known Branch River and Saxton River sites along the Wairau (Alpine) and Awatere strike-slip faults, South Island, New Zealand, reveal that fault-related deformation patterns expressed in the topography at these sites are markedly less structurally complex along the higher-displacement (hundreds of kilometers), structurally mature Wairau fault than along the Awatere fault (∼13–20 km total slip). These differences, which are generally representative of the surface traces of these faults, provide direct evidence that surface faulting becomes structurally simpler with increasing cumulative fault offset. We also examine the degree to which off-fault deformation (OFD) is expressed in the landscape at the Saxton River site along the less structurally mature Awatere fault. Significantly greater amounts of OFD are discernible as a wide damage zone (∼460 m fault-perpendicular width) in older (ca. 15 ka), more-displaced (64–74 m) fluvial terraces than in younger (ca. 1–7 ka), less-displaced (<55 m) terraces; no OFD is discernible in the lidar data on the least-displaced (<35 m) terraces. From this, we infer that OFD becomes progressively more geomorphically apparent with accumulating displacement. These observations imply that (1) the processes that accommodate OFD are active during each earthquake, but may not be evident in deposits that have experienced relatively small displacements; (2) structures accommodating OFD will become progressively geomorphically clearer with increasing displacement; (3) geomorphic measurements of overall fault zone width taken in deposits that have experienced small displacements will be underestimates; and (4) fault slip rates based on geomorphic surface offsets will be underestimates for immature faults if based solely on measurements along the high-strain fault core.

Multidimensional Study on Spall Behavior of High-Purity Copper Under Sliding Detonation

The spall behaviors of high-purity copper samples with different heat treatment histories were investigated using optical microscopy and X-ray computer tomography (XRCT). The spall samples were obtained by sliding detonation experiments at low pressures (2 to 4 GPa). It was found that the spall planes created by sliding detonation in this experiment are similar to the spall planes created by plate impact test, except for more secondary damage residual around the main spall plane. The results of damage degree, the shape, and the distributions of voids obtained by the means of metallography (2D) and XRCT (3D) statistics were consistent. For similar microstructure, the maximum damage degree and damage zone width increase with increasing shock stress. Whereas the ranges of voids distribution parallel to the shock stress direction decreases with the increasing of shock stress. For the shock stress is similar, the shape of voids in annealed samples are closed to spheres, their mean flatness is 0.51. The voids in samples with thermo-mechanical treatment histories are sheet like with mean flatness 0.16. The difference in grain size (40 and 9 μm) may be the main reason of such difference.

Rapid imbibition of water in fractures within unsaturated sedimentary rock

The spontaneous imbibition of water and other liquids into gas-filled fractures in variably-saturated porous media is important in a variety of engineering and geological contexts. However, surprisingly few studies have investigated this phenomenon. We present a theoretical framework for predicting the 1-dimensional movement of water into air-filled fractures within a porous medium based on early-time capillary dynamics and spreading over the rough surfaces of fracture faces. The theory permits estimation of sorptivity values for the matrix and fracture zone, as well as a dispersion parameter which quantifies the extent of spreading of the wetting front. Quantitative data on spontaneous imbibition of water in unsaturated Berea sandstone cores were acquired to evaluate the proposed model. The cores with different permeability classes ranging from 50 to 500mD and were fractured using the Brazilian method. Spontaneous imbibition in the fractured cores was measured by dynamic neutron radiography at the Neutron Imaging Prototype Facility (beam line CG-1D, HFIR), Oak Ridge National Laboratory. Water uptake into both the matrix and the fracture zone exhibited square-root-of-time behavior. The matrix sorptivities ranged from 2.9 to 4.6mms−0.5, and increased linearly as the permeability class increased. The sorptivities of the fracture zones ranged from 17.9 to 27.1mms−0.5, and increased linearly with increasing fracture aperture width. The dispersion coefficients ranged from 23.7 to 66.7mm2s−1 and increased linearly with increasing fracture aperture width and damage zone width. Both theory and observations indicate that fractures can significantly increase spontaneous imbibition in unsaturated sedimentary rock by capillary action and surface spreading on rough fracture faces. Fractures also increase the dispersion of the wetting front. Further research is needed to investigate this phenomenon in other natural and engineered porous media.

