1. Department of Geosciences, Princeton University, Princeton, New Jersey 08544, USA
2. Laboratoire de Géophysique Interne et Tectonophysique, UMR CNRS, C5559 et Université de Savoie , 73376 Le Bourget-du-Lac, France
3. Present address: Exxon Production Research, PO Box ST-2158, Houston, Texas 77252, USA.

Correspondence to: Allan M. Rubin¹ Correspondence and requests for materials should be addressed to A.M.R. (e-mail: Email: arubin@princeton.edu).

Abstract

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.