Australia and India are conventionally thought to be contained in a single plate divided from an Arabian plate by the Owen Fracture Zone. We propose instead that motion along the nearly aseismic Owen Fracture Zone is negligible and that Arabia and India are contained within a single Indo‐Arabian plate, divided from the Australian plate by a diffuse boundary. This boundary, which trends E‐W from the Central Indian Ridge near Chagos Bank to the Ninetyeast Ridge, and north along the Ninetyeast Ridge to the Sumatra Trench, is a zone of concentrated seismicity and deformation heretofore characterized as “intraplate”. Plate motion inversions and an F‐ratio test show that relative motion data along the Carlsberg Ridge are fit significantly better by the new model. The misclosure of the Indian Ocean triple junction is reduced by 40%. The rotation vector of Australia relative to Indo‐Arabia is consistent with the seismologically observed ∼2 cm/yr of left‐lateral strike‐slip along the Ninetyeast Ridge, N‐S compression in the Central Indian Ocean, and the N‐S extension near Chagos. This boundary, possibly initiated in late Miocene time, may be related to the opening of the Gulf of Aden and the uplift of the Himalayas. The convergent segment of this boundary may represent an early stage of convergence preceding the initiation of subduction.

The real time W phase source inversion algorithm was independently running at three organizations (USGS, PTWC and IPGS) at the time of the 2011 off the Pacific coast of Tohoku Earthquake. Valuable results for tsunami warning purposes were obtained 20 min after the event origin time. Within the next hour, as more data became available, the W phase solutions improved, and converged to a common result (M w ≈ 9.0, dip ≈ 14• ). A post-mortem W phase analysis using data selection based on pre-event noise confirmed the M w = 9.0 result and yielded a best double couple given by (strike/dip/rake = 196• /12• /85 • ). We also ran the algorithm with increasingly longer periods (T ≈ 1500 sec) to test for the possibility of additional slow slip. The seismic moment remained stable, confirming the prior results.

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The coupling between plate motions and mantle convection is investigated using a fully dynamic numerical model consisting of a thin non-Newtonian layer which is dynamically coupled to a thick Newtonian viscous layer. The non-Newtonian layer has a simple power-law rheology characterized by power-law index n and stiffness constant pp. A systematic investigation of steady, single cell configurations demonstrates that under certain conditions (n > 7 being one of them) the nonNewtonian layer behaves like a mobile tectonic plate. Time-dependent calculations with multicellular configurations show the ability of the plate-mantle coupling model to adjust the number of plates and their sizes in accordance with the flow in the Newtonian layer. These calculations show that the geometry and number of plates d o not necessarily resemble the planform of convection below.