Earthquakes

Dr. Pamela Gore
Georgia Perimeter College

Objectives

1. Demonstrate an understanding of the forces that cause earthquakes.
2. Contrast the different types of faults (normal, reverse, thrust, strike-slip), and describe the type of stress that produces each.
3. Explain how plate movement relates to the formation of earthquakes.
4. Describe the world-wide distribution pattern of earthquake activity.
5. Describe internal (inside the Earth) and external (on the surface of the Earth) processes associated with earthquakes.
6. Discuss the principle of the seismograph.
7. List and contrast the basic types of earthquake waves (body waves - P & S, surface waves - Love & Rayleigh).
8. Distinguish between the focus and epicenter of an earthquake.
9. Explain the earthquake magnitude scale and the Modified Mercalli scale of earthquake intensity.
10. Explain how the epicenter of an earthquake can be located.
11. Describe the effects of earthquake activity.
12. Describe the forces that cause the formation of tsunamis.

Earthquakes

A fault is a crack or fracture in the Earth's crust along which movement has occurred. Movement along a fault produces earthquakes and seismic waves.

Animation by Charlie Watson, Seismo-Watch

An earthquake is the vibration of the Earth, produced by the rapid release of energy. Energy radiates out from the focus.

The focus is the place within the Earth where the rock breaks, producing an earthquake.

Energy moving outward from the focus of an earthquake travels in the form of seismic waves.

What causes the rock to break?

Stress - a force or directed pressure acting on a rock

Types of stress:

1. Compressional stress - pushing together
2. Tensional stress - pulling apart
3. Shear stress - sliding

When stresses are applied to a rock, the rock will be deformed.

When stress is applied, rocks initially respond by deforming elastically. These are reversible changes. When the stress is removed, the rock returns to its original size and shape.

Once the elastic limit of the rock is exceeded by the stress, the rock will:

1. Deform plastically, resulting in permanent changes in size and shape (such as folding), or
2. Fracture (i.e., break).

Brittle materials break during elastic deformation.

Rocks will deform plastically and will fold under conditions of high temperatures and pressures, like those deep within the crust of the Earth.

Rocks nearer the surface of the Earth, where temperatures and pressures are lower, rocks will behave like a brittle solid and will fracture once their elastic limit is exceeded. This produces a fracture or fault.

Compressional stress leads to the folding of rocks, reverse faults, and thrust faults (see types of faults, below).

Tensional stress leads to the formation of normal faults . Shear stress leads to the formation of strike-slip faults .

Types of faults

1. Dip-slip faults
   Movement along dip-slip faults is vertical; one side moves up and the other side moves down.
   1. Normal fault (hanging wall moves down with respect to the footwall). Forms due to tensional stress.
   2. Reverse fault (hanging wall moves up with respect to the footwall). Forms due to compressional stress.
      1. Thrust fault - a low angle reverse fault. Associated with folded mountain belts such as the Appalachian Valley and Ridge.

   
   Types of dip-slip faults.

   
   Normal Faults. Cambrian Kinsers Formation, Thomasville Quarry, Pennsylvania

   
   Thrust fault. Pennsylvanian Crab Orchard Mountain Group, near Ozone, Tennessee, Route 40/75 eastbound.

2. Strike-slip faults
   Movement along strike-slip faults is horizontal. Forms due to shear stress.
   1. Left-lateral strike-slip fault
   2. Right-lateral strike-slip fault

   How do you tell which is which?
   

   Look across the fault to the other side.
   

   Did it move to the LEFT or to the RIGHT?

   
   Diagram of the strike-slip movement along the San Andreas Fault.

   
   A fence, near Point Reyes, California, offset 8.5 feet by displacement on the San Andreas Fault during the 1906 earthquake (photo by G.K. Gilbert).

For animated images of these types of faults, go to this web site: http://www.iris.edu/gifs/animations/faults.htm.

Seismographs

• The first seismograph was developed by the Chinese almost 2000 years ago.

• Basic components of a seismograph:
  Suspended mass hanging from a support that is attached to (and moves with) the ground.
  Inertia keeps the suspended mass stationary while the ground moves below it.
  The movement is recorded on a rotating drum or magnetic tape.
  

  

  Seismographs at the U.S. Geological Survey Hawaiian Volcano Observatory.

   
  Seismic records of Hawaiian earthquakes on seismographs at U.S. Geological Survey Hawaiian Volcano Observatory Museum.

Types of seismic waves

1. Body waves
   1. P-waves
      Primary, pressure, push-pull
      Travel fastest of the seismic waves
      Travel through solids and liquids

      

   2. S-waves
      Secondary, shaking, shear, side-to-side
      Travel through solids only

      

2. Surface waves
   Often referred to as L-waves or long waves.
   Complex motion. Up-and-down and side-to-side.
   Slowest.
   Causes damage to structures during an earthquake.

