1. What are Earthquakes?
The shaking or trembling caused by the sudden release of energy
Usually associated with faulting or breaking of rocks
Continuing adjustment of position results in aftershocks

2. Objectives Relate earthquake activity to plate tectonics
Define earthquake, and identify the focus and epicenter of an earthquake.
Describe the types of waves emitted during an earthquake.
Distinguish between earthquake intensity and magnitude.
Review some current methods of earthquake prediction.

3. definitions
Earthquake = Vibration of the Earth produced by the rapid release of energy
Seismic waves = Energy moving outward from the focus of an earthquake
Focus= location of initial slip on the fault; where the earthquake origins
Epicenter= spot on Earth’s surface directly above the focus
A drop of water – analogy between ripples in the water and seismic waves

4. Why do earthquakes occur?
focus
epicenter
Fractures, faults
Energy released
and propagates in all directions as seismic waves causing
earthquakes
Why do rock break?
Review three types of stress: tension, compression, shear
Three types of deformation resulting: elastic, plastic and brittle
We’re concerned with the brittle deformation – causes faults, fractures through which energy, created by the friction, is released
Define Focus, epicenter and seismic waves

5. What is the Elastic Rebound Theory?
Explains how energy is stored in rocks
Rocks bend until the strength of the rock is exceeded
Rupture occurs and the rocks quickly rebound to an undeformed shape
Energy is released in waves that radiate outward from the fault

6. The Focus and Epicenter of an Earthquake
The point within Earth where faulting begins is the focus, or hypocenter
The point directly above the focus on the surface is the epicenter

7. Seismographs record earthquake events
At convergent boundaries, focal depth increases along a dipping seismic zone called a Benioff zone

8. Where do earthquakes occur: https://www. youtube. com/watch
1) Most earthquakes occur along the edge of the oceanic and continental plate
2) Along faults: normal, reverse, transform

9. Where Do Earthquakes Occur and How Often?
~80% of all earthquakes occur in the circum-Pacific belt
most of these result from convergent margin activity
~15% occur in the Mediterranean-Asiatic belt
remaining 5% occur in the interiors of plates and on spreading ridge centers
more than 150,000 quakes strong enough to be felt are recorded each year

10. The Economics and Societal Impacts of EQs
Damage in Oakland, CA, 1989
Building collapse
Fire
Tsunami
Ground failure

11. What are Seismic Waves?
Response of material to the arrival of energy fronts released by rupture
Two types:
Body waves
P and S
Surface waves
R and L

12. Body Waves: P and S waves https://www.youtube.com/watch?v=KZaI4MEWdc4
P or primary waves
fastest waves
travel through solids, liquids, or gases
compressional wave, material movement is in the same direction as wave movement
S or secondary waves
slower than P waves
travel through solids only
shear waves - move material perpendicular to wave movement

13. Surface Waves: R and L waves
Travel just below or along the ground’s surface
Slower than body waves; rolling and side-to-side movement
Especially damaging to buildings

14. Seismic waves: forms http://seplessons.ucsf.edu/node/110 lab activity
P-waves:
called compressional, or push-pull waves
Propagate parralel to the direction in which the wave is moving
Move through solids, liquids
S-waves:
Called shear waves
Propagate the movement perpendicular
to the direction in which the wave is
moving
Surface waves (L-waves or long waves).
Complex motion
Up-and-down and side-to-side
Slowest
Most damage to structures, buildings
P-waves: the bell analogy: waves felt by both sound

15. Seismic waves: properties
Velocity: function of the physical properties of the rock the wave is traveling through
Velocity increases with rock density
Velocity changes when passing from one material to another (increases/decreases)
Liquids: S-waves do not get transmitted through liquid; P-waves slow down
Why is this important?
If we know the velocity of the wave, we can infer
the type of rock it traveled through- that’s how we map
the interior of the Earth!!!

16. Calculating Velocity Velocity = speed in a given direction
Velocity = distance / time = meter/second
If a seismic wave travels 45 meters in 2 seconds from the focus, how fast is the seismic wave traveling?
v = 45m / 2s = 22.5 m/s out from focus

17. Measuring earthquakes
Seismometers: instruments that detect seismic waves
Seismographs
Record intensity, height
and amplitude of seismic
waves
Earliest seismograph was invented in China ~ 2000 years ago (or 136 AD)
Easy to make; can make your own at home

18. How is an Earthquake’s Epicenter Located?
Seismic wave behavior
P waves arrive first, then S waves, then L and R
Average speeds for all these waves is known
After an earthquake, the difference in arrival times at a seismograph station can be used to calculate the distance from the seismograph to the epicenter.

