ALBERT

Description: Under extreme conditions like those encountered during earthquake slip, frictional melt is likely to occur. It has been observed on ancient faults that the melt is mostly extruded toward local extensional jogs or lateral tension cracks. In the case of laboratory experiments with a rotary shear apparatus, melt is extruded from the sample borders. When this happens, a thin and irregular melt layer is formed whereby the normal load is still in part supported by contact asperities under an incipient yield condition (as in dry friction models), but also, in the interstices between asperities, by the pressure of the viscous fluid wetting the interface. In addition, roughness of the surface is dynamically reshaped by the melting process of an inhomogeneous material (polymineralic rock). In particular, we argue that the roughness of the melting surface decreases with melting rate and temperature gradient perpendicular to the fault. Taking into account the above conditions, we obtain an expression for the average melt layer thickness and viscous pressure that may be used in estimates of friction in the presence of melt. We argue that the ratio of melt thickness to roughness depends on sliding velocity; such a ratio may be used as a gauge of slip-rate during fossil earthquakes on faults bearing pseudotachylite (solidified melt). Finally, we derive an improved analytical solution for friction in the presence of melt including the effect of roughness evolution.

Description: We acknowledge Takehiro Hirose for providing sample HVR 687. Stefan Nielsen was granted by the MIUR project FUMO, and Giulio Di Toro and Stefan Nielsen were granted by the European Research Council Starting Grant Project 205175 (USEMS) and CA.RI.PA.RO. We thank Leonardo Tauro for thin section preparation, Luca Peruzzo, Steven Smith and Piergiorgio Scarlato for SEM facilities and Toshi Shimamoto for his constant support. Finally, we are grateful to Nick Beeler and Chris Marone for their constructive reviews.

Description: Published

Description: 299–310

Description: 3.1. Fisica dei terremoti

Description: JCR Journal

Description: open

Description: We estimate the static stress drop on small exhumed strike-slip faults in the Lake Edison granodiorite of the central Sierra Nevada (California). The sub-vertical strike-slip faults were exhumed from 4-15 km depth, and were chosen because they are exposed in outcrop along their entire tip-to-tip lengths of 8-12 m. Slip nucleated on joints and accumulated by ductile shearing (forming quartz mylonites from early quartz vein filling in joints) and successive brittle faulting (forming epidote-bearing cataclasites). The occurrence of thin, 〈 1 mm wide, pseudotachylytes along some small faults throughout the study area suggests that some portion of the brittle slip was seismic. We suggest that the contribution of seismic slip to the total slip along the studied cataclasite-bearing small faults may be estimated by the length of epidote-filled, rhombohedral dilatational jogs (rhombochasms) distributed semi-periodically along the length of the faults. The interpretation that slip recorded by rhombochasms occurred in single events is based on evidence that: 1) epidote crystals are randomly oriented and undeformed within the rhombochasm; and 2) cataclasite structure in principal slip zones does not include clasts of previous cataclasite. We thereby constrain both the rupture length and slip. Based on these measurements, we calculate stress drops ranging over 90-250 MPa, i.e., one to two orders of magnitude larger than typical seismological estimates for earthquakes, but similar in magnitude to recent observations of small (〈 M2) earthquakes from the San Andreas Fault Observatory at Depth (SAFOD). These inferred seismic ruptures occurred along small, deep-seated faults, and, given the calculated stress drops and observations that brittle faults exploited joints sealed by quartz-bearing mylonite, we conclude that these were “strong” faults.

Description: In press

Description: 2.3. TTC - Laboratori di chimica e fisica delle rocce

Description: JCR Journal

Description: open

Description: Abstract—Fault surface roughness is a principal factor influencing earthquake mechanics, and particularly rupture initiation, propagation, and arrest. However, little data currently exist on fault surfaces at seismogenic depths. Here, we investigate the roughness of slip surfaces from the seismogenic strike-slip Gole Larghe Fault Zone, exhumed from ca. 10 km depth. The fault zone exploited pre-existing joints and is hosted in granitoid rocks of the Adamello batholith (Italian Alps). Individual seismogenic slip surfaces generally show a first phase of cataclasite production, and a second phase with beautifully preserved pseudotachylytes of variable thickness. We determined the geometry of fault traces over almost five orders of magnitude using terrestrial laser-scanning (LIDAR, ca. 500 to\1 m scale), and 3D mosaics of high-resolution rectified digital photographs (10 m to ca. 1 mm scale). LIDAR scans and photomosaics were georeferenced in 3D using a Differential Global Positioning System, allowing detailed multiscale reconstruction of fault traces in Gocad . The combination of LIDAR and high-resolution photos has the advantage, compared with classical LIDARonly surveys, that the spatial resolution of rectified photographs can be very high (up to 0.2 mm/pixel in this study), allowing for detailed outcrop characterization. Fourier power spectrum analysis of the fault traces revealed a self-affine behaviour over 3–5 orders of magnitude, with Hurst exponents ranging between 0.6 and 0.8. Parameters from Fourier analysis have been used to reconstruct synthetic 3D fault surfaces with an equivalent roughness by means of 2D Fourier synthesis. Roughness of pre-existing joints is in a typical range for this kind of structure. Roughness of faults at small scale (1 m to 1 mm) shows a clear genetic relationship with the roughness of precursor joints, and some anisotropy in the selfaffine Hurst exponent. Roughness of faults at scales larger than net slip ([1–10 m) is not anisotropic and less evolved than at smaller scales. These observations are consistent with an evolution of roughness, due to fault surface processes, that takes place only at scales smaller or comparable to the observed net slip. Differences in roughness evolution between shallow and deeper faults, the latter showing evidences of seismic activity, are interpreted as the result of different weakening versus induration processes, which also result in localization versus delocalization of deformation in the fault zone. From a methodological point of view, the technique used here is advantageous over direct measurements of exposed fault surfaces in that it preserves, in cross-section, all of the structures which contribute to fault roughness, and removes any subjectivity introduced by the need to distinguish roughness of original slip surfaces from roughness induced by secondary weathering processes. Moreover, offsets can be measured by means of suitable markers and fault rocks are preserved, hence their thickness, composition and structural features can be characterised, providing an integrated dataset which sheds new light on mechanisms of roughness evolution with slip and concomitant fault rock production.

