1. EARTHQUAKES

2. Earthquakes & Plate Tectonics
It is a vibration in the crust due to the release of stress
Causes a shift in the rocks along a fault or plate boundary
Explained by the Elastic Rebound Theory
“Locked” rocks build up stress, fracture, & slip back to original position
Slipping causes release of energy = seismic waves

3. Focus is the location (within the earth) along the fault where slippage occurs
Epicenter (what you hear on the news) is the point, on the surface, directly above the focus
Where most damage occurs

4. Approximately 90 % of all earthquake foci are at depths of less than 100 km (62 miles)

5. 3 Major Earthquake Zones
Pacific Ring of Fire
Mid-oceanic ridges
Eurasian-Melanesian mountain belt
EQ occur along plate boundaries

6. Recording Earthquakes
Seismographs detect motion of the crust
Creates series of wavy lines called seismogram

7. 3 Types of Seismic Waves
BODY= a. Primary waves- move together & apart like accordion (fastest)
b. Secondary waves- move at right angles to direction of wave travel like a snake
SURFACE= Love & Rayleigh waves- move up & down like ocean waves (slowest) w/ side-to-side component
These cause most damage
Earthquakes
seismic waves animation

8. Locating an Earthquake
To find epicenter of an earthquake, analyze difference between arrival of P & S waves
P waves travel 1.7 X faster than S waves
If S waves arrive shortly after P waves = close
If S waves arrive long after P waves = far away

9. Locating an Earthquake
Need seismographic data from 3 different locations
Distances to epicenter plotted on map & their intersection determines epicenter

10. Earthquake Measurement
Magnitude expressed by Richter Scale
Measures energy released by EQ as measured by the amplitude
(height) of the seismic wave
increase by 1 = 31.7 times
more energy
Ex. EQ = 7 had 31.7 X more energy released than an EQ = 6
Largest EQ = 9.5, Chile, 1960

11. Earthquake Measurement
Mercalli scale expresses intensity or amount of damage done
Indicated by Roman numerals I to XII
Ex. II = low intensity described as “Felt only by a few persons at rest, especially on upper floors of buildings”

12. Earthquake Damage
Determined by type of ground that building is located on
Loose soil & rock = more damage
Most injuries from collapsing buildings & flying debris
May cause landslides, fires, & explosions

13. EQ occur along plate borders, little “tears” in the crust create faults
One of the world’s
most active = San Andreas Fault System (made of 100s of faults) in California, it separates the N Am. & Pacific plates
800 mi long
10 mi deep

14. Seismic Activity in the U.S.
Red/orange = areas of high seismic activity
Yellow = medium seismic activity
Blue/white= low seismic activity

15. Tsunami
Created when an the epicenter of the earthquake is on the ocean floor
Displaces (and pushes) a huge volume of water
Creates massive “tidal waves” of moving water

16. As a tsunami approaches the shore
Wavelength decreased & wave height increase

17. December 2004 Tsunami Disaster

22. Earthquake Detection Scientists try to detect changes in the crust by:
instruments used to detect movement
immobile area = seismic gaps
(fault is locked)
Future earthquake locations

23. Evidence of Pending Earthquake
Slight tilting of ground just prior to EQ
Increase in strain in rocks
may inject water in cracks to reduce strain
Change in electrical & magnetic properties
Increased natural gas seepage
Decrease in speed of P waves from distant earthquakes

24. 10 things you didin't know about earthquakes