A scaling law to characterize fault-damage zones at reservoir depths

We analyze fracture-density variations in subsurface fault-damage zones in two distinct geologic environments, adjacent to faults in the granitic SSC reservoir and adjacent to faults in arkosic sandstones near the San Andreas fault in central California. These damage zones are similar in terms of width, peak fracture or fault (FF) density, and the rate of FF density decay with distance from the main fault. Seismic images from the SSC reservoir exhibit a large basement master fault associated with 27 seismically resolvable second-order faults. A maximum of 5 to 6 FF/m (1.5 to 1.8 FF/ft) are observed in the 50 to 80 m (164 to 262 ft) wide damage zones associated with second-order faults that are identified in image logs from four wells. Damage zones associated with second-order faults immediately southwest of the San Andreas Fault are also interpreted using image logs from the San Andreas Fault Observatory at Depth (SAFOD) borehole. These damage zones are also 50–80 m wide (164 to 262 ft) with peak FF density of 2.5 to 6 FF/m (0.8 to 1.8 FF/ft). The FF density in damage zones observed in both the study areas is found to decay with distance according to a power law . The fault constant is the FF density at unit distance from the fault, which is about 10–30 FF/m (3.1–9.1 FF/ft) in the SSC reservoir and 6–17 FF/m (1.8–5.2 FF/ft) in the arkose. The decay rate ranges from 0.68 to 1.06 in the SSC reservoir, and from 0.4 to 0.75 in the arkosic section. This quantification of damage-zone attributes can facilitate the incorporation of the geometry and properties of damage zones in reservoir flow simulation models.

An innovative approach to thin coal seam mining of complex geological conditions by pressure regulation

The hard rock parting contained in thin coal seams constrains the development of high-efficiency mining due to high level of equipment wear and low mining advance rate. To achieve safe and high-efficiency mining in thin coal seam with such complex geological conditions, an innovative mining approach using pressure regulation was proposed to substitute traditional deep-hole blasting technique to pre-break the coal and hence promote the flaking of hard parting during mining. Firstly, the strata pressure behaviors in the thin coal seam containing hard iron sulphide concretion parting were investigated by field monitoring and modeling study. Then, the feasibility of abutment pressure regulation via supporting force adjustment to pre-break the coal was proved by numerical experiments. The results show that the existence of hard iron sulphide concretions resulted in localized stress concentration by 30% and the profile of damage zone transforms to be radial and discontinuous. However, it does not enlarge the size of effective damage zone much. The results also reveal that the width of damage zone increases linearly with the decrease of supporting force at the logarithmic scale. This provides a theoretical guidance for the fully-mechanised thin coal seam mining face with hard rock parting. The implementation of pressure regulation in actual mining demonstrates that it can effectively enhance the mining advance rate and significantly reduce mining machine wear.

Pulsed versus continuous wave CO2 laser excisions of 100 oral fibrous hyperplasias: A randomized controlled clinical and histopathological study

BackgroundIn experimental animal studies, pulsing the CO2 laser beam has been shown to reduce the thermal damage zone of excised oral mucosal tissue. However, there is still controversy over whether this is borne out under clinical conditions.ObjectiveTo compare the outcome following excisional biopsies of fibrous hyperplasias using a pulsed (cf) versus a continuous wave (cw) CO2 laser mode regarding the thermal damage zone, duration of surgeries, intra‐ and postoperative complications, postoperative pain sensation, scarring and/or relapse during the initial 6 months.Materials and MethodsOne hundred Swiss‐resident patients with a fibrous hyperplasia in their buccal mucosa were randomly assigned to the cw mode (5 W) or the cf mode (140 Hz, 400 microseconds, 33 mJ, 4.62 W) group. All excisions were performed by one single oral surgeon. Postoperative pain (2 weeks) was recorded by visual analogue scale (VAS; ranging from 0 to 100). Intake of analgesics and postoperative complications were recorded in a standardized study form. The maximum width of the collateral thermal damage zone was measured (µm) in excision specimens by one pathologist. Intraoral photographs at 6‐month follow‐up examinations were evaluated regarding scarring (yes/no).ResultsMedian duration of the excision was 65 seconds in the cw and 81 seconds in the cf group (P = 0.13). Intraoperative bleeding occurred in 16.3% of the patients in the cw and 17.7% of the cf group. The median value of the thermal damage zone was 161(±228) μm in the cw and 152(± 105) μm in the cf group (P = 0.68). The reported postoperative complications included swelling in 19% and minor bleeding in 6% without significant differences between the two laser modes. When comparing each day separately or the combined mean VAS scores of both groups between Days 1–3, 1–7, and 1–15, there were no significant differences. However, more patients of the cw group (25%) took analgesics than patients of the cf group (9.8%) resulting in a borderline significance (P = 0.04). Scarring at the excision site was found in 50.6% of 77 patients after 6 months, and more scars were identified in cases treated with the cf mode (P = 0.03).ConclusionsExcision of fibrous hyperplasias performed with a CO2 laser demonstrated a good clinical outcome and long‐term predictability with a low risk of recurrence regardless of the laser mode (cf or cw) used. Scarring after 6 months was only seen in 50.6% of the cases and was slightly more frequent in the cf mode group. Based on the findings of the present study, a safety border of 1 mm appears sufficient for both laser modes especially when performing a biopsy of a suspicious soft tissue lesion to ensure a proper histopathological examination. Lasers Surg. Med. 46:396–404, 2014. © 2014 Wiley Periodicals, Inc.

