EARTHQUAKES

Chapter 8. notes

An earthquake is a trembling or shaking of the ground caused by the sudden release of energy stored in the rocks beneath the earth’s surface.

Elastic rebound theory (Reid)

1.rock with stress acting on it

2.Stress hava caused strain in the rock. Strain builds up for a long period of time, until the strain gets beyond the rock’s braking point.

3. Rocks brake suddenly, releasing energy with rock movement along a fault. They can move horizontally or vertically. Energy release happens in the form of seismic waves.

Seismic waves

 

After:http://www.geog.ouc.bc.ca/physgeog/contents/10m.html

The point within the earth where seismic waves originate is called the focus of the earthquakes. This is the center of the earthquake, the point of the initial movement along the fault.  

EPICENTER:

Is the point on the earth’s surface  directly above the focus.

There are two types of seismic waves:

1. BODY WAVES:

These waves are traveling through the earth’s interior spreading from the focus in every direction (like sound waves in air). They are not responsible for the damages on human structures.

There are two kind:

a. P WAVE (primary)

is a compressional (longitudinal) in which the wave travels parallel to the direction of wave propagation. This is a very fast wave traveling with 4-7 km/sec speed. Because it is fast usually this is the first (primary) wave to arrive to a recording station after an earthquake.

 

After: http://www.educ.uvic.ca/Faculty/jtinney/earth%20science/Earthq.html#Earthquake

S WAVE (secondary)

It is slower, Transfer wave that travels 2-5 km/ sec. An S wave is propagates by a shearing motion, much like a shaken rope.  The rock vibrates perpendicular to the wave direction.

 

After: http://www.educ.uvic.ca/Faculty/jtinney/earth%20science/Earthq.html#Earthquake

They both pass easily through solid rocks, however S wave can not pass through fluid. They are usually faster in dense rock and slower in unconsolidated sediments.

SURFACE WAVES

They travel on the surface away from the epicenter (like water waves if we throw a pebble into the water.  They are slow waves, but they cause all the damage, because they cause ground movement, and since they are slow it takes longer to pass through.

LOCATION AND MEASURING EARTHQUAKES

Seismograph

Is the equipment which are able to record of the earth motion usually in the form of wiggle line drawn on a strip of paper. 

After:http://www.geo.mtu.edu/UPSeis/locating.html

The paper record is called SEISMOGRAM.

   

After:http://www.geo.mtu.edu/UPSeis/locating.html

There is a network of seismographs throughout the earth to record and study earthquakes. (also nuclear bomb explosions)

Within minutes after an earthquake happen somewhere distant seismographs begin to pick up seismic waves. A large one would be detected everywhere on the earth.

Because of the difference in the speed of the different waves, people can locate earthquakes, and also study their strength and effects.

 

After:http://www.geo.mtu.edu/UPSeis/locating.html

Determination of the earthquake location

Seismogram tells how fare the earthquake happened, but does not give direction.

   

Depth of the earthquakes

The max. depth is 670 km. They are classified into three groups.

1. Shallow                                  0-70 km      85%

2. intermediate focus            70-350 km           12%

3. Deep focus                     350-700 km          3%

The shallow focus quakes are more common, because rocks are more brittle on the surface.

 

EARTHQUAKE STRENGTH

The strength or size of the earthquake is measured in two ways

1. INTENSITY SCALE

This is a measure of the damage caused by the earthquake on humans and buildings.

Intensities are pressed in roman numerals I-XII on the modified MERCALLI SCALE, where higher numbers indicate more damage. 

VI. everybody feels it, the are scared run 0utdoors (damaged chimneys)

VII. Damage slight. Partial collapse in weak badly designed buildings

VIII. Damage slight in well designed building, worse in poorly designed ones. so on.

XII. damage is total.

There is a problem with using this scale, because it is only measures the amount of damage caused in human structures, and there is big difference in building designs. Underlying geology is also important. Houses built on solid rocks are much better for surviving than unconsolidated sediment. 

