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Earthquakes occur in crust or mantle that is below a critical temperature for the tectonic strain-rate such that stress builds up to the breaking point before it can relax due to creep. The limiting temperature depends on pres- sure, which is taken into account by finding a critical homologous temper- ature THc = T/TM above which earthquakes cease (where T, TM are tem- perature and average melting temperature of constituent minerals). The G layer of ocean plates is an uppermost mantle layer of thickness 50 km that is thought to be composed of harzburgite and depleted peridotite from which basalt has been removed to form ocean crust. Thus it has a higher melting temperature than the peridotite of the surrounding mantle, or the lower halves of plates. We find that THc for ocean plates is ~0.55. For California earth- quakes, it is also close to 0.55. Thicknesses of seismicity in deep subduction zones, determined from 2D polynomial fits to a relocated earthquake catalog, are ~50km, which suggests that the earthquake channel is confined to the G layer. We construct thermal models to find homologous temperatures that take into account the variation of thermal parameters as a function of temperature, pressure and mineral content based on laboratory and equation of state es- timates for pyrolite and harzburgite, and latent heat from phase changes. We find that seismicity thicknesses in subducted slabs are also, on average, confined to TH < 0.55 \pm 0.05. The cutoff for deep earthquakes is not sharp. However they appear unlikely to occur if homologous temperature is high TH > 0.55. Exceptions to the rule are anomalously deep earthquakes such as those beneath the Hawaiian hotspot where THc >0.66. These can be explained if volcanic strain-rates are 2 to 3 orders of magnitude higher than the strain-rates associated with tectonic earthquakes.