Aim: How can we explain Earthquakes?

What is an Earthquake? A natural rapid shaking of the lithosphere, due to release of energy stored in rocks Most caused by movements along faults Some caused by moving of magma in the lithosphere and with volcanic eruptions

Energy stored in rocks is given off as seismic (earthquake) waves. What are seismic Waves? Energy stored in rocks is given off as seismic (earthquake) waves. Scientists study waves to find info about Earth’s interior.

Where do earthquakes start? Earthquake starts at the focus – the point from which the waves are emitted The location on Earth’s land or water surface directly above the focus is called the epicenter

How do we measure earthquakes? Scientists measure and record earthquake waves using a seismograph

What are the different types of Earthquake waves? 3 types of seismic waves are produced AT THE SAME TIME but each behaves differently within earth

P-waves (Primary Waves) Particles vibrate parallel to the direction of movement Travel the FASTEST Can pass through solids, liquids, and gases.

S-waves (secondary waves) Particles vibrate perpendicular to the direction they are traveling Second fastest wave Can only pass through SOLIDS

Surface Waves Produced when a P- or S-wave comes to Earth’s surface. Travel only on Earth’s solid surface. Cause much of the surface shaking and damage.

Velocity of waves Depends on properties of materials they are passing through. Increase density and pressure, increase the speed

EARTHQUAKE SHADOW ZONES At a certain angle from the epicenter both P and S waves disappear P waves can again be detected beyond a certain point NO S WAVES EVER APPEAR AGAIN THE BAND IN WHICH NO WAVES ARE OBSERVED IS CALLED THE EARTHQUAKE’S SHADOW ZONE However, scientists noticed something unusual. When the angle around the earth’s circumference from the epicenter is more than 102 (This is a distance of about 11,000 kms.) both the P and S waves disappear. Then the P waves can again be detected if the station is more than l43 (l6,000 kms.). NO S WAVE ARE EVER OBSERVED. THE RESULT IS A BAND OF ABOUT 41 OR 4500 KMS. wide in which no waves are observed. This region is called the earthquake’s shadow zone. Location of the shadow zone will depend on the focus of the EQ. Each EQ produces its own shadow zone.

EARTHQUAKE SHADOW ZONES P WAVES DISAPPEAR FROM 105º -140º S WAVES DISAPPEAR FROM 105º NEVER TO APPEAR AGAIN

Regents Questions

The study of how seismic waves change as they travel through Earth has revealed that Earth’s outer core is liquid because S-waves are not transmitted through this layer P-waves travel more slowly than S-waves through Earth’s crust Seismic waves travel more slowly though the mantle because it is very dense Earth’s outer core is solid because P-waves are not transmitted through this layer

What is the direct cause of most earthquakes? Movement of bedrock along a fault line Gravitational pull on bedrock by the Moon Deposition of sediment in lakes and oceans Heat exchange between the crust and the atmosphere

Sometimes faster and sometimes slower Earthquakes generate P-waves and S-waves. Compared to the speed of the S-waves in a given Earth material, the speed of P-waves is Always slower Always faster Always the same Sometimes faster and sometimes slower