Great thrust earthquakes should increase the potential to do work, and thus island arcs may be subject to large strain variations during the first few decades following an earthquake. As the stress returns to preearthquake levels, the strain differences across an island arc may become much more uniform. This suggests a method for qualitatively assessing long-term earthquake risk and for earthquake prediction. A continuous extension of some island arcs during both the preseismic and the seismic stages of ground displacement may be responsible for the origin of the active interarc basins described by Karig.