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
Place on earth’s surface directly above focus Place where rock begins to separate during an earthquake. Generally underground Crack in the earth’s crust along which movement occurs
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
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 • Increase density and pressure – greater the velocity • 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? 80 800 8000 (choose one) • In a day? 80 8008000 • 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)
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 (1)
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. 6.1-6.9 – 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. Measuring Earthquakes (2)
Frequency of Earthquakes Worldwide National Earthquake Information Center, U.S. Geological Survey.
Indian Ocean – 2004 12 26 9.0 Japan - 2011 03 11 9.0
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