How and Where Earthquakes Occur


    1.    Compare and contrast primary, secondary, and surface waves.

    2.    Explain how an earthquake epicenter is located using seismic wave information.

    3.    Describe how seismic wave studies indicate the structure of the Earth's interior.

Vocabulary words:

    seismic wave    focus    primary wave    epicenter    surface wave    inner core    outer core    mantle    crust


    U.S. Geological Survey, Home page

Notes: (10.1)

More than 3 million earthquakes occur every year.  Earthquakes are caused by the reshaping of Earth through the movement of Earth's tectonic plates.  Most earthquakes occur along plate boundaries or along cracks in the lithosphere called faults.  To understand the formation of faults it is important to remember that all rocks have the quality of elasticity (ability to return to shape).

Seismic waves: Body waves

( gif/eqfig2.gif)

The energy of the earth is released in seismic waves which travel outward from the focus.

        Focus- point where the energy is released  

        Epicenter place on the surface of the Earth above the focus

        Primary (P) waves type of seismic wave that vibrates rock back and forth

        Secondary (S) waves cause particles to move up and down

Seismometer( images/pswaves_sm.gif)

        Surface waves travel out from the epicenter (2 Types)

( quakefigs/waves1.gif)