The Ryukyu Trench is situated southeast of Kyushu where the Philippine Sea plate begins its subduction beneath Japan. While earthquakes have occurred several hundred kilometers northwest of the Ryukyu Trench, most earthquakes are at significant depths along this subduction zone. Thirteen magnitude 5-plus earthquakes have occurred at shallow depths of less than 50 kilometers (31.1 miles) within 100 kilometers (62.1 miles) of the Kumamoto earthquake over the past century, according to the USGS. In addition to the two events in April, a shallow magnitude 6.6 earthquake occurred in 2005 off the north coast of Kyushu. Two other events occurred in 1975 that were of magnitude 5.8 and 6.1 at distances of 40 kilometers (24.9 miles) and 65 kilometers (40.4 miles) to the northwest. The graphic below from the USGS illustrates the depth of the Ryukyu Trench at 20 kilometer (12.4 miles) increments in orange. The depth of the trench beneath Kumamoto city is roughly 140 plus kilometers (87.0 miles).