What causes earthquakes and why do tsunamis often follow a large earthquake?
Earthquakes are caused by the sudden movement of rocks along weak zones (faults) or by pressures from volcanic activity. Tectonic plates divide the Earth’s crust into blocks which move and build up stress in rocks. These stressed rocks can sometimes suddenly snap, vertically or sideways, causing shaking and vibration. Japan lies on the Pacific ‘ring of fire’ a region of high earthquake and volcanic activity. Japan is particularly prone to earthquakes as it lies on the junction of several colliding tectonic plates. The Philippine Sea Plate and the Pacific Plate are moving in a north west direction and are being pushed (subducted) beneath the Eurasian plate and North American plate under the region of Japan. This forms an unstable oceanic trench (subduction zone), and creates an Island Arc or string of volcanoes from the rise of magma from the melting crust.
Tsunami is a Japanese word combining ‘harbour’ and ‘wave’. Shock waves from large earthquakes, volcanic eruptions, or landslides can displace ocean waters which travel out as a wave or series of waves. When the wave reaches shallow water near land its top slows down, but faster moving deeper water catches up and forms a higher wave. Tsunamis travel up to 950 km/hour in deep water and 100 to 300 km/hour across a continental shelf.
See and hear more about earthquakes and tsunamis below.
Go to our Plate Tectonic processes page for more geological information.
(This post was written in conjunction with Dr Lin Sutherland, Senior Research Fellow, Australian Museum)