Can the Okhotsk Plate be discriminated from the North American plate?

Abstract

The plate geometry in northeast Asia has been a long‐standing question, with a major issue being whether the Sea of Okhotsk and northern Japanese islands are better regarded as part of the North American plate or as a separate Okhotsk plate. This question has been difficult to resolve, because earthquake slip vectors along the Kuril and Japan trenches are consistent with either Pacific‐North America or Pacific‐Okhotsk plate motion. To circumvent this difficulty, we also use slip vectors of earthquakes along Sakhalin Island and the eastern margin of the Japan Sea and compare them to the predicted Eurasia‐Okhotsk and Eurasia‐North America motions. For a model with a separate Okhotsk plate, we invert 10 Eurasia‐Okhotsk and 255 Pacific‐Okhotsk slip vectors with Pacific‐North America and Eurasia‐North America NUVEL‐1 data. Alternatively, for a model without an Okhotsk plate, those Eurasia‐Okhotsk and Pacific‐Okhotsk data are regarded as Eurasia‐North America and Pacific‐North America data, respectively. The model with an Okhotsk plate fits the data better than one in which this region is treated as part of the North American plate. Because the improved fit exceeds that expected purely from the additional plate, the data indicate that the Okhotsk plate can be resolved from the North American plate. The motions on the Okhotsk plate's boundaries predicted by the best fitting Euler vectors are generally consistent with the recent tectonics. The Eurasia‐Okhotsk pole is located at northernmost Sakhalin Island and predicts right‐lateral strike slip motion on the NNE striking fault plane of the May 27, 1995, Neftegorsk earthquake, consistent with the centroid moment tensor focal mechanism and the surface faulting. Along the northern boundary of the Okhotsk plate, the North America‐Okhotsk Euler vector predicts left‐lateral strike slip, consistent with the observed focal mechanisms. On the NW boundary of the Okhotsk plate, the Eurasia‐Okhotsk Euler vector predicts E‐W extension, discordant with the limited focal mechanisms and geological data. This misfit may imply that another plate is necessary west of the Magadan region in southeast Siberia, but this possibility is hard to confirm without further data, such as might be obtained from space‐based geodesy.