Locating an earthquake epicenter

Online activity which illustrates how seismic waves are used to determine the magnitude of an earthquake, and to locate its epicenter.

Examine the seismograph and determine the elapsed time between the arrival of the first P-wave and the first S-wave.

Use a time-distance graph (see diagram in text).
Knowing the S - P time, you can determine the distance to the epicenter from the seismic station.

On a map, draw a circle around the seismic station.
Radius of circle = distance to epicenter.

Repeat for two other seismic stations. (Triangulation.)
The three circles will meet at a point; that point is the epicenter.

Measuring Earthquakes

Modified Mercalli Scale

Measures earthquake intensity. Describes damage to structures.
Ranges from I (felt by only a few) to XII (total destruction). Intensity of an earthquake depends on:
• Distance to epicenter
• Building materials and design
• Type of ground material - soil, rock, etc.

Richter Scale

Measures earthquake magnitude.
Determined by measuring the amplitude of the largest waves on the seismogram.

A logarithmic scale.
Each number on the Richter Scale is ten times greater in wave amplitude.
And each number on the Richter Scale involves an energy release about 32 times as great.

(See table in text comparing Richter magnitudes and approximate number per year.)

More precise ways of measuring earthquakes

Ms = based on measurements of surface waves with a period of about 20 seconds
Ml = local magnitude, based on waves with period of 0.1 to 3.0 seconds
mb = magnitude of body waves
Mw = moment magnitude. total amount of seismic wave energy released in an earthquake, based on waves with a period of about 200 seconds. Measures the earthquake source.

Major Historical Earthquakes

There is a 100% chance of an earthquake today!

Table of Most Destructive Known Earthquakes on Record in the World Earthquakes with 50,000 or More Deaths

Selected large earthquakes in the U.S.

1886 Charleston, South Carolina (was felt in Atlanta area; toppled chimneys).

1906 San Francisco, CA (fires) 8.25

1964 Anchorage, AK (8.4 - 8.6).
Strongest earthquake to hit US this century.
Ground flowed. Thixotropic. Landslides.

1989 San Francisco, California (Loma Prieta) 7.1.

Locations of Earthquakes

Ninety percent of the world's earthquakes occur in specific areas that are the boundaries of the Earth's major crustal plates. Shown on the map are the epicenter locations of earthquakes of magnitude 4.5 or greater that occurred from 1978 through 1987.
Image courtesy of U.S. Geological Survey.

World seismicity, 1990-2000. Each colored dot represents an earthquake. The color of the dot corresponds to the depth of the earthquake. See colored depth scale along the right edge of the image. Shallow earthquakes are shown in orange. Image courtesy of U.S. Geological Survey.

From an examination of the map above, you can see that earthquakes seem to be concentrated in the following areas:

1. Around the Pacific Ocean rim
2. Along the mid-ocean ridge
3. In a belt from the Mediterranean area and southern Europe through the Middle East and into the Himalayas and China
4. In several belts in Asia and the southern Pacific Ocean (Indonesia, Philippines, Japan, New Zealand)
5. In eastern Africa
6. In a belt across the western, southern, and eastern part of North America
7. In the Carribbean
8. In Hawaii

Map of the Earth's tectonic plates, courtesy of U.S. Geological Survey.

Some occur far from plate edges.

We are familiar with earthquakes in California and Alaska, and in other parts of the world such as Asia, the Mediterranean area, and the Middle East, but it is important to keep in mind that earthquakes can occur in Georgia and in other parts of the "tectonically stable" Midwest, eastern and southeastern United States.

Northwestern Georgia lies within the East Tennessee Seismic Zone, and small earthquakes occur there fairly commonly.

Seismicity in the southeastern U.S.

Seismic hazard map of Georgia, courtesy of the U. S. Geological Survey.

Click here to see map of recent earthquake activity in the U.S.

Results of Earthquakes

1. Vibration of the ground
   Causes damage to structures

2. Thixotropic sediment causes ground to flow

3. Tsunami - seismic sea waves.
   Earthquakes under the ocean
   Waves travel at 500 - 950 km/hr
   Fast, high energy waves, but not tall (less than 1 m) in deep sea
   Long wave length (100 - 700 km)
   Not distinguishable in open ocean
   When they enter shallow water, they slow down, the water stacks up.
   First sign = withdrawl of water from coast
   5 - 30 min later, a BIG wall of water arrives (100 ft)
   Kills many, floods coastal cities, washes boats inland
   Tsunami Warning Center in Honolulu.

