What is delta? Give an example.

As in geography i think you are meaning a river delta (if not i’m sorry)

a river delta is often shaped as a triangle and is at the part of the river where the water dispenses into the body of water.

if you want a more composed answer here is a google definition:

A river delta is a landform created by deposition of sediment that is carried by a river as the flow leaves its mouth and enters slower-moving or stagnant water. This occurs where a river enters an ocean, sea, estuary, lake, reservoir, or (more rarely) another river that cannot carry away the supplied sediment.

Geologists use  seismic waves to  locate an earthquake's epicenter..