Description: USEMS project, European Research Council

Description: Published

Description: 2345-2363

Description: 3.1. Fisica dei terremoti

Description: JCR Journal

Description: reserved

Description: Earthquakes are the result of a combination of (1) physico-chemical processes operating in fault zones. which allow ruptures to nucleate and rock friction to decrease with increasing slip or slip rate, and (2) of the geometrical complexity of fault zones. In this review paper, we summarize recent experimental findings from high velocity (conducted at about 1 m/s slip rate, or typical seismic slip rates) rock friction experiments with an emphasis on potential dynamic weakening mechanisms (melt lubrication, nano-powder lubrication, etc.) and how these mechanisms might be recognized by means of microstructural and mineralogical studies in exhumed fault zones. We discuss how earthquake source parameters (coseismic fault strength, weakening distances, energy budgets, etc.) might be derived from the field and laboratory experiments. Additionally, we discuss what needs to be considered in terms of fault zone geometry and morphology (focusing on fault surface roughness) in order to develop models of realistic fault surfaces and present theoretical considerations for microphysical modeling of laboratory data at seismic slip rates, with an emphasis on the case of melt lubrication. All experimental data and, in the case of melt lubrication, microphysical models indicate that faults must be very weak (mu 〈 0.1) during coseismic slip. Moreover, experiments have shown that the slip weakening distance during coseismic slip is on the order of a few tens of centimeters at most under natural conditions, consistent with inferences from field observations. Finally, we discuss open questions, future challenges and opportunities in the field of earthquake mechanics. (C) 2012 Elsevier Ltd. All rights reserved.

Description: Published

Description: 2-36

Description: 3.1. Fisica dei terremoti

Description: JCR Journal

Description: restricted

Description: The elastic displacement and stress fields due to rectangular faults and opening-mode fractures within an anisotropic homogeneous half-space are derived in this paper. The solution is expressed in terms of the mathematically elegant and computationally powerful Stroh formalism and can be applied to the generally anisotropic half-space or a transversely isotropic half-space with any oriented isotropic plane. For any flat fault or opening-mode fracture of polygonal shape, one needs only to carry out a simple line integral from 0 to in order to express the fault-induced response. Numerical examples are presented to demonstrate the effect of the anisotropy and fault orientation on the internal and surface responses of the half-space. Our results prove that both rock anisotropy and fault orientation could dramatically change the fields in the domain and one needs to consider these properties as accurately as possible to be able to predict the response in the domain precisely. Anisotropy of the rock mass may alter the dominant displacement and stress components at observation points in the model domain as compared to the isotropic case.

Description: Based upon the fundamental solution to a single straight dislocation segment, a complete set of exact closed-form solutions is presented in a unified manner for elastic displacements and strains due to general polygonal dislocations in a transversely isotropic half-space. These solutions are systematically composed of two parts: one representing the solution in an infinite transversely isotropic medium and the other accounting for the influence of the free surface of the half-space. Numerical examples are provided to illustrate the effect of material anisotropy on the elastic displacement and strain fields associated with dislocations. It is shown that if the rock mass is strongly anisotropic, surface displacements calculated using an isotropic model may result in errors greater than 20%, and some of the strain components near the fault tip may vary by over 200% compared with the transversely isotropic model. Even for rocks with weak anisotropy, the strains based on the isotropic model can also result in significant errors. Our analytical solutions along with the corresponding MATLAB source codes can be used to predict the static displacement and strain fields due to earthquakes, particularly when the rock mass in the half-space is best approximated as transversely isotropic, as is the case for most sedimentary basins. Online Material: MATLAB scripts to calculate rectangular and triangular dislocations in a transversely isotropic half-space.

Description: Sandstone injectites ranging from 〈30 m to 〉1 km in outcrop length intrude the Cretaceous Mowry Formation in the vicinity of Sheep Mountain anticline (Bighorn Basin, Wyoming, USA). These injectites were sourced from the Peay Sandstone Member of the overlying Cretaceous Frontier Formation and represent a significant possible fluid pathway through impermeable shales. Sand injection occurred along dikes and sills, interacting with bedding discontinuities and preexisting joints in the Mowry Formation during the early folding of Sheep Mountain anticline. We argue that, in contrast to the passive sweeping of sediments into fissures characteristic of Neptunian dike formation, downward intrusion of the Peay sand was forceful and made possible by a highly stratified horizontal stress field resulting from the deposition, burial, and lithification history of the rock units in the area. The internal structure of the injectites is dominated by two sets of mutually offsetting deformation bands. The deformation bands have shear and compaction components, exhibiting significant porosity loss, as well as cataclasis and minor pressure solution. After formation of the deformation bands, subsequent faulting was localized along the margins of deformation bands, evidenced in the field by slickensided surfaces. A detailed kinematic analysis of slickenline lineations yields shortening and extension axes consistent with deformation band formation during early Laramide–oriented shortening, and continuing through the folding of Sheep Mountain anticline. Beyond the formation and deformation of these sandstone injectites, this study highlights the importance of mechanical stratigraphy in the containment of hydraulic fractures.