Answer:
they are caused from earthquakes
Step-by-step explanation:
Earthquakes and underground explosions can release a lot of energy. That energy ripples away from its source in a variety of ways. Some of those vibrations will move forward and back through the material they travel through . Other waves travel just like ocean waves, where they make the material they pass through move up and down compared to the direction the wave is traveling. And while some of these waves travel deep within the planet, still others move only along the surface. Studying where these various flavors of waves are and how they move not only can help scientists pinpoint where an earthquake or explosion occurred, but also can shed light on the structure of our inner planet.