1. Tension – stretches rock, they become thin in the middle

2. Compression – pushes rock together, squeezes rock until it folds or breaks

3. Shearing – masses of rock slip, they move past in opposite directions

Earthquakes

8.1 What Is an Earthquake?

• Focus is the point within Earth where the rock breaks.

• Epicenter is the location on the surface directly above the focus.

An earthquake is the shaking & trembling of rock produced by the rapid release of energy

Focus and Epicenter

• Faults are fractures in Earth where movement has occurred.

Faults

1. Normal – due to tension, fault is at an angle, at diverging boundaries, hanging wall slips down footwall

2. Reverse or thrust – due to compression, fault is at an angle, hanging wall slides up and over footwall

3. Strike-slip or transform – due to shearing, 2 plates slip past each other sideways, at transform boundaries

Normal Fault: The hanging wall has slipped down in comparison to the foot wall.

Gravity causes the hanging wall to slip down. Normal Faults are from layers being pulled apart.

Also known as a GRAVITY FAULT.

ReverseFault: The hanging wall has slipped up in comparison to the foot wall. When layers are pushed together this is the kind of fault that occurs.

Also known as a THRUST FAULT.

Strike Slip Fault: Two layers of rock are shifted horizontally or parallel to the fault plane.

  

Focus, Epicenter, and Fault

Slippage Along a Fault

1. Fault-block mountains – uplifted block of rock createdbetween 2 normal faults

2. Anticlines – upward fold = ridges

3. Synclines – downward fold = valleys

Fault-block mountains in Juniper Canyon and Yosemite

Cause of Earthquakes

8.1 What Is an Earthquake?

Elastic Rebound Hypothesis• Most earthquakes are produced by the rapid

release of elastic energy stored in rock that has been subjected to great forces.

• When the strength of the rock is exceeded, it suddenly breaks, causing the vibrations of an earthquake.

Elastic Rebound Hypothesis

Earthquake Waves

8.2 Measuring Earthquakes

• P waves

- Have the greatest velocity of all earthquake waves

- a.k.a. primary waves

- Are push-pull waves that push (compress) and pull (expand) in the direction that the waves travel

- Travel through solids, liquids, and gases

http://www.jclahr.com/science/earth_science/tabletop/pslnkmv.html#

Earthquake Waves

8.2 Measuring Earthquakes

• S waves

- Seismic waves that travel along Earth’s outer layer

- a.k.a. secondary waves

- Slower velocity than P waves

- Shake particles at right angles to the direction that they travel or back & forth

- Travel only through solids

http://www.jclahr.com/science/earth_science/tabletop/sslnkmv.html#

http://sunshine.chpc.utah.edu/labs/seismic/index.htm

Surface waves are seismic waves that travel along Earth’s outer layer.-Make most severe ground movements.

http://jclahr.com/alaska/aeic/waves/index.htmlhttp://www.classzone.com/books/earth_science/terc/content/visualizations/es1002/es1002page01.cfm

Seismic Waves

Earthquake Waves

8.2 Measuring Earthquakes

Seismographs are instruments that record earthquake waves.

Seismograms are the drawn records

Seismograph

Seismogram

Measuring Earthquakes

8.2 Measuring Earthquakes

Mercalli Scale• Rates damage at a given place• Uses Roman numerals 1-12

Richter Scale

• Does not estimate adequately the size of very large earthquakes

• Based on the amplitude of the largest seismic wave

• Each unit equates to roughly a 32-fold energy increase

Modified Mercalli Scale

Intensity Verbal Description Magnitude Witness Observations

I Instrumental 1 to 2 Detected only by seismographs

II Feeble 2 to 3 Noticed only by sensitive people

III Slight 3 to 4 Resembling vibrations caused by heavy traffic

IV Moderate 4Felt by people walking; rocking of free standing

objects

V Rather Strong 4 to 5 Sleepers awakened and bells ring

VI Strong 5 to 6 Trees sway, some damage from overturning and

falling object

VII Very Strong 6 General alarm, cracking of walls

VIII Destructive 6 to 7 Chimneys fall and there is some damage to buildings

IX Ruinous 7Ground begins to crack, houses begin to collapse

and pipes break

X Disasterous 7 to 8 Ground badly cracked and many buildings are

destroyed.There are some landslides

XI Very Disasterous 8Few buildings remain standing; bridges and railways destroyed;water, gas, electricity and telephones out

of action.

XII Catastrophic 8 or greater Total destruction; objects are thrown into the

air,much heaving,shaking and distortion of the ground

Modified Mercalli Scale

Richter scale no. No. of earthquakes per year

Typical effects of this magnitude

< 3.4 800 000 Detected only by seismometers

3.5 - 4.2 30 000 Just about noticeable indoors

4.3 - 4.8 4 800 Most people notice them, windows rattle.

