Streaks of microearthquakes along creeping faults

Crustal faults that produce most of their slip aseismically typically gener ate large numbers of small earthquakes. These events have generally been in terpreted as coming from localized patches of the fault that undergo unstab le (stick-slip) sliding, surrounded by larger regions of stable sliding (cr eep). In published catalogues the microearthquakes often appear to be distr ibuted over large portions of the fault surface. By accurately locating lar ge numbers of microearthquakes from faults of different orientations in Cal ifornia and Hawaii, we show here that instead the locations define highly c oncentrated streaks that are characteristically aligned in the direction of fault slip. The underlying cause of this structural organization of the fa ult surface remains to be determined.