1. Earthquakes
EEn Explain how the rock cycle, plate tectonics, volcanoes, and earthquakes impact the lithosphere.

2. Forces Within Earth
Earthquakes are natural vibrations of the ground caused by plate movement in Earth’s crust or by volcanic eruptions
Can be extremely destructive
Kill thousands of people
Destroy entire cities
Anyone living in an area prone to earthquakes should be aware of the potential danger and how to minimize the damage that they cause.

3. Stress and Strain
Most earthquakes occur when rocks fracture deep within Earth
Fractures form when stress (forces acting on a material) exceed the strength of the rock
3 kinds of stress:
Compression – decreases the volume of a material (squishes it)
Tension – pulls a material apart
Shear – causes a material to twist
The deformation of materials in response to stress is strain

4. Faults
Rocks fail when stress is applied too quickly, or when stress is too great.
The resulting fracture or system of fractures, along which movement occurs, is called a fault
Three basic types of faults:
Reverse fault – form as a result of horizontal compression
Normal fault – caused by horizontal tension
Strike-slip fault – caused by horizontal shear

5. Earthquake Waves
The vibration of the ground during an earthquake are called seismic waves
Every earthquake generates 3 types of seismic waves:
Primary waves (P-waves) – causes rock particles to move back and forth as it passes
Secondary waves (S-waves) – causes rock particles to move at right angles to the direction of the wave
Surface waves – causes rock particles to move both up and down and side to side.

6. Earthquake Waves Surface waves travel along Earth’s surface
P-waves and S-waves pass through Earth’s interior
The first waves generated by a quake spread out from the point of failure of rocks
This point, where an earthquake originates, is the focus of the earthquake; usually several km below the surface
The point on Earth’s surface directly above the focus is the earthquake’s epicenter.

7. Measuring and Locating Earthquakes
More than one million earthquakes occur each year!
More than 90% are not felt and cause little to no damage
The ones that make the news are major seismic events that cause much damage

8. Earthquake Magnitude & Intensity
The amount of energy release during an earthquake is measured by its magnitude
Measured using the Richter Scale – based on the size of the largest seismic waves made by the quake
10 fold scale:
meaning seismic waves of a magnitude-8 earthquake on the Richter scale are 10 times larger than a magnitude-7 and 100 times larger than a magnitude-6.

9. Moment Magnitude Scale
Most seismologists use the moment magnitude scale – takes into account the size of the fault rupture, the amount of movement along the fault, and the rock’s stiffness
Moment Magnitude – based on size of several types of seismic waves

10. Modified Mercalli Scale
Another way to assess earthquakes is to measure the amount of damage
Called the intensity of an earthquake; is determined using the modified Mercalli scale – rates the types of damage and other effects during and after its occurrence.

12. Depth of Focus
Another factor that determines a quake’s intensity is the depth of the quake’s focus
Can be classified as shallow, intermediate, or deep
Deep-focus = smaller vibrations at epicenter
Shallow-focus = larger vibrations at epicenter
A shallow-focus, magnitude-6 quake will have greater intensity than a deep-focus, magnitude-8 quake
Catastrophic quakes with high intensities are almost always shallow-focus quakes

13. Locating an Earthquake
Initially, the exact location of an earthquake’s epicenter and time of quake’s occurrence is not known
But, it can be easily determined using seismograms and travel-time curves

14. Distance to an Earthquake
To determine the location of the epicenter:
The locations of 3 seismic stations are plotted on a map
A circle whose radius is equal to the epicentral distance is plotted around each station
The point of intersection of these circles is the epicenter

15. Time of an Earthquake
The exact arrival times of the P-waves and S-waves at a seismic station can be read from the seismogram
Using a travel-time graph, the time of occurrence can be determined by subtracting the travel time from the known arrival time of the wave.

