Frictional ageing from interfacial bonding and the origins of rate and state friction

Earthquakes have long been recognized as being the result of stick-slip frictional instabilities. Over the past few decades, laboratory studies of rock friction have elucidated many aspects of tectonic fault zone processes and earthquake phenomena. Typically, the static friction of rocks grows logarithmically with time when they are held in stationary contact, but the mechanism responsible for this strengthening is not understood. This time-dependent increase of frictional strength, or frictional ageing, is one manifestation of the 'evolution effect' in rate and state friction theory. A prevailing view is that the time dependence of rock friction results from increases in contact area caused by creep of contacting asperities. Here we present the results of atomic force microscopy experiments that instead show that frictional ageing arises from the formation of interfacial chemical bonds, and the large magnitude of ageing at the nanometre scale is quantitatively consistent with what is required to explain observations in macroscopic rock friction experiments. The relative magnitude of the evolution effect compared with that of the 'direct effect'--the dependence of friction on instantaneous changes in slip velocity--determine whether unstable slip, leading to earthquakes, is possible. Understanding the mechanism underlying the evolution effect would enable us to formulate physically based frictional constitutive laws, rather than the current empirically based 'laws', allowing more confident extrapolation to natural faults.     

Friction falls towards zero in quartz rock as slip velocity approaches seismic rates

An important unsolved problem in earthquake mechanics is to determine the resistance to slip on faults in the Earth's crust during earthquakes. Knowledge of coseismic slip resistance is critical for understanding the magnitude of shear-stress reduction and hence the near-fault acceleration that can occur during earthquakes, which affects the amount of damage that earthquakes are capable of causing. In particular, a long-unresolved problem is the apparently low strength of major faults, which may be caused by low coseismic frictional resistance. The frictional properties of rocks at slip velocities up to 3 mm s(-1) and for slip displacements characteristic of large earthquakes have been recently simulated under laboratory conditions. Here we report data on quartz rocks that indicate an extraordinary progressive decrease in frictional resistance with increasing slip velocity above 1 mm s(-1). This reduction extrapolates to zero friction at seismic slip rates of approximately 1 m s(-1), and appears to be due to the formation of a thin layer of silica gel on the fault surface: it may explain the low strength of major faults during earthquakes.     

The implications of the temperature-independent binding and the temperature-dependent action of interferon

Résumé Des expériences utilisant la températur et la trypsine sont décrites; elles démontrent que la phase initiale de l'action de l'interferon comporte deux étapes. La première consiste dans la fixation de l'interferon. Elle a lieu a 0 °C et n'est pas sous la dépendance du métabolisme cellulaire. La deuxième étape, au cours de laquelle l'interferon lié amorce le développement de la résistance antivirale, dépend du métabolisme cellulaire et est terminée au bout de 45 min. Deux modèles de l'étape sensible à la température sont présentés et discutés.

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This research has been supported by grants from the Connecticut Research Commission and the American Cancer Society.