A plate made by a back-arc basin
The Okinawa Plate is a microplate of about 325,000 square kilometers, or 0.06% of Earth’s surface. It forms a long, narrow strip from southern Kyushu through the Ryukyu Islands to the northeast corner of Taiwan. In the dataset, 3.2% of Japan and 2% of Taiwan lie on it.
The plate exists because the Ryukyu island arc is pulling away from Asia. Behind the arc, the Okinawa Trough is opening as a back-arc basin, so the arc moves differently from the continent behind it. PB2002 draws this moving strip as its own plate. Many textbooks simply include it in the Eurasian Plate.
What happens at its edges
The outer edge is the Ryukyu Trench (also called the Nansei-Shoto Trench), where the Philippine Sea Plate subducts northwest beneath the islands. This 1,593 km boundary is the fastest of all the Philippine Sea Plate’s boundaries, with relative motion of about 5.8-13.5 cm/yr, partly because the opening of the Okinawa Trough adds to the convergence.
The inner edge is the 1,409 km boundary with the Yangtze Plate along the Okinawa Trough. PB2002 classifies most of it as continental rift, with a short spreading-ridge segment, and the opening rate is about 2.1-5.5 cm/yr. At the northern end, a 383 km boundary with the Amur Plate near Kyushu opens more slowly, at about 0.3-1.7 cm/yr.
Earthquakes
The dataset lists 67 earthquakes of magnitude 6.5 or larger on the plate since 1900, none of them magnitude 8. The largest include the 1922 earthquake off Yilan, Taiwan (M7.7), the 1938 earthquake near Miyako Island (M7.7), the 1961 earthquake off Nichinan in Kyushu (M7.5) and the 1966 earthquake near Yonaguni (M7.5). Tsunamis are a concern here too: the 1771 Meiwa tsunami, triggered by an offshore earthquake, devastated Ishigaki and Miyako islands.
Volcanoes
The plate holds 13 volcanoes in the dataset, lined up along the volcanic front of the Ryukyu arc. In southern Kyushu sit Kirishima and the Aira Caldera, whose active vent Sakurajima erupts almost constantly. Offshore, the Kikai Caldera produced a very large eruption about 7,300 years ago. Smaller island volcanoes such as Suwanosejima, Kuchinoerabujima and Nakanoshima continue the chain south toward Okinawa.