Earth Quakes

Movement along a Fault

SEISMIC WAVES Waves of energy that travel outward in all directions from the earthquake focus are called SEISMIC WAVES 3 types of seismic waves are produced AT THE SAME TIME but each behaves differently within earth.

PRIMARY WAVES “P” waves: Also called compression waves Back and forth vibration (click diagram below) Travel fastest (6-8 km/s) Travel through solids, liquids, and gases Travel through all parts of Earth’s interior

SECONDARY WAVE “S” waves: Also called shear waves Up and down vibration (click below) Slower than P waves (4-5 kms./s) TRAVEL THROUGH SOLIDS ONLY DO NOT go though Earth’s OUTER CORE

SURFACE OR LONG WAVES Also called “L” waves Vibrate along the surface Slowest seismic wave (2 km/s) Make the most ground movement

INTERPRETING SEISMIC WAVES YouTube Seismic Waves Seismic waves travel in all directions away from the quake’s focus P-waves travel faster through the Earth… S-waves are slower through Earth… Both types bend as they travel… S -waves DO NOT go through the outer core because is it liquid!

SHADOW ZONE SHADOW ZONE S- Wave SHADOW ZONE EARTHQUAKE SHADOW ZONES NO WAVES HERE- Waves bend away! S-WAVE SHADOW ZONE S-Waves don’t go through liquid outer core! SHADOW ZONE SHADOW ZONE S- Wave SHADOW ZONE

ANALYSIS OF SEISMIC WAVES TELL US ABOUT THE ABOUT EARTH’S INTERIOR P-WAVES ARE BENT as they TRAVEL though Earth! S WAVES CANNOT PASS THROUGH THE LIQUID OUTER CORE!

Seismic waves travel in all directions away from the quake’s focus P-waves arrive other places first (fastest) S-waves arrive at other places next, followed by surface waves (both slower than P-waves) Seismographic stations measure earthquake waves at different places with seismographs Seismograms show when waves arrive!

Alaska Quake Video Finding The Epicenter California Quake Video Use 3 seismograms from different places Plot their distances on a map with circles The circles intersect at the EPICENTER!

Return to P wave slide Return to S wave slide

SEISMOGRAPH SEISMOGRAM

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 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.

LOCATING THE EPICENTER OF AN EARTHQUAKE The difference in travel time between P and S waves can be used to determine the DISTANCE from a station to the epicenter The farther a station is from the epicenter, the GREATER the time interval between the arrival of P and S waves TO DETERMINE THE EXACT LOCATION OF AN EPICENTER, ITS DISTANCE FROM 3 STATIONS MUST BE DETERMINED AND 3 CIRCLES DRAWN

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

LOCATING EARTHQUAKE EPICENTER MUST HAVE DISTANCE FROM 3 SEISMOGRAPH STATIONS TO DETERMINE EPICENTER

EARTH’S INTERIOR

EARTHQUAKES AND FAULTS An earthquake is caused by the sudden movement of bedrock along a fault Fault is a crack in the bedrock strata where the movement occurs! Faults created by plate movement! FAULT

Notice the LAYERS of BEDROCK…the layers are called STRATA FAULTS show the BREAKS in the STRATA This is a NORMAL FAULT REVERSE FAULT TRANSFORM FAULT

SAN ANDREAS FAULT NORTH PLATE WEST SIDE MOVES NORTH NORTH AMERICAN PLATE MOVES SOUTH SOUTH

TRANSFORM FAULT SAN ANDREAS FAULT Calif. EARTHQUAKE Dog Calif. EARTHQUAKE