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GPS shows that the Tonga arc is moving rapidly toward the Pacific plate a little like a windshield wiper rotating clockwise. The Tonga arc occupies a microplate separated from the Australian plate by seafloor spreading centers in the Lau Basin. As the Lau basin opens, the Tonga arc moves eastward out ahead of the Australian plate adding to the convergence rate at the Tonga trench. As Tonga moves eastward, the Pacific plate sinks or thrusts beneath the Tonga arc generating numerous earthquakes within the sinking Pacific plate.