4.9 - 5.4 1400 Everyone notices them, dishes may break, open doors swing.

5.5 - 6.1 500 Slight damage to buildings, plaster cracks, bricks fall.

6.2  6.9 100 Much damage to buildings: chimneys fall, houses move on foundations.

7.0 - 7.3 15 Serious damage: bridges twist, walls fracture, buildings may collapse.

7.4 - 7.9 4 Great damage, most buildings collapse.

> 8.0 One every 5 to 10 years

Total damage, surface waves seen, objects thrown in the air.

Measuring Earthquakes

8.2 Measuring Earthquakes

Moment Magnitude Scale• Derived from the amount of displacement

that occurs along the fault zone

• most widely used measurement for earthquakes because it estimates the energy released by earthquakes.

• Scale ranges from 0-10+ • Measures very large earthquakes

Earthquake Magnitudes

Some Notable Earthquakes

2009 Sichuan, China – 70,000 dead, 10 million homeless

1994 - magnitude 6.7 Northridge Earthquake

Port au Prince, Haiti, Jan. 2010

1. Shaking – landslides, avalances, building & bridge topples, gas &water mains break

2. Liquefaction – loose, soft soil turns intoLiquid mud, buildings fall & sink

3. Aftershocks – smaller eqs that occur after the larger one

4. Tsunami – giant wave created by displaced water

Earthquake Dangers

Liquefaction http://earthquake.usgs.gov/learn/animations/animation.php?flash_title=Liquefaction+Flash+Animation&flash_file=liquefaction&flash_width=450&flash_height=200Earthquake animationhttp://www.pbs.org/wnet/savageearth/animations/earthquakes/index.html

Haiti clip http://www.youtube.com/watch?v=HgQd0K5W0vI

Photo galleryhttp://www.chiff.com/science/earthquake-pictures.htm

Loma Prietahttp://www.vibrationdata.com/earthquakes/lomaprieta.htm

Transamerica Building in San Francisco, CA

Building design to minimize EQ damage

Locating an Earthquake

8.2 Measuring Earthquakes

Earthquake Distance

• Travel-time graphs from three or more seismographs can be used to find the exact location of an earthquake epicenter.

• The epicenter is located using the difference in the arrival times between P and S wave

recordings, which are related to distance.

• About 95 percent of the major earthquakes occur in a few narrow zones.

Earthquake Direction

Earthquake Zones

Locating an Earthquake

Seismic Vibrations

8.3 Destruction from Earthquakes

The damage to buildings and other structures from earthquake waves depends on several factors. These factors include the intensity and duration of the vibrations, the nature of the material on which the structure is built, and the design of the structure.

Earthquake Damage

Seismic Vibrations

8.3 Destruction from Earthquakes

Building Design

- The design of the structure

- Unreinforced stone or brick buildings are the most serious safety threats

- Nature of the material upon which the structure rests

• Factors that determine structural damage

- Intensity of the earthquake

Seismic Vibrations

8.3 Destruction from Earthquakes

Liquefaction• Saturated material turns fluid

• Underground objects may float to surface

Effects of Subsidence Due to Liquefaction

Tsunamis

8.3 Destruction from Earthquakes

Cause of Tsunamis• A tsunami triggered by an earthquake occurs

where a slab of the ocean floor is displaced vertically along a fault.

• A tsunami also can occur when the vibration of a quake sets an underwater landslide into motion.

• Tsunami is the Japanese word for “seismic sea wave.”

Movement of a Tsunami

Tsunamis

8.3 Destruction from Earthquakes

• Large earthquakes are reported to Hawaii from Pacific seismic stations.

Tsunami Warning System

• Although tsunamis travel quickly, there is sufficient time to evacuate all but the area closest to the epicenter.

Other Dangers

8.3 Destruction from Earthquakes

• With many earthquakes, the greatest damage to structures is from landslides and ground subsidence, or the sinking of the ground triggered by vibrations.

Landslides

• In the San Francisco earthquake of 1906, most of the destruction was caused by fires that started when gas and electrical lines were cut.

Fire

Landslide Damage

Discovering Earth’s Layers

8.4 Earth’s Layered Structure

• Velocity of seismic waves increases abruptly below 50 km of depth

• Separates crust from underlying mantle

Shadow Zone • Absence of P waves from about 105 degrees to

140 degrees around the globe from an earthquake

• Can be explained if Earth contains a core composed of materials unlike the overlying mantle

Moho ˇ ´

Earth’s Interior Showing P and S Wave Paths