   Click here to see animated tsunami explanation 1.
   Alternate site for animated tsunami explanation 1.
   Tsunami slide show from NY Times.

4. Fires - Why? Think about broken gas lines, downed electric lines, broken water mains, etc.

5. Landslides

6. Seiches (sloshing in lakes may cause dams to break)

7. Well water levels fluctuate; 1964 Alaska caused wells in SE US to fluctuate by 3.5 m (10 ft).

Earthquake Prediction

There is no sure-fire method of predicting earthquakes. Several methods have been used to try to predict earthquakes before they occur so that lives could be saved. While we expect earthquakes to occur along major faults, particularly in the western US, people cannot be evacuated months or years in advance. Short-term prediction is needed, but so far it has been elusive.

1. Monitor to look for patterns
   Parkfield, California had earthquakes approximately every 22 years (1857, 1881, 1901, 1922, 1934, 1966). In anticipation of the next one, seismographs and electronic-laser ground-motion monitoring devises were put into place, but the expected earthquake has not yet occurred, and is long overdue. The last earthquake in Parkfield was in 1966.
2. Foreshocks - small earthquakes MAY occur before a large one
3. Ground tilt (lasers, creep meters, tilt meters, strain gauges, etc.)
4. Changes in animal behavior
5. Changes in groundwater levels and groundwater chemistry (radon, etc.)
6. Variations in electrical conductivity of the rock
7. Changes in seismic wave velocity
8. Radio emissions from the Earth - first noticed 1989 San Francisco.
9. Anomalous temperature increases using infrared satellite data. There have been several apparent successes in China using this method. Changshu earthquake, 1990; Tainan earthquake, 1991; Hualian earthquake, 1992; Dongsha Earthquake, 1992; Hualian earthquake, 1994. References: China1 - China2

Earthquake Safety

You never know when you might be in an earthquake, even in Georgia. Earthquakes in the eastern and central US tend to be felt over much larger areas than those in western North America. Remember that the 1886 Charleston, SC earthquake toppled chimneys in Atlanta. There is also the potential for very large earthquakes in the New Madrid, Missouri area. Add to that any travel that you might take for business, pleasure, or military service. Many vacation spots are earthquake prone. It is best to be prepared with basic safety knowledge. Your safety and survival may depend on your knowing the proper actions to take. This could well be the most important part of this course for you one day.

Note - These comments address only earthquake safety. Signs of an impending tsunami are also critically important to note in the wake of the December 26, 2004 killer tsunami that swamped south Asia killing more than 100,000 coastal residents and vacationers: principally be alert for the rapid withdrawal of water from a coastline, and move immediately to high ground. With basic knowledge of earthquakes and tsunami, many lives could have been saved.

Things to do before an earthquake occurs

1. Check for hazards in the home.
   1. Fasten shelves securely to walls.
   2. Place large or heavy objects on lower shelves.
   3. Store breakable items, such as glassware and china, in low, closed cabinets with latches.
   4. Do not hang heavy items such as pictures and mirrors above beds or seating areas.
   5. Repair potential fire risks such as defective electrical wiring and leaky gas connections.
   6. Secure water heaters by strapping them to wall studs and bolting them to the floor. If they turn over during an earthquake there may be ruptured gas lines, which can contribute to fire, as well as the potential for a large quantity of scalding water to be released, causing injury.
   7. Repair cracks in ceilings or foundations. Get expert advice if there are signs of structural defects.
   8. Store poisons such as weed killers or pesticides, and flammable products securely in closed cabinets with latches and on bottom shelves.
   9. Consult a professional to find out additional ways you can protect your home, such as bolting the house to its foundation and other structural mitigation techniques.
   10. Take a First Aid-CPR class from your local Red Cross chapter, and keep your training current.
   11. Get training in how to use a fire extinguisher from your local fire department.
2. Identify safe places in each room to be during an earthquake.
   1. Under sturdy furniture such as a heavy desk or table.
   2. Against an inside wall.
   3. Away from glass, which could shatter (such as windows or mirrors), and away from heavy objects, which could fall (pictures, heavy bookcases or furniture, items on shelves or in un-latched cabinets).
   4. Practice "drop, cover, and hold on". Be prepared to move with the furniture if it is moving. Practice protecting your eyes by pressing your face against your arm.
3. Identify safe places outdoors.
   In the open, away from buildings, trees, telephone and electrical lines, overpasses, elevated expressways, or parking garages.
4. Make sure all family members know how to respond after an earthquake.
   1. Teach all family members how to turn off gas, electricity, and water. (Where is the shut-off valve or fuse box/breaker box? Is a special wrench or tool needed? If so, where is it kept?)
   2. Teach children how and when to call 9-1-1 (or your local emergency number for police, or fire department).
   3. Learn which radio stations to tune to for emergency information (AM? FM?). Have a battery-powered radio on hand at home and at work, in case of power outages.
   4. Inform babysitters and caregivers of your plan.
5. Have disaster supplies on hand.
   1. Flashlight and extra batteries and bulbs of the proper size.
   2. Battery-operated radio and extra batteries
   3. First aid kit and manual
   4. Non-perishable food that does not require cooking (canned food)
   5. Bottled water (at least 3 gallons per person).
   6. Hand-operated can opener (not electric)
   7. Essential medicines
   8. Cash and credit cards
   9. Sturdy shoes or boots (you may have to walk over broken glass and concrete or rough terrane)
   10. Develop an emergency communication plan in case family members are separated during an earthquake (when adults are at work and children are at school). Have an out-of-town friend or relative serve as the "family contact." After a disaster, it may be easier to call long distance than locally in the disaster area. Make sure that everyone in the family knows the name, address, and phone number of the contact person.
   11. Develop a plan for reuniting after the disaster. Where will you meet?

