Fault plane solutions of earthquakes on the Nazca plate boundaries and the Easter plate

Thirteen new and fourteen published earthquake fault plane solutions are used to examine the directions of the relative plate motion along the boundaries of the Nazca plate on the East Pacific Rise, Galapagos Spreading Center, and Chile Rise. In general, slip vectors agree well with one another and with the directions of plate motion given by Minster et al. A few solutions that disagree with these inferred directions of plate motion corroborate the independent evidence and suggestions of Forsyth and Herron that two small plates exist along the East Pacific Rise crest. Together, magnetic anomalies, bathymetry, seismicity and the fault plane solutions suggest that the northernmost of these two plates, the Easter plate, is bounded on the north by transform faults and on the south by spreading centers. The existence of the diamond-shaped Easter plate may be related to (1) its close proximity to Easter Island (a proposed hot spot) or, (2) the very high velocities of plate motion and, therefore, the thin lithosphere in this region.