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Coupling is a convenient word that describes a wide variety of interactions or feedback processes, including those that we do not fully understand. Examples in Earth science include ocean-atmosphere coupling, climate-tectonics coupling, and core-mantle coupling. The word is also very popular in discussions of plate boundary earthquake processes.

As a vague expression, fault coupling is a perfectly adequate term, describing some kind of mechanical interaction between rocks of each side of a fault. For those of us who try to infer fault processes from geodetic measurements, coupling usually indicates a state of no or low current slip. If a fault is fully locked, we may say it is “coupled” or “fully coupled.” If a plate boundary fault is slipping at the long-term plate convergence rate, we may say it is “decoupled.” Fault segments that are slipping more slowly than the plate convergence rate are then “partially” coupled. To avoid awkward expressions such as “negatively” or “overly” coupled, an equivalent description has been used in the literature; that is, to define a “coupling ratio” with values ranging from negative to greater than unity. For describing kinematics, these expressions would not be wrong. However, with one additional step, our usage of the word coupling can lead to confusion. That step is to describe a fault that is not slipping as “strongly coupled.”

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Eos, Transactions, American Geophysical Union, v. 85, issue 18, p. 180-181

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