Crustal faults that produce most of their slip aseismically typically generate large numbers of small earthquakes. These events have generally been interpreted as coming from localized patches of the fault that undergo unstable (stick-slip) sliding, surrounded by larger regions of stable sliding (creep). In published catalogues the microearthquakes often appear to be distributed over large portions of the fault surface. By accurately locating large numbers of microearthquakes from faults of different orientations in California and Hawaii, we show here that instead the locations define highly concentrated streaks that are characteristically aligned in the direction of fault slip. The underlying cause of this structural organization of the fault surface remains to be determined.
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