Identification of reactivation and increased permeability associated with a fault damage zone using a multidisciplinary approach

We evaluate the fault damage zone associated with a reactivated long-strike length, small-offset normal fault in the Permian Cedar Mesa Sandstone, southeastern Utah. This fault is characterized by a single slip surface and a 9-m wide damage zone containing deformation bands and veins. Field observations include cross-cutting relationships, permeability increase, rock strength decrease, and ultraviolet-light-induced mineral fluorescence within the damage zone. These field observations, combined with the interpreted structural diagenetic sequence from petrographic analysis, suggest a deformation history of reactivation and several generations of mineralization. All deformation bands and calcite veins fluoresce under ultraviolet light, indicating fluid pathway connectivity and a shared mineralization history. Pre-existing structures act as loci for younger deformation and mineralization events, so this fault and its damage zone illustrate the importance of the fault damage zone to subsurface fluid flow.We model a simplified stress history to understand the importance rock properties and variations in differential and effective mean stress have on the structures within the damage zone. The moderate confining pressures, possible variations in pore pressure, and porous, fine-grained nature of the Cedar Mesa Sandstone produces a fault damage zone characterized by enhanced permeability and mineralization.

A Comparison of Seismicity Characteristics and Fault Structure Between Stick–Slip Experiments and Nature

Fault zones contain structural complexity on all scales. This complexity influences fault mechanics including the dynamics of large earthquakes as well as the spatial and temporal distribution of small seismic events. Incomplete earthquake records, unknown stresses, and unresolved fault structures within the crust complicate a quantitative assessment of the parameters that control factors affecting seismicity. To better understand the relationship between fault structure and seismicity, we examined dynamic faulting under controlled conditions in the laboratory by creating saw-cut-guided natural fractures in cylindrical granite samples. The resulting rough surfaces were triaxially loaded to produce a sequence of stick–slip events. During these experiments, we monitored stress, strain, and seismic activity. After the experiments, fault structures were imaged in thin sections and using computer tomography. The laboratory fault zones showed many structural characteristics observed in upper crustal faults, including zones of localized slip embedded in a layer of fault gouge. Laboratory faults also exhibited a several millimeter wide damage zone with decreasing micro-crack density at larger distances from the fault axis. In addition to the structural similarities, we also observed many similarities between our observed distribution of acoustic emissions (AEs) and natural seismicity. The AEs followed the Gutenberg–Richter and Omori–Utsu relationships commonly used to describe natural seismicity. Moreover, we observed a connection between along-strike fault heterogeneity and variations of the Gutenberg–Richter b value. As suggested by natural seismicity studies, areas of low b value marked the nucleation points of large slip events and were located at large asperities within the fault zone that were revealed by post-experimental tomography scans. Our results emphasize the importance of stick–slip experiments for the study of fault mechanics. The direct correlation of acoustic activity with fault zone structure is a unique characteristic of our laboratory studies that has been impossible to observe in nature.

