Seismotectonics of the Explorer region

Copyrighted by American Geophysical Union.The Explorer region offshore western Canada is a tectonically complex area\ud surrounded by the Pacific, North America, and Juan de Fuca plates. Existing tectonic\ud models for the region differ fundamentally. Proposed plate configurations range from\ud multiple independent plate fragments to an Explorer plate now fused to North America\ud along the continental margin and cut by Pacific–North America transform faults in the\ud west.We present new seismological data constraining the region’s current tectonics.We use\ud three-component regional waveforms to determine the source parameters of 84 earthquakes\ud with magnitude greater than 4. Combined with 34 Harvard centroid moment tensor\ud solutions, they represent the region’s largest earthquake source parameter data set obtained\ud by robust waveform modeling techniques. In addition, we perform joint epicenter\ud determination to relocate larger earthquakes recorded since 1918. The source parameters\ud and improved locations provide a consistent tectonic picture. Earthquake slip vector\ud azimuths along the Pacific plate boundary change smoothly and are significantly less\ud northerly oriented than the Pacific-North America plate motion direction, requiring an\ud independent Explorer plate. The present-day Pacific-Explorer boundary is formed by\ud transform faults subparallel to the Revere-Dellwood-Wilson fault. Plate motion vectors\ud indicate that theWinona block is part of the Explorer plate. Current Explorer motion is more\ud northerly than indicated by magnetic anomalies prior to 2 Ma, implying a recent change,\ud possibly coinciding with a northwestward ridge jump near Explorer plate’s northern end\ud transferring the Winona block from the Pacific to the Explorer plate. In response to\ud these plate motion changes the region north of the western Sovanco fracture zone was\ud assimilated into the Pacific plate. The region around the eastern Sovanco fracture zone,\ud characterized by broadly distributed seismicity, is composed of well-defined sets of\ud conjugate faults bounding rotating crustal blocks. Earthquake fault strikes agree with the\ud dominant northwest-southeast fault sets; however, the conjugate sets must be also active to\ud fully accommodate present-day Explorer plate motion. The SW portion of the strike-slip\ud Nootka fault zone, the Explorer-Juan de Fuca plate boundary, is well defined by focused\ud seismicity; however, its full extent under Nootka Island remains unresolved. The Explorer–\ud North America boundary shows sporadic low-magnitude seismicity. Our Explorer–North\ud America rotation pole predicts convergence varying from negligible at the boundary’s\ud northwest end to 2 cm/yr at the SE end. This convergence can be accommodated either by\ud subduction or by crustal thickening extending to the North American continent. We favor\ud subduction based on low deformation rates observed by onshore GPS sites. The present\ud Explorer plate system configuration is a result of stepwise reorientation of the Explorer\ud ridge system, each step successively reducing the subduction rate relative to North\ud America