FREQUENT shallow earthquakes within the rift zones of the Hawaiian volcano Kilauea have been interpreted as resulting from stress changes associated with a shallow magma conduit system. Here, by using a precise earthquake relocation technique, we show that what had been imaged as a diffuse cloud of seismicity in the Upper East Rift in 1991 is in fact a narrow ribbon, defining vertical strike-slip faults that extend 2.5 km along the rift, but less than 100 to 200 m vertically. This extreme aspect ratio in the seismicity is unexpected and has not been recognized in other fault systems. The observed earthquake depths, left-lateral focal mechanisms, limited vertical extent and increasing moment release rate since 1983 can be explained in terms or a owing stress concentration above the deeper, aseismic portion of Kilauea's rift system, which deforms in response to the seaward displacement of the south flank of the volcano. The shallow background seismicity within Kilauea's rifts can therefore be tied to large-scale motions of the volcanic edifice, and need not be interpreted as resulting from magma migration. The observed pattern of seismicity may also have implications for the mechanical erosion of locked patches along major strike- slip faults elsewhere.
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