Aftershocks occurring on faults in the far-field of a large earthquake rupture can generally be accounted for by changes in static stress on these faults caused by the rupture(1,2). This implies that faults interact, and that the timing of an earthquake can be affected by previous nearby ruptures(3-6). Here we explore the potential of small earthquakes to act as 'beacons' for the mechanical state of the crust. We investigate the static-stress changes resulting from the 1992 Landers earthquake in southern California which occurred in an area of high seismic activity stemming from many faults. We first gauge the response of the regional seismicity to the Landers event with a new technique, and then apply the same method to the inverse problem of determining the slip distribution on the main rupture from the seismicity. Assuming justifiable parameters, we derive credible matches to slip profiles obtained directly from the Landers mainshock(7,8). Our results provide a way to monitor mechanical conditions in the upper crust, and to investigate processes leading to fault failure.
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