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Relative Motion of Major Tectonic Plates
- Bird defined an idealized plate as a rigid region of lithosphere that rotates about an Euler pole without internal deformation. To distinguish between distinct plates and zones of distributed deformation, he applied both "Strain Rate Contrast" and "Velocity Threshold" criteria. Plates were identified based on:
- Past published research with high priority given to data from "Paleo-Oceanographic Mapping Project"
- Topographic lineaments that suggest active faulting
- Moment tensors of shallow earthquakes and epicenter catalogs were used to define boundaries in complex areas, such as the North America-South America or India-Australia boundaries
- Locations of subaerial volcanoes to highlight extensional boundaries and subduction zones
- GPS data to formulate the best velocity estimates for plates with internal islands, such as those in the southwest Pacific
- Bird acknowledges that some regions exhibit distributed deformation where distinct plates cannot be accurately defined with current data. He designated 13 "orogens" (such as the Alps-Himalayas and the Philippines) that demonstrate large internal deformation and require special attention.