Since 2003, 39 small earthquakes have been detected off the coast of central Oregon in the nominally locked part of the Cascadia subduction zone, where very little seismic activity has been recorded in spite of a paleoseismic record of great subduction events. Although the regional earthquake bulletin reports depths of 29 and 28 km for the two largest events (Mw 4.9 and Mw 4.7, which occurred in 2004), analysis by Tréhu et al. (2008) indicates that they were low-angle thrust events that occurred on the plate boundary at depths of 9–11 and 16 km, respectively. Because of sparse onshore station coverage, most of the smaller events have large location uncertainties. Double-difference relative location of 30 of these earthquakes reveals two tight clusters approximately 30 km apart; each cluster is associated with one of the two larger events. Within each cluster, relocation reduces the hypocenter depth spread from >15 km to <3 km, with uncertainties on the order of 0.1 km. The relocations, combined with independent absolute hypocenter locations for two 2008 events using a deployment of land and ocean-bottom seismometers, suggest that the seismicity occurred at plate boundary depths, possibly on the Cascadia megathrust. This concentrated activity in the seismogenic zone may represent patches on the fault plane with anomalous frictional characteristics, possibly caused by subducted topographic features, which can affect the propagation of a large megathrust rupture.

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