The determination of rock friction at seismic slip rates (about 1 m s(-1)) is of paramount importance in earthquake mechanics, as fault friction controls the stress drop, the mechanical work and the frictional heat generated during slip(1). Given the difficulty in determining friction by seismological methods(1), elucidating constraints are derived from experimental studies(2-9). Here we review a large set of published and unpublished experiments (similar to 300) performed in rotary shear apparatus at slip rates of 0.1-2.6 ms(-1). The experiments indicate a significant decrease in friction (of up to one order of magnitude), which we term fault lubrication, both for cohesive (silicate-built(4-6), quartz-built(3) and carbonate-built(7,8)) rocks and non-cohesive rocks (clay-rich(9), anhydrite, gypsum and dolomite(10) gouges) typical of crustal seismogenic sources. The available mechanical work and the associated temperature rise in the slipping zone trigger(11,12) a number of physicochemical processes (gelification, decarbonation and dehydration reactions, melting and so on) whose products are responsible for fault lubrication. The similarity between (1) experimental and natural fault products and (2) mechanical work measures resulting from these laboratory experiments and seismological estimates(13,14) suggests that it is reasonable to extrapolate experimental data to conditions typical of earthquake nucleation depths (7-15 km). It seems that faults are lubricated during earthquakes, irrespective of the fault rock composition and of the specific weakening mechanism involved

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.

[1] Frictional melt is implied in a variety of processes such as seismic slip, ice skating, and meteorite combustion. A steady state can be reached when melt is continuously produced and extruded from the sliding interface, as shown recently in a number of laboratory rock friction experiments. A thin, low-viscosity, high-temperature melt layer is formed resulting in low shear resistance. A theoretical solution describing the coupling of shear heating, thermal diffusion, and extrusion is obtained, without imposing a priori the melt thickness. The steady state shear traction can be approximated at high slip rates by the theoretical formunder a normal stress s n , slip rate V, radius of contact area R (A is a dimensional normalizing factor and W is a characteristic rate). Although the model offers a rather simplified view of a complex process, the predictions are compatible with experimental observations. In particular, we consider laboratory simulations of seismic slip on earthquake faults. A series of highvelocity rotary shear experiments on rocks, performed for s n in the range 1-20 MPa and slip rates in the range 0.5-2 m s À1 , is confronted to the theoretical model. The behavior is reasonably well reproduced, though the effect of radiation loss taking place in the experiment somewhat alters the data. The scaling of friction with s n , R, and V in the presence of melt suggests that extrapolation of laboratory measures to real Earth is a highly nonlinear, nontrivial exercise.

Abstract:The article is to complete the description of the special mapping method which theoretical basis and principles were published in . With reference to data on the Ulirba site located in Priolkhonie (Western Pribaikalie), the content of special mapping is reviewed in detail. The method is based on paragenetical analysis of abundant jointing which specific feature is the lack of any visible displacement indicators. There are three stages in the special mapping method (Fig. 3) as follows:Stage I: Preparation and analysis of previously published data on the regional fault structure (Fig. 1, А-Г), establishment of a networks of stations to conduct structural geological monitoring and mass measurements of joints, record of rock data (Fig. 2, А), general state of the fault network (Fig. 1, Д-З), fracture density (Fig. 2, Б) and, if any, structures of the above-jointing level (Fig. 1, Е, З; Fig. 2 , А).Stage II is aimed at processing of field data and includes activities in four groups (II.1-II.4) as follows: Group II.1: construction of circle diagrams, specification of characteristics of joint systems and their typical scatters (Fig. 4, А), identification of simple (generally tipple) paragenesises, and determination of dynamic settings of their formation (translocal rank) ( Table 1), evaluation of densities and complexity of the joint networks, analysis of their spacial patterns within the site under mapping, and identification of the most intensively destructed zones in the rock massif (Fig. 2, Б-В). Group II.2: comparison of jointing diagrams with reference ones showing joint poles (Fig. 4, Б-В; Е-З; Л-Н), and, in case of their satisfactory correlation, making a conclusion of potential formation of a specific joint pattern in the local zone of strike-slip, normal faulting or reverse faulting (Fig. 4, Г-Д, И-К, О-П; Fig. 5; Fig. 7, Б), and determination of relative age relationships between such zones on the basis analysis of the scatter of joint systems, shearing angles and other relevant information. Group II.3: construction of a circle diagram for the specified mapping site with local fault poles (Fig. 8, Б), identification of conjugated systems and dynamic settings of their formation (Fig. 2), plotting the information onto the schematic map of the location under study, and marking the transregional fault zones (Fig. 7, В-К) with observation sites showing similar settings and paragenesises of local faults. Group II.4: comparison between diagrams of fault poles of local ranks with reference patterns selected according to the availability of conjugated pairs of fractures (Fig. 9, Б-Г); based on the above comparison, decision making on potential formation of a paragenesis of local faults in the strike-slip, normal and reserve/thrust fault zones (Fig. 9, Д-Ж), and delineation of boundaries of such zones in the schematic map by connecting the observation sites with similar solutions (Fig. 7, Л-Н).Stage III is aimed at interpreting and includes comprehensive analyses of mapping results and priori information, co...