Earthquake & Types
Context:
A powerful 7.5-magnitude earthquake recently struck northeastern Japan, injuring at least 30 people and forcing thousands to evacuate.
The Japan Meteorological Agency initially issued tsunami warnings which were later downgraded and lifted.
Japan is situated in the Ring of Fire (an area in the Pacific Basin known for frequent volcanic eruptions and earthquakes) accounting for about 20% of the world's earthquakes of magnitude 6.0 or greater.
Tsunami
A tsunami is a series of waves that are caused by a disturbance (such as an undersea earthquake, a volcanic eruption or a meteor strike).
A disturbance that moves the water vertically (a sudden drop or a sudden rise) can generate a tsunami, like ripples generated when a stone is thrown into a pond.
How do earthquakes cause tsunamis?
Tsunamis are often formed when there is a large vertical displacement of Earth’s crust, particularly at boundaries of tectonic plates (fault lines).
Denser oceanic plates slip under continental plates to cause a subduction earthquake.
A subduction earthquake is a particularly effective generator of tsunamis.
Vertical movement of the crust displaces a large volume of water.
As gravity acts to restore equilibrium, waves of water (tsunamis) race away from the epicentre.
Effects of water depth
The speed of a tsunami is directly related to water depth, so as depth decreases, speed of the tsunami also decreases.
Energy flux of a tsunami is dependent upon its speed and wave height, so in shallow water the height of the wave increases.
The rate at which energy is dissipated from a wave is inversely proportional to its wavelength, so tsunamis travel great distances (e g right around the globe) with limited energy loss