to release the accumulated strain energy. Occurrence and characteristics Megathrust earthquakes are almost exclusive to tectonic subduction zones and are often associated with the Pacific and Indian Oceans. These subduction zones are not only responsible for megathrust earthquakes but are also largely responsible for the volcanic activity associated with the Pacific Ring of Fire. Since the earthquakes associated with these subduction zones deform the ocean floor, they often generate a significant series of tsunami waves. Subduction zone earthquakes are also known to produce intense shaking and ground movements for significant periods of time that can last for up to 3-5 minutes. In the Indian Ocean region, the Sunda megathrust is located where the Indo-Australian Plate is subducting under the Eurasian Plate and extends off the coasts of Myanmar, Sumatra, Java and Bali before terminating off the northwestern coast of Australia. This subduction zone was responsible for the 2004 Indian Ocean earthquake and tsunami. In Japan, the Nankai megathrust under the Nankai Trough is responsible for Nankai megathrust earthquakes and associated tsunamis. In North America, the Juan de Fuca Plate is subducting under the North American Plate creating the Cascadia subduction zone which stretches from mid Vancouver Island, British Columbia to Northern California. This subduction zone was responsible for the 1700 Cascadia earthquake. The Aleutian Trench, of the southern coast of Alaska, and the Aleutian Islands, where the North American Plate overrides the Pacific Plate, has generated many major earthquakes throughout history, several of which generated Pacific-wide tsunamis, including the 1964 Alaska earthquake; at magnitude 9.2, it remains the largest recorded earthquake in North America, and the second-largest earthquake instrumentally recorded in the world. The largest recorded megathrust earthquake was the 1960 Valdivia earthquake, estimated magnitude 9.4–9.6, centered off the coast of Chile along the Peru-Chile trench, where the Nazca Plate is subducting under the South American Plate. This megathrust region has regularly generated extremely large earthquakes. The largest megathrust event within the last 20 years was the magnitude 9.1 Tōhoku earthquake. A study reported in 2016 found that the largest megathrust quakes are associated with downgoing slabs with the shallowest dip, so-called flat slab subduction. Compared with other earthquakes of similar magnitude, megathrust earthquakes have a longer duration and slower rupture velocities. The largest megathrust earthquakes occur in subduction zones with thick sediments, which may allow a fault rupture to propagate for great distances unimpeded. See also Lists of earthquakes