2. MAGNITUDE SCALE

The second method is to measure the Magnitude of an earthquake (RICHTER SCALE).   In this method people calculate the energy released during the quake. People are measuring the amplitude of one of the wiggles on the seismogram (commonly used in the US). After measuring the wiggle amplitude and correcting for the distance and the specific seismograph, scientist can assign a number called MAGNITUDE.  

Richter Scale

Energy Released
in Joules

Comment

2.0

6.3 x 10 7

Smallest earthquake detectable by people.

5.0

2.0 x 10 12

Energy released by the Hiroshima atomic bomb.

6.0 - 6.9

6.3 x 1013
to
1.4 x 1015

 About 120 shallow earthquakes of this magnitude occur each year.

6.7

7.1 x 1014

 Northridge, California earthquake 1994.

7.0

2.0 x 1015

 Major earthquake.

7.4

7.9 x 1015

 Turkey earthquake August 17, 1999. More than 12,000 people killed.

7.6

1.6 x 1016

 Deadliest earthquake this century. Tangshan, China, 1976. About 250,000 people died.

8.3

1.8 x 1017

 San Francisco earthquake of 1906.

8.6

5.0 x 1017

 Most powerful earthquake recorded in the last 100 years. Southern Chile 1960. Claimed 5,700 lives.

After:http://www.geog.ouc.bc.ca/physgeog/contents/10m.html

The richter scale goes from 1 -8.6, and there is no upper limit. It is a logarithmic scale the difference between 1 and 2 is ten times. Earthquake with larger amplitude than 9 is unlikely to occur because it seems that this is the upper limit of elastic strength beyond which rocks will brake.

Erathquakes in USA    

Only a few location are earthquake free.

Most of the large earthquakes occur in the western part of the US.

EFFECT OF EARTHQUAKES

1.GROUND MOTION

This is the trembling and shaking of the land that can cause the buildings to vibrate. In large quakes the motion is visible.

Because proper building construction can reduce the damage in a great deal, building codes has to be very strict in special areas. Their location also has to be controlled. (hard rock!)

2.FIRE

Particularly serious problem after an earthquake, because of the frequently broken gas and water pipes, and fallen electrical wires. (San Francisco 1906)

3.LANDSLIDES

They can be triggered by shaking the ground (1959 Madison Canyon) 1970 Peru huge mud flows in the Andes!!

4. PERMANENT DISPLACEMENT OF THE SURFACE

5.AFTERSHOCKS

Small earthquakes following the big ones. Although they are smaller they still can cause damage in the weakened constructions.

6. TSUNAMIS

Seaquake; initiate huge wave.... (seismic sea waves) Hurricane can have a wavelength of 400 m, however a tsunami may have a wavelength of 100 miles and may be moving 450 miles/hour. The wave height near the shore can be 15-30 m.

Because of the long wavelength of the tsunamis it will not withdraw fast, but the water will rise for 5-10 minutes, causing flood.

7. LIQUEFACTION

In areas where unconsolidated sediment ids water filled, during the quake the sediment turns into liquid that is not capable to support buildings. Buildings will collapse an sewage tanks and other underground constructions will float up to the surface.

DISTRIBUTION OF EARTHQUAKES

Related to plate boundaries most of the time

EARTHQUAKE PREDICTION

-MONITORING OF SLIGHT CHANGES ALONG FAULT ZONES.

-BEFORE LARGE QUAKES, SMALL CRACKS CAN FORM ALONG THESE ZONES; THE PROPERTIES THE ROCKS CAN BE CHANGED, BECAUSE OF THE SMALL CRACKS.

-WATER LEVELS IN WELLS MAY RISE

-SURFACE OF THE EARTH TILTS AND SLIGHTLY CHANGES ELEVATION

-CHINESE SCIENTISTS CLAIM PREDICTION BASED ON ANIMAL BEHAVIORS: HORSES BECAME SKITTISH, SNAKES LIVE THEIR HOLES

(IN 1975 CHINESE PREDICTED AN EARTHQUAKE 5 HOURS BEFORE)