What are Earthquakes

What are Earthquakes?

The shaking or trembling caused by the sudden release of energy
Usually associated with faulting or breaking of rocks
Continuing adjustment of position results in aftershocks

What is the Elastic Rebound Theory?

Explains how energy is stored in rocks
Rocks bend until the strength of the rock is exceeded
Rupture occurs and the rocks quickly rebound to an undeformed shape
Energy is released in waves that radiate outward from the fault

The Focus and Epicenter of an Earthquake

The point within Earth where faulting begins is the focus, or hypocenter
The point directly above the focus on the surface is the epicenter

Seismographs record earthquake events

 At convergent boundaries, focal depth increases along a dipping seismic zone called a Benioff zone

Where Do Earthquakes Occur and How Often?

~80% of all earthquakes occur in the circum-Pacific belt
most of these result from convergent margin activity
~15% occur in the Mediterranean-Asiatic belt
remaining 5% occur in the interiors of plates and on spreading ridge centers
more than 150,000 quakes strong enough to be felt are recorded each year

The Economics and Societal Impacts of EQs

Building collapse
Fire
 Tsunami
Ground failure

What are Seismic Waves?

Response of material to the arrival of energy fronts released by rupture
Two types:
 Body waves
P and S
Surface waves
R and L

Body Waves: P and S waves

Body waves
P or primary waves
fastest waves
travel through solids, liquids, or gases
compressional wave, material movement is in the same direction as wave movement
S or secondary waves
slower than P waves
travel through solids only
shear waves - move material perpendicular to wave movement

Surface Waves: R and L waves

Surface Waves
Travel just below or along the ground’s surface
Slower than body waves; rolling and side-to-side movement
Especially damaging to buildings

How is an Earthquake’s Epicenter Located?

     Seismic wave behavior
P waves arrive first, then S waves, then L and R
Average speeds for all these waves is known

After an earthquake, the difference in arrival times at a seismograph station can be used to calculate the distance from the seismograph to the epicenter.

How is an Earthquake’s Epicenter Located?

Time-distance graph showing the average travel times for P- and S-waves. The farther away a seismograph is from the focus of an earthquake, the longer the interval between the arrivals of the P- and S- waves

How is an Earthquake’s Epicenter Located?

Three seismograph stations are needed to locate the epicenter of an earthquake
A circle where the radius equals the distance to the epicenter is drawn
The intersection of the circles locates the epicenter

How are the Size and Strength of an Earthquake Measured?

subjective measure of the kind of damage done and people’s reactions to it
isoseismal lines identify areas of equal intensity
Modified Mercalli Intensity Map
1994 Northridge, CA earthquake, magnitude 6.7

How are the Size and Strength of an Earthquake Measured?

Magnitude
Richter scale measures total amount of energy released by an earthquake; independent of intensity
Amplitude of the largest wave produced by an event is corrected for distance and assigned a value on an open-ended logarithmic scale

What are the Destructive Effects of Earthquakes?

Ground Shaking
amplitude, duration, and damage increases in poorly consolidated rocks

Can Earthquakes be Predicted?

Earthquake Precursors
changes in elevation or tilting of land surface, fluctuations in groundwater levels, magnetic field, electrical resistance of the ground
seismic dilatancy model
seismic gaps

Can Earthquakes be Predicted?

Earthquake Prediction Programs
include laboratory and field studies of rocks before, during, and after earthquakes
monitor activity along major faults
produce risk assessments

Can Earthquakes be Controlled?

Graph showing the relationship between the amount of waste injected into wells per month and the average number of Denver earthquakes per month
Some have suggested that pumping fluids into seismic gaps will cause small earthquakes while preventing large ones