If intermediate and deep earthquakes occur in the coldest portions of the downgoing slabs of lithosphere, then different lengths of seismic zones represent different temperatures in the slabs. As the slab descends through the aesthenosphere, it warms primarily by conduction of heat through its upper and lower surfaces. Isotherms are advected downwards to distances approximately proportional both to the rate of subduction and to the square of the thickness of the lithosphere. Consequently, the lengths of seismic zones should be approximately proportional to the product of the rates and the squares of these thicknesses. As these thicknesses are approximately proportional to the square root of the age of the lithosphere, the lengths ought therefore to be approximately proportional to the product of the convergence rates times the ages. Although there is considerable scatter, observed lengths are approximately proportional to such products, and are not simply related to the rate, the age or the thickness alone. The data crudely fit the relationship: length = rate x age/lO. Using this relationship, we infer that the Philippine sea and Pacific plates move slowly, if at all, with respect to one another and that the Farallon plate may have been too young to be subducted to a great distance beneath western North America in the Palaeogene. Calculations of temperatures at the depths of the deepest events suggest that these cut-off temperatures increase from about 600 100°C at 200 km to 830 k 50°C at 650 km depth, but the cut-off potential temperature is approximately constant. Assuming that the strength is a thermally activated parameter, and using the activation energy for olivine, a crude estimate of activation volume is obtained from the dependence of the cut-off temperature on depth. 42

By using magnetic anomaly data from a detailed geophysical survey west of the Juan de Fuca rise between longitudes 143°W–134°W and latitudes 42°N–48°N the history of spreading at the Pacific‐Farallon spreading center in this region can be reconstructed for the period 35–20 m.y. ago (anomalies 12–5E). During this time period, relative migration of spreading axes separated by transform faults resulted in the elimination of the offset represented by the Surveyor fracture zone. Magnetic anomalies in the southern part of the region require eastward jumps of spreading centers of between 40 and 50 km, and those in the northern part imply westward jumps of up to 70 km. The locations of the spreading center jumps migrate along spreading axes with time, concurrently with northward or southward jumps of transform faults, and leave zones of extensively sheared crust with unidentifiable magnetic anomaly patterns in the crust between old and new spreading centers. Such a process may account for the disturbed zone of magnetic anomalies between the Murray and Molokai fracture zones and could be common to all ridge jumps. If so, it suggests that the new spreading centers do not begin simultaneously over long lengths but instead develop in a manner somewhat similar, but not identical, to a crack propagating through a solid.

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