Earthquakes

Earthquakes - Transcript

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 Damage in Oakland CA 1989

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 Swaves 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



Intensity 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