16. Seismic Belts
Seismologists have collected and plotted locations of epicenters
The global distribution of these epicenters reveals an interesting pattern
They are not randomly distributed
Most earthquakes are associated with tectonic plate boundaries
80% occur along the Circum-Pacific Belt
15% occur along the Mediterranean-Asian Belt
Most of the remaining occur at ocean ridges

17. Earthquakes and Society
Most earthquake damage results from prolonged shaking of the ground by surface waves (can last longer than a minute)
Many structures cannot withstand this violent motion
Collapsing buildings are responsible for many earthquake-related deaths

18. Some Earthquake Hazards
The damage caused by an earthquake is directly related to the strength or quality of the structures involved
The most severe damage occurs to unreinforced buildings
Wooden structures and high-rise steel-frame buildings sustain a lot less damage
Some buildings in earthquake-prone areas (California) rest on large rubber structures that absorb most of the vibrations made during a quake.

19. Structural Failure
Buildings are destroyed as the ground beneath them shakes
In some cases, the supporting walls of the ground floor fail and cause the upper floors to fall and collapse as they hit the ground or lower floors – called “pancaking”
In other cases, the height of the building is related to structural failure
In the 1985 Mexico City earthquake, most buildings between 5 and 15 stories collapsed or were destroyed.
Similar structures that were shorter or taller sustained only minor damage
The shaking had the same period of vibration as the natural sway of the intermediate (5-15 stories) buildings = caused them to sway violently

20. Land and Soil Failure Earthquakes can wreak havoc on Earth itself
In sloping areas, earthquakes may trigger massive landslides
Soil liquefaction can also cause trees and houses to fall over or to sink into the ground
Earthquake waves can be amplified as they travel through a soil
Soft materials have little resistance to deformation, so seismic waves are amplified in such materials
Wave size and earthquake intensity are greatest in soft, unconsolidated sediments and small in hard, resistant rocks such as granite

21. Fault Scarps
Fault movements associated with earthquakes can produce areas of great vertical offset where the fault intersects the ground surface – called fault scarps.

22. Tsunami
Another type of earthquake hazard is a tsunami – a large ocean wave generated by vertical motions of the seafloor during an earthquake
These motions displace the entire column of water overlying the fault, creating bulges and depressions in the water
The disturbance then spreads out from the epicenter in form of extremely long waves
In the open ocean, the wave height is generally less than 1m
In shallow water, the height can exceed 30 m
These enormous wave heights and the speed at which they travel make tsunamis dangerous threats to coastal cities both near and far from the quake’s epicenter.

23. Seismic Risk Most earthquakes occur in areas called seismic belts
The probability of future quakes is much greater in these belts than elsewhere around the globe
Past seismic activity is a reliable indicator of future earthquakes and can be used to generate seismic-risk maps
Alaska, Hawaii and some western states are at high seismic risk
Some central and eastern states are at risk as well
These regions have suffered disastrous earthquakes in the past and probably will again in the future.

24. Earthquake Prediction
To minimize the damage and deaths caused by quakes, seismologists are searching for ways to predict these events
Prediction is largely based on probability studies
Probability of an earthquake occurring is based on 2 factors:
The history of earthquakes in an area
The rate at which strain builds up in the rocks

25. Earthquake History
Earthquake recurrence rates can indicate that the fault involved ruptures repeatedly at regular intervals to generate similar quakes
For example: the earthquake recurrence rate at Parkfield, CA shows that a sequence of quakes of approx mag-6 shook the area ~ every 22 years from 1857 to 1966; this indicates a 90% probability that a major quake will rock the area within the next few decades

26. Earthquake History
Probability forecasts are also based on the location of seismic gaps – sections of active faults that haven’t experienced significant earthquakes for a long period of time
Example: a seismic gap in the San Andreas fault that cuts through San Francisco
This section of the fault hasn’t ruptured since the devastating earthquake in 1906, so seismologists predict a 67% probability that the San Francisco area will experience a mag-7 or higher quake within the next 30 years.

27. Strain Accumulation
The rate at which strain builds up in rocks is another factor used to determine the earthquake probability along a section of a fault
Two important factors in earthquake probability studies:
1. Strain accumulated in a particular part of a fault
2. How much strain was released during the last quake
Another factor is how much time has passed since an earthquake has struck that section

28. Prediction
Earthquake prediction is still a relatively new branch of geology.
Being able to predict these destructive events can prevent damage to property, possibly reduce the number of injuries as a result of the quake, and, most importantly, save many lives.