Spatial variations in damage zone width along strike-slip faults: An example from active faults in southwest Japan

Field investigations reveal spatial variations in fault zone width along strike-slip active faults of the Arima–Takatsuki Tectonic Line (ATTL) and the Rokko–Awaji Fault Zone (RAFZ) of southwest Japan, which together form a left-stepping geometric pattern. The fault zones are composed of damage zones dominated by fractured host rocks, non-foliated and foliated cataclasites, and a fault core zone that consists of cataclastic rocks including fault gouge and fault breccia. The fault damage zones of the ATTL are characterized by subsidiary faults and fractures that are asymmetrically developed on each side of the main fault. The width of the damage zone varies along faults developed within granitic rocks of the ATTL and RAFZ, from ∼50 to ∼1000 m. In contrast, the width of the damage zone within rhyolitic tuff on the northwestern side of the ATTL varies from ∼30 to ∼100 m. The fault core zone is generally concentrated in a narrow zone of ∼0.5–∼5 m in width, consisting mainly of pulverized cataclastic rocks that lack the primary cohesion of the host rocks, including a narrow zone of fault gouge (<0.5 m) and fault-breccia zones either side of the fault. The present results indicate that spatial variations in the width of damage zone and the asymmetric distribution of damage zones across the studied strike-slip faults are mainly caused by local concentrations in compressive stress within an overstep area between left-stepping strike-slip faults of the ATTL and RAFZ. The findings demonstrate that fault zone structures and the spatial distribution in the width of damage zone are strongly affected by the geometric patterns of strike-slip faults.

Significance of brittle deformation in the footwall of the Alpine Fault, New Zealand: Smithy Creek Fault zone

The Smithy Creek Fault represents a rare exposure of a brittle fault zone within Australian Plate rocks that constitute the footwall of the Alpine Fault zone in Westland, New Zealand. Outcrop mapping and paleostress analysis of the Smithy Creek Fault were conducted to characterize deformation and mineralization in the footwall of the nearby Alpine Fault, and the timing of these processes relative to the modern tectonic regime. While unfavorably oriented, the dextral oblique Smithy Creek thrust has kinematics compatible with slip in the current stress regime and offsets a basement unconformity beneath Holocene glaciofluvial sediments. A greater than 100 m wide damage zone and more than 8 m wide, extensively fractured fault core are consistent with total displacement on the kilometer scale. Based on our observations we propose that an asymmetric damage zone containing quartz–carbonate–chlorite–epidote veins is focused in the footwall. Damage zone asymmetry likely resulted from the fact that the hanging wall was mostly deformed at greater depth than the footwall, rather than resulting from material contrasts across the fault plane. Kinematic inversions on mineralized fractures within the damage zone suggest veins formed in the current stress regime, from fluids comparable to those now circulating in the footwall. The Smithy Creek Fault zone is therefore a rare exhumed example of the modern footwall hydrothermal system, and of a structure actively accommodating footwall deformation near the Alpine Fault zone. Two significantly less mature, subvertical faults having narrow (20 cm or less) damage zones and similar orientations to nearby strike-slip segments of the Alpine Fault crosscut the mineralized zone at Smithy Creek. We envisage that hydrothermal mineralization strengthened the fault core, causing it to widen as later slip was partitioned into the (now) weaker surrounding damage zone. With progressive alteration, formation of favorably oriented faults became preferable to reactivation of the existing structure.

Damage and seismic velocity structure of pulverized rocks near the San Andreas Fault

AbstractA combination of seismic refraction tomography, laboratory ultrasonic velocity measurements, and microstructural observations was used to study the shallow velocity structure of a strand of the San Andreas fault (SAF) just south of Littlerock, California. The examined site has a strongly asymmetric damage structure with respect to the SAF core. The conglomerates to the southwest show little to no damage, whereas a ~100 m wide damage zone exists to the northeast with a ~50 m wide zone of pulverized granite adjacent to the fault core. Seismic P‐wave velocities of the damaged and pulverized granite were investigated over a range of scales. In situ seismic velocity imaging was performed on three overlapping profiles normal to the SAF with lengths of 350 m, 50 m, and 25 m. In the laboratory, ultrasonic velocities were measured on centimeter‐ to decimeter‐sized samples taken along the in situ profiles. The samples were also investigated microstructurally. Micro‐scale fracture damage intensifies with increasing proximity to the fault core, allowing a subdivision of the damage zone into several sections. Laboratory‐derived velocities in each section display varying degrees of anisotropy, and combined with microfracture analysis suggest an evolving damage fabric. Pulverized rocks close to the fault exhibit a preferred fault‐parallel orientation of microfractures, resulting in the lowest P‐wave velocity orientated in fault‐perpendicular direction. Closest to the fault, pulverized rocks exhibit a gouge‐like fabric that is transitional to the fault core. Comparison of absolute velocities shows a scaling effect from field to laboratory for the intact rocks. A similar scaling effect is absent for the pulverized rocks, suggesting that they are dominated by micro‐scale damage. Fault‐parallel damage fabrics are consistent with existing models for pulverized‐rock generation that predict strong dynamic reductions in fault‐normal stress. Our observations provide important constraints for theoretical models and imaging fault damage properties at depth using remote methods.

