Journal #60
What are earthquakes?
Why do earthquakes occur?
Earthquakes
Earthquakes are one of the most destructivenatural disasters.
Earthquakes- defined as movements of theground that are caused by a sudden release ofenergy when rocks along a fault move.
Earthquakes usually occur when rocks understress suddenly shift along a fault.
A fault is a break in a body of rock alongwhich one block slides relative to another.
Why Do Earthquakes happen?
The rocks along both sides of a fault are commonlypressed together tightly.
Friction prevents them from moving past eachother.
In this immobile state, a fault is said to be locked.
Parts of fault remain locked until the stressbecomes so great they suddenly move past eachother.
This slippage causes trembling and vibrations ofan earthquake.
Elastic Rebound (pg 295)
Earthquakes are a result of elastic rebound.
Elastic rebound is the sudden return of elasticallydeformed rock to its undeformed shape.
The rocks on each side of a fault are movingslowly.
If the fault is locked, stress in the rocks increases.
When the rocks are stress passed a point, theyfracture at their weakest point and rebound(spring back) to their original shape.
Anatomy of an Earthquake
Focus- where the first motion of an earthquakeoccurs
The point on the Earth’s surface directly above thefocus is called the epicenter
Anatomy of an Earthquake
Earthquakes that usually cause the mostdamage usually have a shallow focus.
Earthquakes that have a deep focus, most oftheir energy dissipates before thevibrations reach the surface.
Seismic Waves
As rocks along a fault slip into new positions, the rocksrelease energy in the form of vibrations called SeismicWaves.
These waves travel outward in all directions from thefocus.
Similar to when you drop a stone into a pool of stillwater.
Eathquakes produce 2 main types of waves:
Body waves- travel through the body of a medium
Surface waves- travel along the surface of a body ratherthan through the middle.
Body Waves
P Waves- (Primary waves or compression waves)
Are the fastest seismic waves
The first waves of an earthquake to be detected
Cause particles of rock to move in a back andforth motion that is parallel to the direction inwhich the waves are traveling.
Move through solids, liquids and gases.
The more rigid the material is, the faster the Pwaves travel through it.
Body Waves
S Waves- (Secondary or Shear waves)
Second-fastest waves and arrive at detection sites after Pwaves
Cause particles of rocks to move in a side-to-sidedirection that is perpendicular to the direction inwhich the waves are traveling
S waves can only travel through solids
Surface Waves
Surface waves form from motion along a shallow faultwhen P and S waves reach the surface
Although surface waves are the slowest moving waves,they may cause the greatest damage during anearthquake.
Love waves- move rock side to side and perpendicular
Rayleigh waves- cause the ground to move in anelliptical, rolling motion.
Seismic Waves and Earth’s Interior
The composition of the material through which Pwaves and S waves travel affects the speed anddirection of the waves.
Example: P waves travel fastest through materials thatare very rigid and are not easily compressed.
By studying the speed and direction of seismic waves,scientists can learn more about the makeup andstructure of the Earth’s Interior.
Shadow Zones
Shadow Zones- are locations on Earth’s surfacewhere no body waves from an earthquake can bedetected.
They exist because the materials that make upEarth’s interior are not uniform in rigidity.
When seismic waves travel through materials ofdiffering rigidities, the speed of the waves changedirection as they pass through different materials.
Earthquakes and Plate Tectonics
Earthquakes are the result of stresses in Earth’slithosphere.
Most earthquakes occur in 3 main tectonicenvironments
These are generally located at or near tectonicplates boundaries, where stress on the rock isgreatest.
Convergent-Oceanic Environments
At convergent plate boundaries, plates movetoward one another and collide.
The plate that is denser subducts.
As the plates move, the overriding plate scrapesacross the tope of the subducting plate, andearthquakes occur.
This can occur between continental and oceanicplates
Divergent Oceanic Environments
At the divergent plate boundaries that make up themid ocean ridges, plates are moving away fromeach other.
This spreading motion causes earthquakes alongthe ocean ridges.
Continental Environments
Earthquakes occur at locations where 2continental plates converge, diverge or movehorizontally.
The rock surrounding the boundary experiencesstress.
The stress may cause mountains to form and alsocauses frequent earthquakes.
Fault Zones
Fault zones form at plate boundaries because of theintense stress that results when the plates separate,collide, subduct or slide past each other.
Ex: North Anatolian fault zone
When enough stress is built up, movement occursalong one or more of the individual faults in the faultzone and sometimes causes major earthquakes.
Earthquakes away from plateboundaries
Not all earthquakes result from movement alongplate boundaries
Some faults were formed millions of years agowithin continental crust
Ex: New madrid, Missouri in 1811
Your Assignment:Vocabulary Squares
Earthquake
Elastic rebound
Focus
Epicenter
Body wave
Surface wave
P wave
S wave
Shadow zone
Fault zone
* you do not have to give examples of the ones in bold