Things to do during an earthquake

1. Take cover under a piece of sturdy or heavy furniture, hold on, and move with the furniture if it moves.
2. If no sturdy furniture is available, stand against an inside wall and hold on.
3. Shield your eyes from flying glass by pressing your arm over your eyes.
4. Stay inside. Breaking window glass, falling bricks and building stone or ornamentation, falling trees, falling electrical lines, etc. make it hazardous to be outside during an earthquake.
5. If you are in an office building or a high-rise building, expect the fire alarms and sprinklers to go off during a quake.
6. If you are in bed, hold on and stay there, protecting your head with a pillow.
7. If you are outdoors, move into the open, away from buildings, street lights, trees, and utility wires. Stay there until the shaking stops.
8. If you are in a moving vehicle, move to a clear area away from buildings, trees, overpasses, or utility wires. Stop quickly and stay in the vehicle. Once the shaking has stopped, proceed with caution. Avoid roads, bridges or ramps that might have been damaged by the quake.

Things to do after an earthquake

1. Check the scene. Check yourself and others for injuries. Look for any unsafe situations or active hazards such as fires, downed power lines, sparks, leaking gas, unstable structures, frightened animals, moving vehicles, etc. Protect yourself from further danger by putting on long pants, a long-sleeved shirt, sturdy shoes, and work gloves.
   1. Check for gas leaks--If you smell gas or hear a hissing noise, open a window and quickly leave the building. Do not turn on or off any electrical devices or cell phones while in an area with leaking gas. Turn off the gas at the outside main valve if you can and call the gas company from a neighbor's home, or using your cell phone outside. If you turn off the gas, it must be turned back on by a professional.
   2. Check for electrical system damage - sparks, broken or frayed wires, fallen electrical lines outside, or if you smell hot insulation. Turn off the electricity at the main fuse box or circuit breaker. If you have to step in water to get to the fuse box or circuit breaker, call an electrician first for advice.
   3. Check walls, chimneys, ceilings, pictures or mirrors on wall, bookcases, etc. to see if they are in danger of falling and injuring someone. Stay out of damaged buildings.
   4. Watch pets or animals in the vicinity closely. The behavior of pets may change dramatically after an earthquake. Normally quiet and friendly cats and dogs may become aggressive or defensive. Tie dogs or place them in a fenced yard.
   5. Call for help.
2. Care for injured or trapped persons. Give first aid where appropriate. Do not move seriously injured persons unless they are in immediate danger of further injury.
3. Check with your neighbors to see if anyone is injured or trapped. Help anyone who may require assistance, particularly the elderly, children, people with infants, and those with disabilities.
4. Be prepared for aftershocks. Aftershocks may cause additional damage and may bring weakened structures down. Aftershocks may occur hours, days, weeks, or even months after the quake.
5. Check for sewage and water lines damage. If water pipes are damaged, avoid using water from the faucets. Drink bottled water or melt ice cubes. Contact the water company. If you suspect sewage lines are damaged, avoid using the toilets and call a plumber.
6. If the power is out, listen to a battery-operated radio or television for instructions and the latest emergency information.
7. Use the telephone to report life-threatening emergencies only.
8. Clean up spilled medicines, bleaches or gasoline or other flammable liquids immediately. Leave the area if you smell gas or fumes from other chemicals.
9. Open closet and cupboard doors cautiously because contents may have shifted.

Earthquakes for Kids

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Page created by Pamela J.W. Gore
Georgia Perimeter College,
Clarkston, GA

Page created March 4 - 9, 2005
Modified July 5, 2007
Updated May 16, 2008