1. Earthquake Properties

2. Physical Properties
Focus – point of rock failure  where the earthquake originates
Epicenter - The point on the earth's surface directly above the earthquake's focus

3. Seismic Waves
Vibrations generated during an earthquake

4. Types of Earthquake Waves
Surface waves
Travel on Earth’s surface outward from the epicenter in a circular motion pattern
Complex motion
Slowest velocity of all waves

5. Types of Earthquake Waves
Body waves - travel through interior of Earth
1. Primary (P) waves
Push-pull (compressional) motion
Travel through solids, liquids, and gases
Greatest velocity of all earthquake waves

6. Types of Earthquake Waves
Body waves - travel through interior of Earth
2. Secondary (S) waves
"Shake" motion
Travel only through solids
Slower velocity than P waves

7. A seismogram records wave amplitude vs. time

8. The machine that produces seismograms is called a seismograph.

9. Locating an epicenter requires 3 seismograph stations.

10. Distance from the epicenter to a station is determined by the difference in arrival time between P and S waves.

11. Calculate distances from 3 stations and see where the circles intersect.
300 km
500 km
100 km

12. Measuring The Size of An Earthquake
Intensity – qualitative; determined by effects on people, structures, and the environment
Magnitude – quantitative; measures energy released at the source of the earthquake

13. Magnitude Scales
Richter scale - measures earthquake magnitude  based on size of largest wave
Moment magnitude scale - accounts for size of fault rupture, amount of movement at fault, and rocks’ stiffness  more precise
Each step represents a 10-fold increase in wave size, or a 30-fold increase in energy released
A magnitude 6 earthquake is 10 times bigger than a magnitude 5 earthquake and releases 30 times more energy.
A magnitude 7 earthquake is 100 times bigger than a magnitude 5 earthquake and releases 900 times more energy.

15. Moment Magnitude
                                                          
Moment = µ A D
µ = shear modulus
A = LW = area D = average displacement during rupture
Source: USGS
The seismic moment is a measure of the size of an earthquake based on the area of fault rupture, the average amount of slip, and the force that was required to overcome the friction sticking the rocks together that were offset by faulting.

16. Magnitude From Amplitude

17. Let’s try it! An earthquake has been recorded on 3 seismograms.
Can you locate its epicenter?
Can you determine its magnitude?

18. Abbreviated Modified Mercalli Intensity Scale
Witness observations I
Felt by very few people; barely noticeable II
Felt by a few people, especially on upper floors.
III
Noticeable indoors, especially on upper floors, but may not be recognized as an earthquake. Hanging objects swing. IV
Felt by many indoors, by few outdoors. May give the impression of a heavy truck passing by. V
Felt by almost everyone, some people awakened. Small objects move. Trees and poles may shake. VI
Felt by everyone. Difficult to stand. Some heavy items of furniture move, plaster falls. Slight damage to chimneys possible.
VII
Slight to moderate damage in well-built, ordinary structures. Considerable damage to poorly built structures. Some walls may fall.
VIII
Little damage in specially built structures. Considerable damage to ordinary buildings, severe damage to poorly built structures. Some walls collapse. IX
Considerable damage to specially built structures, buildings shifted off foundations. Noticeable cracks in ground. Wholesale destruction. Landslides. X
Most masonry and frame structures and their foundations destroyed. Ground badly cracked. Landslides. Wholesale destruction. XI
Total damage. Few, if any, structures standing. Bridges destroyed. Wide cracks in ground. Waves seen on ground.
XII
Total damage. Waves seen on ground. Objects thrown up into air.

20. Intensity Intensity  when focus depth 
Intensity  when distance from epicenter 
Also depends on what ground is made of – why?
Let’s see it!

21. Did You Feel It?