The Brittle‐Plastic Transition, Earthquakes, Temperatures, and Strain Rates

Maximum depths of earthquakes in different settings are commonly thought to lie within a transition, many kilometers in width, from brittle deformation at shallow depths to plastic deformation by high‐temperature creep at greater depth. A review of temperatures and strain rates, both of which are low in intraplate settings, shows faster deformation in warmer tectonically active regions and highest strain rates in regions where the movement of magma affects strain rates and temperatures are especially high. Although intraplate earthquakes appear to occur in oceanic lithosphere only where colder than ~600°C, in tectonically active regions, where strain rates are several orders of magnitude higher than intraplate settings, earthquakes occur where temperatures exceed 600°C and perhaps 800°C. The temperature cutoff increases by ~100°C as strain rates increase by ~2 to 2.5 orders of magnitude. Nowhere, however, do any two of average strain rates, background stresses, and temperatures seem to be determined well enough that, when combined with laboratory‐based flow laws for high‐temperature creep, they can place a tight constraint on the third of these quantities. At the same time, the pattern of higher temperatures at higher strain rates follows the general forms of power law and Peierls creep that are appropriate for lithospheric conditions.