Shallow-focus and deep-focus earthquakes
Topic: Interior of the Earth: Earthquakes, Volcanoes and the Layered Earth · NCERT: Class 11, Ch 4 "Distribution of Oceans and Continents"
Meaning
Earthquakes are grouped by the depth of their focus, the point where the energy is released. Shallow-focus quakes start 0-70 km down, intermediate ones 70-300 km down and deep-focus ones 300-700 km down. Under mid-ocean ridges the foci are shallow, because new crust forms there and the plates pull apart near the surface. Along the rim of the Pacific and the Alpine-Himalayan belt the foci are deep, because one plate sinks below another. This difference was an early clue to sea-floor spreading and subduction, where one plate slides down under another.
Example
Quakes along the Mid-Atlantic Ridge have shallow foci. Under Japan, on the Pacific rim, foci get deeper as the Pacific plate sinks into the mantle.
Don't confuse with
- Magnitude: this is the energy a quake releases. A shallow quake is not always weak, and a deep quake is not always strong. Focus depth and size are separate things.