The islands of Japan are primarily the result of several large oceanic movements occurring over hundreds of millions of years from the mid-Silurian to the Pleistocene as a result of the subduction of the Philippine Sea Plate beneath the continental Amurian Plate and Okinawa Plate to the south, and subduction of the Pacific Plate under the Okhotsk Plate to the north. Japan was originally attached to the eastern coast of the Eurasian continent. The subducting plates, being deeper than the Eurasian plate, pulled Japan eastward, opening the Sea of Japan around 15 million years ago. The Strait of Tartary and the Korea Strait opened much later. Japan is situated in a volcanic zone on the Pacific Ring of Fire. Frequent low intensity earth tremors and occasional volcanic activity are felt throughout the islands. Destructive earthquakes, often resulting in tsunamis, occur several times a century. Hot springs are numerous and have been developed as resorts.