Sensitivity of fluid flow to deformation-band damage zone heterogeneities: A study using fault facies and truncated Gaussian simulation

We demonstrate that truncated Gaussian simulation (TGS), which is typically used for modeling of sedimentary rocks, can be employed to reproduce detailed damage zone structure as observed in outcrops. The basic modeled units employed are fault facies classified according to deformation density. Published damage zone field maps are re-drawn as fault facies maps and used for deriving geostatistical descriptions of model input parameters. We apply the modeling method for damage zones related to three scenarios: an isolated fault, branching faults and double-tip interacting faults. Constrained by the resulting TGS models, a series of damage zone permeability models are generated by systematically modulating five modeling factors related to different heterogeneity scales. Single-phase flow simulations reveal that fault facies proportion and damage zone width are the most influential factors, followed by deformation band frequency. Deformation band permeability and fault facies extent are the least important factors. Modifying fault facies proportion and damage zone width mainly change the flow retardation/enhancement in the models, whereas modifying deformation band frequency, deformation band permeability and fault facies extent mainly change the flow tortuosity in the models. Finally, we examine hierarchical modeling and upscaling procedures to incorporate our fine-scale models into flow simulation models.

Spatial distribution of deformation bands in damage zones of extensional faults in porous sandstones: Statistical analysis of field data

The distribution of deformation bands in damage zones of extensional faults in porous sandstones has been analyzed using 106 outcrop scanlines along which the position and frequency of deformation bands have been recorded. The analysis reveals a non-linear relationship between damage zone width and fault throw, a logarithmic decrease in deformation band frequency away from the fault core, as well as a fractal spatial distribution associated with clustering of the deformation bands. Furthermore, damage zones appear wider in the hanging wall than in the footwall, although the deformation band density is similar on both sides. Statistical trends derived from the database imply that fault growth in porous sandstones can be considered as a scale invariant process. From an initial process zone, the damage zone grows by a constant balance between the development of new deformation bands in the existing damage zone and the creation of new bands outside. Moreover, as the width of the damage zone increases throughout the active lifetime of a fault, the distribution of the deformation bands in the damage zone remains self-similar. Hence band distribution and damage zone width for seismically mapped faults can be predicted from the relationships found in this paper.

Pathomorphological distinction between Er:YAG and diode lasers on the excisional biopsy of the oral mucosa

Background - In oral pathology, laser devices can provide important advantages, especially in the treatment of certain lesions. However, there is controversy about the use of some wavelengths in the analysis of suspected dysplastic or neoplastic lesions, raising doubt about the laser's suitability for use in biopsy procedures. The present pilot study evaluates the pathomorphological characteristics and suitability of Er:YAG and diode lasers for performing excisional biopsies in the oral mucosa with special emphasis on the extent of the thermal damage zone created. Material and Methods - 10 patients were randomly assigned to one diode or one Er:YAG laser groups. The Er:YAG laser(2940nm) was used in a pulsed wave mode 200mJ/35Hz with a power of 7 W. Power settings for the diode laser(810 nm) were 3 W in a pulsed mode. The thermal damage zone of the two lasers and intraoperative and postoperative complications were assessed and compared.All biopsy specimens were evaluated by optical microscopy by threeblinded pathologists. After establishing the pathomorphological diagnosis of the lesion assessed, the pathologists measured the maximal width of the peripheral thermal damage zone in the specimen in μm and classified it using appropriate index. Results - The peripheral thermal damage zone on the borders of the excisional biopsies was significantly smaller with the Er:YAG laser compared to the diode laser regarding values in μm or pathomorphological index scores. No postoperative complications occurred in any of the two groups. Conclusions - The Er:YAG laser seems to be appropriate for excisional biopsies of benign oral mucosal lesions. The Er:YAG laser offers clear advantages in terms of smaller thermal damage zones over the diode laser. Although in some samples thermal damage was minimally visible, in all samples histological evaluation was clearly possible. The study demonstrated that the Er:YAG laser can be safely used in oral biopsy investigations while ensuring a successful histological evaluation, which is fundamental to correct clinical management.

