EARTHQUAKES

What is an Earthquake? Sudden movement in the earth’s crust which releases energy Movement occurs along areas of weakness in the earth’s crust Areas of weakness are generally cracks in the rock

Crack in the earth’s crust along which movement occurs Place where rock begins to separate during an earthquake. Generally underground Place on earth’s surface directly above focus

SEISMIC WAVES When earthquakes occur, waves of energy seismic waves travel outward from the earthquake focus. 3 types of seismic waves are produced AT THE SAME TIME but each behaves differently within earth.

P WAVES Primary waves or compression waves vibrate parallel to the direction of movement. (push-pull) Travel faster than any other wave (6-8 km./s) Travel through solids, liquids, and gases

P Waves Longitudinal: –Particles move parallel to the direction wave travels

S WAVES Shear wave or secondary waves vibrate back and forth perpendicular to the direction the wave is moving (side-side) Slower than P waves (4-5 kms./s) TRAVEL THROUGH SOLIDS ONLY

S Waves Transverse waves –Particles move perpendicular to direction of wave motion

SURFACE OR LONG WAVES Vibrations travel along earth’s surface in a circular motion at relatively slow speeds (2 kms/s) like waves in a pond Do more damage because they produce more ground movement

Seismic Waves are either: Body waves: travel through the earth –P-waves –S-waves or Surface waves (Long waves): –Form when p and s waves reach the surface –Move slowly –Cause the most damage

How are earthquakes detected? Seismograph: instrument which detects motion seismogram: recording of the earthquake

seismograph

Seismogram

P WAVES TRAVEL FASTER THAN S WAVES AS DISTANCE FROM EPICENTER INCREASES -THE GREATER THE TIME INTERVAL BETWEEN P AND S WAVES

VELOCITY OF SEISMIC WAVES Velocity depends on the material they are passing through a.Increase density and pressure – greater the velocity b.Waves are refracted or bent as waves pass through material with different densities

EARTHQUAKE SHADOW ZONES At an angle of 103° (distance 11,000 kms) from the epicenter both P and S waves disappear P waves can again be detected at l42° (16,000 kms.) NO S WAVES EVER APPEAR AGAIN THIS BAND OF 39° IN WHICH NO WAVE ARE OBSERVED IS CALLED THE EARTHQUAKE’S SHADOW ZONE

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

ANALYSIS OF SEISMIC WAVES HAVE RESULTED IN THE INFERENCE ABOUT EARTH’S INTERIOR S WAVES CANNOT PASS THROUGH THE LIQUID OUTER CORE

How many… In a day? (choose one) In a day? That’s about one every ten seconds. But we don’t feel all of them, do we? No, most are smaller earthquakes.

How is the strength of an earthquake measured? Mercalli scale –Based upon reports of people in effected area Richter Scale: –Based upon size of waves on seismogram –Logarithmic scale (ex. An earthquake of 5 has 10 times as much movement as a 4)

Measuring Earthquakes (1) Mercalli Index Measure of intensity based on descriptions of structural damage and what is felt Scale of I to XII Often dependent on the proximity to EQ focus –One EQ will have many Mercalli scores Examples II. A few people might notice movement if they are at rest and/or on the upper floors of tall buildings. VI. Everyone feels movement. People have trouble walking. Objects fall from shelves. X. Most buildings and their foundations are destroyed. XII. Almost everything is destroyed. Objects are thrown into the air.

Measuring Earthquakes (2) Richter Magnitude Measures the energy released from an earthquake 1 – 10 scale Every EQ has a unique magnitude, but effects will vary due to distance, ground conditions, etc. Examples Less than 3.5 – Generally not felt, but recorded – Can be destructive in areas up to about 100 kilometers across where people live. 8 or greater – Great earthquake. Can cause serious damage in areas several hundred kilometers across.

DescriptorMagnitudeAnnual average Great8 or higher1 Major7– Strong6– Moderate5–5.91,319 2 Light4–4.9c. 13,000 Minor3–3.9c. 130,000 Very minor2–2.9c. 1,300,000 Frequency of Earthquakes Worldwide National Earthquake Information Center, U.S. Geological Survey.

Indian Ocean – Japan

Body Wave Review P Waves Are Primary waves They arrive first They are the fastest They are compressional waves The travel through anything S Waves Are Secondary waves They arrive second They are Shear waves They travel through Solids only