# Ch. 8 Earthquakes.

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Ch. 8 Earthquakes

Types of Stress 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

8.1 What Is an Earthquake? Earthquakes
 An earthquake is the shaking & trembling of rock produced by the rapid release of energy  Focus and Epicenter • Focus is the point within Earth where the rock breaks. • Epicenter is the location on the surface directly above the focus.  Faults • Faults are fractures in Earth where movement has occurred.

Kinds of faults Normal – due to tension, fault is at an angle, at diverging boundaries, hanging wall slips down footwall Reverse or thrust – due to compression, fault is at an angle, hanging wall slides up and over footwall 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

Fault-block mountains –
uplifted block of rock created between 2 normal faults Anticlines – upward fold = ridges Synclines – downward fold = valleys

Fault-block mountains in Juniper Canyon and Yosemite

8.1 What Is an Earthquake? Cause of Earthquakes
 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

8.2 Measuring Earthquakes
Earthquake Waves 8.2 Measuring Earthquakes • P waves - Are push-pull waves that push (compress) and pull (expand) in the direction that the waves travel - Travel through solids, liquids, and gases Have the greatest velocity of all earthquake waves a.k.a. primary waves

8.2 Measuring Earthquakes
Earthquake Waves 8.2 Measuring Earthquakes • S waves Seismic waves that travel along Earth’s outer layer a.k.a. secondary waves - Shake particles at right angles to the direction that they travel or back & forth - Travel only through solids - Slower velocity than P waves

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

Seismic Waves

8.2 Measuring Earthquakes
Earthquake Waves 8.2 Measuring Earthquakes  Seismographs are instruments that record earthquake waves. Seismograms are the drawn records

Seismograph

Seismogram

8.2 Measuring Earthquakes
Mercalli Scale Rates damage at a given place Uses Roman numerals 1-12  Richter Scale • Based on the amplitude of the largest seismic wave • Each unit equates to roughly a 32-fold energy increase • Does not estimate adequately the size of very large earthquakes

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 4 Felt 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 7 Ground 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 8 Few 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

No. of earthquakes per year
Richter scale no. No. of earthquakes per year Typical effects of this magnitude < 3.4 Detected only by seismometers 30 000 Just about noticeable indoors 4 800 Most people notice them, windows rattle. 1400 Everyone notices them, dishes may break, open doors swing. 500 Slight damage to buildings, plaster cracks, bricks fall. 6.2  6.9 100 Much damage to buildings: chimneys fall, houses move on foundations. 15 Serious damage: bridges twist, walls fracture, buildings may collapse. 4 Great damage, most buildings collapse. > 8.0 One every 5 to 10 years Total damage, surface waves seen, objects thrown in the air.

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
magnitude 6.7 Northridge Earthquake

Port au Prince, Haiti, Jan. 2010

Earthquake Dangers Shaking – landslides, avalances,
building & bridge topples, gas & water mains break Liquefaction – loose, soft soil turns into Liquid 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
Photo gallery Loma Prieta Earthquake animation Haiti clip

Building design to minimize EQ damage
Transamerica Building in San Francisco, CA

8.2 Measuring Earthquakes
Locating an Earthquake 8.2 Measuring Earthquakes  Earthquake Distance • The epicenter is located using the difference in the arrival times between P and S wave recordings, which are related to distance.  Earthquake Direction • Travel-time graphs from three or more seismographs can be used to find the exact location of an earthquake epicenter.  Earthquake Zones • About 95 percent of the major earthquakes occur in a few narrow zones.

Locating an Earthquake

8.3 Destruction from Earthquakes
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

8.3 Destruction from Earthquakes
Seismic Vibrations 8.3 Destruction from Earthquakes  Building Design • Factors that determine structural damage - Intensity of the earthquake - Unreinforced stone or brick buildings are the most serious safety threats - Nature of the material upon which the structure rests - The design of the structure

8.3 Destruction from Earthquakes
Seismic Vibrations 8.3 Destruction from Earthquakes  Liquefaction • Saturated material turns fluid • Underground objects may float to surface

Effects of Subsidence Due to Liquefaction

8.3 Destruction from Earthquakes
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

8.3 Destruction from Earthquakes
Tsunamis 8.3 Destruction from Earthquakes  Tsunami Warning System • Large earthquakes are reported to Hawaii from Pacific seismic stations. • Although tsunamis travel quickly, there is sufficient time to evacuate all but the area closest to the epicenter.

8.3 Destruction from Earthquakes
Other Dangers 8.3 Destruction from Earthquakes  Landslides • With many earthquakes, the greatest damage to structures is from landslides and ground subsidence, or the sinking of the ground triggered by vibrations.  Fire • In the San Francisco earthquake of 1906, most of the destruction was caused by fires that started when gas and electrical lines were cut.

Landslide Damage

8.4 Earth’s Layered Structure
Discovering Earth’s Layers 8.4 Earth’s Layered Structure  Moho ˇ • 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

Earth’s Interior Showing P and S Wave Paths