Earthquakes

Pamela J. W. Gore
Georgia Perimeter College

Objectives

1. Discuss the principle of the seismograph, seismometer, seismogram.
2. List and contrast the basic types of earthquake waves (body waves - P & S, surface waves - Love & Rayleigh).
3. Distinguish between the focus and epicenter of an earthquake.
4. Describe the world-wide distribution pattern of earthquake activity.
5. Explain the earthquake magnitude scale and the Modified Mercalli scale of earthquake intensity.
6. Explain how the epicenter of an earthquake can be located.
7. Describe the effects of earthquake activity.

What is an earthquake?

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.

(see diagram in text).

What causes the rock to break?

Stress - a force or directed pressure acting on a rock

Types of stress:

1. compressional
2. tensional
3. shear

Strain - the result of stress or deformation

elastic deformation - when stresses are removed, rock returns to original shape

plastic deformation - permanent deformation. when stresses are removed, rock stays bent

rupture - breakage and fracturing of the rock, causing an earthquake.

Brittle materials break during elastic deformation.

Seismographs

Movement along a fault produces earthquakes and seismic waves.

A seismograph detects seismic waves.

• 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

      

      Animation used with permission from The Tech Museum of Innovation, San Jose, CA.

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

      

      Animation used with permission from The Tech Museum of Innovation, San Jose, CA.

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.
   Types of surface waves = Love waves (L-waves) and Rayleigh waves

Locating an earthquake 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.
(see textbook).

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

1811 & 1812 - Three large earthquakes, the largest in U.S. history occurred near New Madrid, Missouri.
Rattled doors in Washington, DC and rang bells in Boston.
Was felt in Atlanta area (Indian Territory).

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 due to liquifaction of the soil. A vibration or jolt ot water-ricn or clay-rich soils caused them to flow. They became thixotropic, a term used to refer to a solid that flows when vibrated or jolted. Landslides or earthflows occurred.

1989 San Francisco, California (Loma Prieta) 7.1.

Locations of Earthquakes

• around the Pacific Ocean (circum-Pacific belt is major site of large earthquakes)
• Mediterranean Sea area through Iran and on to the Himalayas
• Asia (Indonesia, Himalayan region)
• Mid-ocean ridges

Some occur far from plate edges.

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.

4. Fires - Why?

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

1. Monitor to look for patterns
   Parkfield, California had earthquakes every 22 years
   in anticipation of the next one, seismographs were put into place, but it has not yet occurred. overdue.
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. Radio wave emissions from the Earth - first noticed 1989 San Francisco.

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

Page created March 31, 1996
Modified January 10, 1997
Modified November 25, 1998
Modified July 17, 1999
Modified Nov 1, 2000
Links updated August 15, 2009