Historic triangulation data have been analysed to determine whether intraplate seismicity is associated with ongoing ductile deformation in the lower crust. The model we have attempted to test is basically analogous to strain accumulation and release along plate-boundary strike-slip faults like the San Andreas Fault in California. That is, beneath an elastic-seismogenic upper crust ???20 km thick, strain is preferentially localized within ductile shear zones in the lower crust due to broad-scale plate driving forces. The localized lower-crustal ductile strain causes stress and strain to accumulate elastically in the brittle crust which is eventually released in crustal earthquakes. At greater depths, this localized shear deformation probably develops into pervasive ductile flow. Numerous geodetic measurements along the San Andreas Fault confirm that earthquakes in the brittle upper crust are produced by the release of elastic strain that results from ongoing ductile shear or slip in the lower crust1,2. We have found evidence of high rates of crustal deformation in southern New York which suggest that localized ductile shear is occurring in the lower crust. ?? 1985 Nature Publishing Group.