19. How is an Earthquake’s Epicenter Located?
Time-distance graph showing the average travel times for P- and S-waves. The farther away a seismograph is from the focus of an earthquake, the longer the interval between the arrivals of the P- and S- waves

20. How is an Earthquake’s Epicenter Located?
Three seismograph stations are needed to locate the epicenter of an earthquake
A circle where the radius equals the distance to the epicenter is drawn
The intersection of the circles locates the epicenter

21. Earthquake size: two ways to measure
Magnitude: Richter Scale
Measures the energy released by fault movement
related to the maximum amplitude of the S wave measured from the seismogram
Logarithmic-scale; quantitative measure
For each whole number there is a 31.5 times increase in energy
eg. an increase from 5 to 7 on the Richter scale = an increase in energy of 992 times!!
Write Table 11-1 on the board, to help relate to Richter scale

22. How are the Size and Strength of an Earthquake Measured?
Magnitude
Richter scale measures total amount of energy released by an earthquake; independent of intensity
Amplitude of the largest wave produced by an event is corrected for distance and assigned a value on an open-ended logarithmic scale

23. Frequency of Occurrence of Earthquakes
Descriptor
Magnitude
Average Annually
Great
8 and higher
1 ¹
Major
17 ²
Strong
134 ²
Moderate
1319 ²
Light
13,000 (estimated)
Minor
130,000 (estimated)
Very Minor
1,300,000 (estimated)
¹ Based on observations since ² Based on observations since 1990.

24. How are the Size and Strength of an Earthquake Measured?
Intensity
subjective measure of the kind of damage done and people’s reactions to it
isoseismal lines identify areas of equal intensity
Modified Mercalli Intensity Map
1994 Northridge, CA earthquake, magnitude 6.7

25. 2) Intensity: Mercalli Scale:
What did you feel?
Assigns an intensity or rating to measure an earthquake at a particular location (qualitative)
I (not felt) to XII (buildings nearly destroyed)
Measures the destructive effect
Intensity is a function of:
Energy released by fault
Geology of the location
Surface substrate: can magnify shock waves e.g. Mexico City (1985) and San Francisco (1989)

26. What are the Destructive Effects of Earthquakes?
Ground Shaking
amplitude, duration, and damage increases in poorly consolidated rocks

27. Earthquake damage Ground Failure - constructions collapse
Fires - from broken gas and electrical lines
Landslides - EQ's triggered; occur in hilly/mountainous areas.
Liquefaction - water-saturated, unconsolidated materials flow
Tsunami (seismic sea waves; "tidal" waves) - can grow up to 65 m
***

28. Tsunami
A tsunami is a large ocean wave that is caused by sudden motion on the ocean floor.
This sudden motion could be an earthquake, a powerful volcanic eruption, or an underwater landslide.
The impact of a large meteorite could also cause a tsunami.
Tsunamis travel across the open ocean at great speeds and build into large deadly waves in the shallow water of a shoreline.

29. Most Destructive Known Earthquakes on Record in the World
Date
Location
Deaths
Magnitude
Comments
May 31, 1970
Peru
66,000
7.9
$530,000,000
damage, great rock
slide, floods.
July 27, 1976
China, 
Tangshan
255,000 (official)
7.5
Estimated death toll as high as 655,000.
Sept 19, 1985
Mexico
Michoacan
9500
(official)
8.0  
Estimated death toll as high as 30,000
Old lake bed magnified shock waves by 500%
2001 Jan 26
India
20,023
7.7
166,836 injured,
600,000 homeless .
2004 Dec 26
Sumatra
283,106
9.0
Deaths from earthquake
and tsunami

30. Landslides: May 30, 1970 Peru disaster
Magnitude: 7.9
A large mass of ice and rock slid from a vertical face on Nevado Huascaran, the highest peak in Peru
Debris reached a velocity of 280 km/hr
traveled 11 km horizontally in about 4 minutes at a mean velocity of 165 km/hr.
Buried the towns of Yungay and Ranrahirca, The death toll in both villages was 20,000.
A large mass of ice and rock separated from a vertical face on Nevado Huascaran, the highest peak in Peru
The slab fell 1000 m vertically, disintegrated, slid down the lateral slopes of the mountain, and then was launched off a 'ski jump'.
Debris reached a velocity as high as 280 km/hr
buried the towns of Yungay and Ranrahirca, having traveled 11 km horizontally in about 4 minutes at a mean velocity of 165 km/hr.
The death toll in both villages was 20,000.

31. India, Gujarat earthquake Jan 26, 2001

32. Can Earthquakes be Predicted?
Earthquake Precursors
changes in elevation or tilting of land surface, fluctuations in groundwater levels, magnetic field, electrical resistance of the ground
seismic dilatancy model
seismic gaps

33. Can Earthquakes be Predicted?
Earthquake Prediction Programs
include laboratory and field studies of rocks before, during, and after earthquakes
monitor activity along major faults
produce risk assessments

34. Can Earthquakes be Controlled?
Graph showing the relationship between the amount of waste injected into wells per month and the average number of Denver earthquakes per month
Some have suggested that pumping fluids into seismic gaps will cause small earthquakes while preventing large ones