Approximately 100 heat flow measurements in the San Andreas fault zone indicate (1) there is no evidence for local factional heating of the main fault trace at any latitude over a 1000‐km length from Cape Mendocino to San Bernardino, (2) average heat flow is high (∼2 HFU, ∼80 mW m−2) throughout the 550‐km segment of the Coast Ranges that encloses the San Andreas fault zone in central California; this broad anomaly falls off rapidly toward the Great Valley to the east, and over a 200‐km distance toward the Mendocino Triple Junction to the northwest. As others have pointed out, a local conductive heat flow anomaly would be detectable unless the frictional resistance allocated to heat production on the main trace were ≲100 bars. Frictional work allocated to surface energy of new fractures is probably unimportant, and hydrologic convection is not likely to invalidate the conduction assumption, since the heat discharge by thermal springs near the fault is negligible. Explanations for the low dynamic friction fall into two intergradational classes: those in which the fault is weak all of the time and those in which it is weak only during earthquakes (possibly just large ones). The first class includes faults containing anomalously weak gouge materials and faults containing materials with normal frictional properties under near‐lithostatic steady state fluid pressures. In the second class, weakening is caused by the event (for example, a thermally induced increase in fluid pressure, dehydration of clay minerals, or acoustic fluidization). In this class, unlike the first, the average strength and ambient tectonic shear stress may be large, ∼1 kbar, but the stress allocated to elastic radiation (the apparent stress) must be of similar magnitude, an apparent contradiction with seismic estimates. Unless seismic radiation is underestimated for large earthquakes, it is difficult to justify average tectonic stresses on the main trace of the San Andreas fault in excess of ∼200 bars. The development of the broad Coast Range heat flow anomaly southward from Cape Mendocino suggests that heat flow increases by a factor of 2 within 4 m.y. after the passage of the Mendocino Triple Junction. This passage leaves the San Andreas transform fault zone in its wake; the depth of the anomalous sources cannot be much greater than the depth of the seismogenic layer. Some of the anomalous heat may be supplied by conduction from the warmer mantle that must occur south of the Mendocino transform (where there is no subducting slab), and some might be supplied by shear heating in the fault zone. With no contribution from shear heating, extreme mantle upwelling would be required, and asthenosphere conditions should exist today at depths of only ∼20 km in the northernmost Coast Ranges. If there is an appreciable contribution from shear heating, the heat flow constraint implies that the seismogenic layer is partially decoupled at its base and that the basal traction is in the sense that resists right lateral motion on the fault(s). As a result of thes...

L e f f i n g w e l l 's contraction-crack theory of ice-wedge polygons in permafrost has been examined from the point of view of mechanics. A nonlinear viscoelastic model of thermal stress in permafrost leads to results consistent with the theory within the limits of existing information on polygon dimensions, crack depths, temperature, and mechanical properties of ice and permafrost. Stresses that cause crack ing are evidently generated not only by low temperature but also by rapid cooling. The size of the polygons can be explained in terms of the stress-perturbation due to a single crack and the distribution of mechanical flaws. The polygonal patterns can be classified accord ing to whether or not the intersections are predominantly orthog onal. It is proposed that orthogonal polygons evolve by progres sive subdivision, nonorthogonal ones by successive branching of cracks attaining high propagation velocities.Much of the discussion is general and applies directly to other types of contraction-crack polygons such as columnar basalt joints and mud cracks. 1 List of Symbols E i Principal components of distortional strain tensor i f Principal components of distortional strain-rate tensor Principal components of distortional stress tensor Principal components of distortional stress-rate tensor (x , % > tz Principal components of thermal strain Tx , Ty , Tj Principal components of thermal stress © Temperature referred to mean annual temperature as zero a Linear coefficient of thermal expansion K Elastic bulk modulus Y Young's modulus fj, Elastic shear modulus v Poisson's ratio rj Quasi-viscous parameter rj Viscosity X, ¡jf, rf Viscoelastic parameters t Time $ Time required for stress to reach 90 per cent of equilibrium value under a step increase of strain rate from zero stresst Horizontal thermal stress ( t " , or t z) T* = t ( x ) -pgx Sum of horizontal thermal stress and lithostatic stress ro = r(0 ) = r*(0 ) Horizontal thermal stress at ground surface, x = 0 Sx , Sv , S a , Sxy Tensor components of stress perturbation due to a long straight crack (collectively : Sti) Say = Sy(0, y) ^-component of stress-relief at ground surface (x = 0) Soz = S s(0, y) ^-component of stress-relief at ground surface (x = 0) S Combined horizontal stress-relief of all pre-existing cracks a " Stress depth," thickness of surficial layer with uniform stress b Crack depth p Density g Acceleration of gravity k Crack-edge stress-intensity factor k© Contribution to crack-edge intensity factor due to thermal stress np Contribution to crack-edge intensity factor due to weight of overburden G Crack extension force Gc Value of G necessary for initiation of fast fracture Go Value of G necessary for sustained crack propagation K0 Value of k necessary for sustained crack propagation A Intensity parameter for step approximation to t * ( x ) B Intensity parameter for linear approximation to t * ( x ) y = K /A y /b or k/ B \ / b Normalized crack-edge stress-intensity factor N Nominal tensile strength N -e Tensile strength of a flaw SOME M ECHANICAL ...