Influence of brittle shear damage on accuracy of the two-step method in prediction of structural response to blast loads

A two-step numerical approach, which substantially reduces the modelling and computational effort in analysing structural responses to blast loads, was recently proposed. The method solves the responses of the equivalent SDOF system of a structural component during the blast loading phase to obtain the structural displacement and velocity at the end of the blast loading duration. Using these displacements and velocities as initial conditions, a detailed FE model is developed to solve the free-vibration response of the structure. It has been demonstrated that this approach yields very good predictions of structural displacement and longitudinal reinforcement stress at the mid span of RC beams. The accuracy in predicting the stresses in hoop reinforcements near the supports however varies from case to case. One possible reason for this inconsistency in predicting the stresses in hoop reinforcements near the supports is because of the brittle shear damage occurring near the structure supports during the loading phase, which is not considered in the second step free-vibration analysis in the proposed two-step method. To further improve the accuracy of the two-step method, in this paper, the influence of possible brittle shear damage during the blast loading phase is estimated, which will be included in the second step free-vibration analysis to improve the prediction accuracy of the two-step method. Pressure–Impulse diagrams for generic RC beams are generated for straightforward evaluation of the initial damage and the width of the initial damage zone at the end of the blast loading phase for inclusion in the second-step analysis. It is demonstrated that including damage caused by blast loads in the loading phase in the second step free-vibration analysis improves the prediction accuracy of the beam responses to blast loadings.

Seismic velocity constraints on the material properties that control earthquake behavior at the Quebrada‐Discovery‐Gofar transform faults, East Pacific Rise

Mid‐ocean ridge transform faults (RTFs) vary strongly along strike in their ability to generate large earthquakes. This general observation suggests that local variations in material properties along RTFs exert a primary control on earthquake rupture dynamics. We explore these relationships by examining the seismic structure of two RTFs that have distinctly different seismic coupling. We determine the seismic velocity structure at the Gofar and Quebrada faults on the East Pacific Rise (EPR) using P wave traveltime tomography with data from two active‐source wide‐angle refraction lines crossing the faults. We image low‐velocity zones (LVZs) at both faults, where P wave velocities are reduced by as much as 0.5–1.0 km/s (∼10–20%) within a several kilometer wide region. At the Gofar fault, the LVZ extends through the entire crust, into the seismogenic zone. We rule out widespread serpentinization as an explanation for the low velocities, owing to the lack of a corresponding signal in the locally measured gravity field. The reduced velocities can be explained if the plate boundary region is composed of fault material with enhanced fluid‐filled porosity (1.5–8%). Local seismic observations indicate that the high‐porosity region lies within a ∼10 km long portion of the fault that fails in large swarms of microearthquakes and acts as a barrier to the propagation of large (M ∼ 6.0) earthquakes. Tomographic images of fault structure combined with observed earthquake behavior suggest that EPR transform segments capable of generating large earthquakes have relatively intact gabbro within the seismogenic zone, whereas segments that slip aseismically or via earthquake swarms are composed of highly fractured, ≥2 km wide damage zones that extend throughout the crust.

Fault zone structure and seismic slip localization in dolostones, an example from the Southern Alps, Italy

