 Energy travels as seismic waves which are vibrations caused by earthquakes.  All earthquakes start beneath the earth’s surface

 The focus of an earthquake is where rocks first begin to move.  Seismic waves travel outward from earthquakes focus.  The epicenter is the point right above the focus on the earth’s surface.

 If two earthquakes occur at the same time the one with a shallow focus will be more powerful  Earthquakes produce three kind of waves: primary, secondary, and surface.

 Primary waves are the fastest waves and the first to reach a location  P waves go through the earth at a speed of about 3 miles per second

 Secondary waves are the second wave to reach a location after the initial wave.  Secondary waves can travel through rock but unlike primary waves they cant travel through liquid or gases.

 Surface waves are seismic waves that move along earth’s surface  Surface waves are what make the ground roll up and down or shake from side to side.

 Seismograph is a instrument that constantly records ground movement  Scientists use seismographs to measure thousands of earthquakes per year  A seismogram is the recording produced.

 To locate a earthquakes epicenter scientists must have seismograms from at least three seismograph locations  1. Scientists find the difference between arrival times of primary and secondary waves at each 3 stations

 2. The time difference is used to determine the distance from epicenter from each station  The greater the distance the farther away from the epicenter

 3. A circle is drawn around each station, where all the circles meet is the epicenter.  The seismograph also helps scientists determine focus, depth, magnitude and strength