What Causes Earthquakes?

 EARTHQUAKES ARE A VIOLENT SHAKING OF THE EARTH'S CRUST CAUSED BY ENERGY RELEASED DURING TECTONIC MOVEMENT.  

Most of the time the friction between the plates prevents them from moving.  Eventually the stress becomes so great that it overcomes the friction and the plates suddenly move causing the earthquake. 

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THE POINT INSIDE THE CRUST WHERE THE PLATES ACTUALLY MOVE IS CALLED THE FOCUS.  The depth of the focus depends on the type of plate boundary is involved.  The deepest earthquakes occur at subduction boundaries where the crust is being pushed down into the mantle. The shallowest earthquakes occur at diverging boundaries.

 THE EPICENTER OF THE EARTHQUAKE IS A POINT ON THE EARTH'S SURFACE DIRECTLY ABOVE THE FOCUS.  The epicenter is usually of more interest for us, because it can be mapped and easily seen.

 

Seismic Waves

When the plates move at the focus, all of the energy that was building up due to friction is suddenly released.  THE TYPE OF ENERGY RELEASED BY EARTHQUAKES IS SEISMIC WAVES.  THERE ARE 3 DIFFERENT TYPES OF SEISMIC WAVES, EACH WITH DISTINCT PROPERTIES WE NEED TO BE AWARE OF.  

P-waves

P-WAVES, OR PRIMARY WAVES ARE THE FASTEST SEISMIC WAVES.  

THEY TRAVEL IN A PUSH-PULL MOTION, AND CAN MOVE THROUGH SOLIDS AND LIQUIDS.

 

S-waves

S WAVES, OR SECONDARY WAVES, ARE ALWAYS THE SECOND WAVES DETECTED.

THEY ARE SLOWER THAN P WAVES

THEY TRAVEL IN A SIDE TO SIDE MOTION AND CAN ONLY TRAVEL THROUGH SOLIDS.

 

L-waves

L WAVES, OR LONGITUDINAL WAVES ARE THE SLOWEST WAVES, BUT THE MOST DESTRUCTIVE TO HUMAN STRUCTURES.

THEIR DESTRUCTIVE POWER IS THE RESULT OF TEHIR VERTICAL UP AND DOWN MOVEMENT CAUSING ROCK TO RISE AND FALL AS THE WAVE PASSES THROUGH IT.

 

The Shadow Zone

EVERY TIME AN EARTHQUAKE OCCURS, ALL 3 SEISMIC WAVES ARE RELEASED AND TRAVEL OUTWARD FROM THE FOCUS IN ALL DIRECTIONS.  AS THEY MOVE DOWN THROUGH THE EARTH'S INTERIOR SEVERAL THINGS HAPPEN TO SEISMIC WAVES THAT EFFECT OUR ABILITY TO DETECT THEM.

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  1. AS P WAVES TRAVEL THROUGH THE INTERIOR THEY ENCOUNTER LAYERS WITH DIFFERENT DENSITIES.  EACH TIME THIS HAPPENS THE WAVE REFRACTS, OR BENDS.  THIS BENDING CAUSES THE PATH OF THE P WAVE TO BE SHIFTED.

 

  1. AS S WAVES TRAVEL THROUGH THE INTERIOR THEY ARE ABSORBED AND BLOCKED BY THE OUTER CORE.  THIS HAPPENS BECAUSE THE OUTER CORE IS LIQUID, AND S WAVES CANNOT GO THROUGH LIQUIDS.

THE COMBINED EFFECT OF P WAVE REFRACTION AND S WAVE ABSORPTION, IS THAT SOME AREAS ON EARTH WILL NOT RECEIVE ANY TYPE OF SEISMIC WAVE.

THE AREA THAT IS NOT HIT BY ANY SEISMIC WAVES IS CALLED THE SHADOW ZONE..  

shadow zone.GIF