Fault zones cutting limestones and dolostones represent significant seismogenic sources worldwide. The structure of an exhumed strike-slip fault zone hosted in dolostones, the Borcola Pass Fault Zone (BPFZ, Italian Southern Alps), was studied by means of field and microstructural analysis. Ambient conditions of faulting were ca. 1.6–1.7 km and 50 °C. The BPFZ consists of a >80 m wide damage zone cut by three systems of sub-vertical secondary faults striking approximately N–S, E–W and NW–SE. N–S and E–W striking faults reactivated pre-existing Jurassic-Paleogene joints with spacing between 0.2 and 0.5 m, whereas NW–SE striking faults were newly formed during post-Paleogene activity associated with movements along the nearby Schio–Vicenza Line. The core of the BPFZ consists of dolostone fault rock lenses bound by slip zones up to 10 cm thick. Both the principal and secondary slip zones consist of cement-supported dolomitic cataclasites and dolomite-filled veins. Some slip zones contain a sub-centimeter thick “vein-like” cataclastic layer (Layer-A) located immediately beneath the slip surface that truncates another cataclasite below (Layer-B). Detailed microstructural and clast size distribution analysis suggests that Layer-A experienced fluidization (cuspate-lobate boundaries, injection structures, strong grain sorting: D < 1 for clast diameters smaller than 300 μm) possibly related to fast fault slip following seismic ruptures. In light of these observations a conceptual model is proposed for the formation of Layer-A, and the structure of the BPFZ is compared to that of an active seismogenic fault cutting carbonates.

Statistical analysis of the relationships between faults attributes

The statistical analysis of fault attributes scaling relationships is discussed. Dependences of length, width of damage zone and thickness of fault core on displacement were studied assuming power law relations. The approximation forms a piecewise‐linear function with few slopes in log‐log scale. The Bayesian Information Criterion (BIC) was used to find the best fit for an optimal number of parameters. Numerical tests show that the best fit was obtained when using power law relations with two slopes. Bayesian analysis of model parameters' probability distribution was performed. For length‐displacement relation (L‐D), the slope decreases from one scale of faults to another. This change occurs at ∼1 m displacement for reverse and normal faults in siliciclastic rocks, at ∼1500 m displacement for strike slip and at ∼300 m displacement for normal faults in non‐siliciclastic rocks. The slope of the damage zone width‐displacement (W‐D) relation decreases at ∼10 m, while it slightly increases for fault core thickness‐displacement (T‐D) relation at ∼10 cm. The result of the probability density of changepoints confirms the calculated changepoints, which correspond to maximal BIC in most cases. We propose an evolutionary growth pattern of faults based on the statistical results, in which faults lengthen during the initial stage. During subsequent overlapping and linkage between the faults, mainly displacement accumulates. Fault damage zone and fault core form early in the process of faulting. In mature faults, the development of damage zone would be slower than for small faults, whereas fault core slightly thickens with further localization.

Does a Pulsed Mode Offer Advantages Over a Continuous Wave Mode for Excisional Biopsies Performed Using a Carbon Dioxide Laser?

Animal studies of excisional biopsies have shown less thermal damage when a carbon dioxide (CO(2)) laser (10.6 μm) is used in a char-free (CF) mode than in a continuous-wave (CW) mode. The authors' aim was to evaluate and compare clinical and histopathologic findings of excisional biopsies performed with CW and CF CO(2) laser (10.6 μm) modes. This prospective randomized controlled clinical trial included 60 patients with similar fibrous hyperplasias of the buccal plane willing to undergo excisional CO(2) laser biopsy. Patients were randomly allocated to the CW (5 W) or CF (140 Hz, 400 μs, 33 mJ) group. Duration of surgery, intra- and postoperative complications, and the width (micrometers) of the histopathologic collateral thermal damage zone were registered as primary outcome variables. Secondary outcome variables were pain (patients filled in a visual analog scale [VAS]) and analgesic intake (recorded by patients). The study group consisted of 36 women and 24 men with a median age of 50.5 years. Median durations of surgery were 74.5 seconds in the CW group and 83.5 seconds in the CF group. Intraoperative venous bleeding occurred in 16.7% of patients in the CW group and in 13.3% of patients in the CF group. Median areas of histopathologic collateral damage zones were similar in the CW group (166.5 μm) and the CF group (162.5 μm). There was no statistically significant difference between the VAS values of the 2 groups. Analgesic intake was recorded by 16.7% of patients in the CW group and by 6.7% of patients in the CF group (P = .23, not significant). No statistically significant correlation was found between areas of thermal damage zones and postoperative VAS scores. In contrast to previous animal studies, no significant difference was found in the widths of thermal damage zones between the CW and CF groups. The VAS values and analgesic intake were low in the 2 groups. The 2 CO(2) laser modes are appropriate for the excision of intraoral mucosal lesions. A safety border of at least 1 mm is recommended regardless of the laser mode used.