Expansion of pore fluid caused by frictional heating might have an important effect on the factional resistance and temperature during an earthquake and a controlling influence on the physics of the earthquake process. When confined water is heated, the pressure increases rapidly (≳10 bars/°C). As Sibson (1973) has pointed out, this could cause a sharp reduction of effective normal stress and dynamic friction on the fault surface. Whether or not this transient stress reduction occurs depends upon the tandem operation of several processes, any of which can break the chain that links frictional heat to frictional stress: the friction must cause an appreciable temperature rise (imposing conditions on the width of the shear zone and rate of conductive transport); the temperature rise must cause an appreciable fluid pressure rise (imposing conditions on the rate of pore dilatation or hydrofracturing, and the rate of Darcian transport); the fluid pressure rise must cause an appreciable reduction of friction (requiring the presence of a continuous fluid phase). Each process depends upon event duration, particle velocity, and the initial value of dynamic friction. With the present uncertainty in the controlling parameters (principally permeability, width of the shear zone, initial stress, and factors controlling transient hydrofracture and pore dilatation) a wide variety of fault behavior is possible. Limits to fault behavior for various ranges of the controlling parameters can be estimated from the governing equations, however, and results can be summarized graphically. If the effective stress law applies and pore dilatation is unimportant, dynamic friction would drop from an initial value of 1 kbar to ∼100 bars when shear strain reached 10 for most earthquakes if the permeability were less than 0.1 μdarcy; the maximum temperature rise would be only ∼150°C irrespective of final strain. If the permeability were ≳100 mdarcies, however, friction would be unaffected by faulting and temperatures could approach melting for shear strains ∼20. For permeabilities ∼1 mdarcy, friction could be reduced appreciably during large earthquakes, but during small ones it could not. Combined with thermal effects, dilatational strain of a few percent of pore volume could lead to virtually frictionless faulting or increasing frictional resistance, dependeing upon its sign; unstable propagation of hydrofractures (after fluid pressure exceeded the least principal stress) could cause a sudden increase in fault friction. Strengthening due to cooling and Darcian flow at the conclusion of an earthquake could occur in seconds or weeks depending upon event duration, transport parameters, and shear zone width; it might influence the redistribution of stress by aftershocks.

Temperature profiles measured in permafrost in northernmost Alaska usually have anomalous curvature in the upper 100 meters or so. When analyzed by heat-conduction theory, the profiles indicate a variable but widespread secular warming of the permafrost surface, generally in the range of 2 to 4 Celsius degrees during the last few decades to a century. Although details of the climatic change cannot be resolved with existing data, there is little doubt of its general magnitude and timing; alternative explanations are limited by the fact that heat transfer in cold permafrost is exclusively by conduction. Since models of greenhouse warming predict climatic change will be greatest in the Arctic and might already be in progress, it is prudent to attempt to understand the rapidly changing thermal regime in this region.

No abstract