Earth's surface is covered by more than a dozen plates(rock slabs), which move several centimeters per year. Earthquakes and volcanic activity are concentrated at plate boundaries.
Basic Knowledge
Plates are rock slabs about 100 km thick that move over the soft asthenosphere beneath them. Their speed is about several centimeters to 10 cm per year(close to the rate at which fingernails grow).
Near Japan, the Pacific Plate, Philippine Sea Plate(oceanic plates), North American Plate, and Eurasian Plate(continental plates) push against one another.
At locations where oceanic plates subduct beneath continental plates, trenches(the Japan Trench, Nankai Trough, and Kuril Trench) form, and massive earthquakes occur there(plate-boundary earthquakes). In contrast, earthquakes caused by the movement of active faults inland are called inland(directly beneath the surface) earthquakes. The Great Hanshin-Awaji Earthquake(1995) was this type.
Plate(A rock slab about 100 km thick that covers Earth's surface)
Trench(A deep depression where an oceanic plate subducts. The Japan Trench is about 8000 m deep)
Plate-boundary earthquake(A massive earthquake caused by the release of strain during subduction. It is likely to cause a tsunami)
Inland earthquake(directly beneath the surface)(A shallow earthquake caused by movement along an active fault. It can cause major local damage)
Active fault(A fault that has moved within the past several hundred thousand years and may move again)
Tsunami(A wave caused when the entire body of seawater moves during a seafloor earthquake. It is fast offshore but becomes higher near the coast)
In Depth (Background and Significance)
The theory of plate tectonics was established in the 1960s. It was based on Wegener's continental drift theory proposed in 1912 and was systematized through studies of seafloor topography, paleomagnetism, and earthquake distribution.
In the Nankai Trough, massive earthquakes of M8〜9 have repeatedly occurred at intervals of 100〜200 years in the past, and the probability of one occurring within the next 30 years is estimated at 70〜80%. During the 2011 Tohoku earthquake(M9.0), a tsunami with a maximum run-up height of more than 40 m was recorded.
In the open ocean, a tsunami travels at about 700 km/h at a water depth of approximately 4000 m, as fast as a jet aircraft. Near the coast, it slows down but grows higher. If you feel an earthquake, deciding to evacuate immediately to higher ground can save your life.
- Plates move several centimeters per year
- Four plates meet near Japan
- Oceanic plates(the Pacific and Philippine Sea plates) subduct
- Plate-boundary earthquakes = massive earthquakes + tsunamis(the Great East Japan Earthquake)
- Inland earthquakes = directly beneath the surface and shallow(the Great Hanshin-Awaji Earthquake)
- The probability of a massive Nankai Trough earthquake within the next 30 years is 70〜80%
- Tsunamis travel at about 700 km/h in the open ocean
Points to Note (Common Confusions)
① Do not confuse plate-boundary earthquakes(M8〜9, tsunamis, and periodic occurrence) with inland earthquakes(around M7, directly beneath the surface, and active faults). ② Of the four plates near Japan, the Pacific Plate and Philippine Sea Plate are oceanic plates, while the North American and Eurasian plates are continental plates. ③ A trench(deep and narrow) and a trough(shallow and broad, such as the Nankai Trough) are different concepts. ④ The speed of a tsunami is determined by water depth; it travels faster in deeper water.
Practice
- Name all four plates near Japan.
- Was the Great East Japan Earthquake an inland earthquake or a plate-boundary earthquake?
- About how fast do plates move per year?