island of Taiwan is separated from the southeast coast of mainland China by the Taiwan Strait, which ranges from at its widest point to at its narrowest. Part of the continental shelf, the Strait is no more than deep, and has become a land bridge during glacial periods. To the south, the island of Taiwan is separated from the Philippine island of Luzon by the -wide Luzon Strait. The South China Sea lies to the southwest, the East China Sea to the north, and the Philippine Sea to the east. Niushan Island in Nanlai village, Aoqian town, Pingtan County, Fuzhou, Fujian is the closest China (PRC)-administered island to Taiwan (main island). Smaller islands of the archipelago include the Penghu islands in the Taiwan Strait west of the main island, with an area of , the tiny islet of Xiaoliuqiu off the southwest coast, and Orchid Island and Green Island to the southeast, separated from the northernmost islands of the Philippines by the Bashi Channel. The islands of Kinmen and Matsu near the coast of Fujian across the Taiwan Strait have a total area of ; the Pratas and Taiping islets in the South China Sea are also administered by the ROC, but are not part of the Taiwanese archipelago. Geology The island of Taiwan was formed approximately 4 to 5 million years ago at a complex convergent boundary between the Philippine Sea Plate and the Eurasian Plate. In a boundary running the length of the island and continuing southwards in the Luzon Volcanic Arc (including Green Island and Orchid Island), the Eurasian Plate is sliding under the Philippine Sea Plate. Most of the island comprises a huge fault block tilted to the west. The western part of the island, and much of the central range, consists of sedimentary deposits scraped from the descending edge of the Eurasian Plate. In the northeast of the island, and continuing eastwards in the Ryukyu Volcanic Arc, the Philippine Sea Plate slides under the Eurasian Plate. The tectonic boundary remains active, and Taiwan experiences 15,000 to 18,000 earthquakes each year, of which 800 to 1,000 are noticed by people. The most catastrophic recent earthquake was the magnitude-7.3 Chi-Chi earthquake, which occurred in the center of Taiwan on 21 September 1999, killing more than 2,400 people. On 4 March 2010 at about 01:20 UTC, a magnitude 6.4 earthquake hit southwestern Taiwan in the mountainous area of