Focus
Also called: Hypocentre · Topic: Interior of the Earth: Earthquakes, Volcanoes and the Layered Earth · NCERT: Class 11, Ch 3 "Interior of the Earth"
Meaning
The focus, also called the hypocentre, is the point inside the earth where an earthquake's energy is first released. From there, seismic waves spread out in all directions. Earthquakes are grouped by how deep the focus is: shallow (0-70 km), intermediate (70-300 km) or deep (300-700 km). How deep the focus is helps explain how strong the shaking feels at the surface. Focus depths also showed where plates sink into the mantle.
Example
In the Bhuj earthquake of 26 January 2001 (Mw 7.7), the energy was released at a focus deep below the Kachchh region of Gujarat. The point on the surface directly above it, the epicentre, was hit hardest.
Don't confuse with
- Epicentre: this is the point on the surface directly above the focus. It feels the waves first and usually suffers the most damage. The focus is underground.