Earthquakes

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Slide 10

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.

Slide 11

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

Slide 12

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

Slide 13

How are the Size and Strength of an Earthquake Measured?

Modified Mercalli Intensity Map

1994 Northridge, CA earthquake, magnitude 6.7

Intensity

subjective measure of the kind of damage done and people’s reactions to it

isoseismal lines identify areas of equal intensity

Slide 14

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 logarithic scale

Slide 15

What are the Destructive Effects of Earthquakes?

Ground Shaking

amplitude, duration, and damage increases in poorly consolidated rocks

Slide 16

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

Slide 17

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

Slide 18

Can Earthquakes be Controlled?