Journal #60
What are earthquakes?
Why do earthquakes occur?
Earthquakes
 Earthquakes are one of the most destructive
natural disasters.
 Earthquakes- defined as movements of the
ground that are caused by a sudden release of
energy when rocks along a fault move.
 Earthquakes usually occur when rocks under
stress suddenly shift along a fault.
 A fault is a break in a body of rock along
which one block slides relative to another.
Why Do Earthquakes happen?
 The rocks along both sides of a fault are commonly
pressed together tightly.
 Friction prevents them from moving past each
other.
 In this immobile state, a fault is said to be locked.
 Parts of fault remain locked until the stress
becomes so great they suddenly move past each
other.
 This slippage causes trembling and vibrations of
an earthquake.
Elastic Rebound (pg 295)
 Earthquakes are a result of elastic rebound.
 Elastic rebound is the sudden return of elastically
deformed rock to its undeformed shape.
 The rocks on each side of a fault are moving
slowly.
 If the fault is locked, stress in the rocks increases.
 When the rocks are stress passed a point, they
fracture at their weakest point and rebound
(spring back) to their original shape.
Anatomy of an Earthquake
 Focus- where the first motion of an earthquake
occurs
 The point on the Earth’s surface directly above the
focus is called the epicenter
Anatomy of an Earthquake
 Earthquakes that usually cause the most
damage usually have a shallow focus.
 Earthquakes that have a deep focus, most of
their energy dissipates before the
vibrations reach the surface.
Seismic Waves
 As rocks along a fault slip into new positions, the rocks
release energy in the form of vibrations called Seismic
Waves.
 These waves travel outward in all directions from the
focus.
 Similar to when you drop a stone into a pool of still
water.
 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 rather
than 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 and
forth motion that is parallel to the direction in
which the waves are traveling.
 Move through solids, liquids and gases.
 The more rigid the material is, the faster the P
waves travel through it.
Body Waves
 S Waves- (Secondary or Shear waves)
 Second-fastest waves and arrive at detection sites after P
waves
 Cause particles of rocks to move in a side-to-side
direction that is perpendicular to the direction in
which the waves are traveling
 S waves can only travel through solids
Surface Waves
 Surface waves form from motion along a shallow fault
when P and S waves reach the surface
 Although surface waves are the slowest moving waves,
they may cause the greatest damage during an
earthquake.
 Love waves- move rock side to side and perpendicular
 Rayleigh waves- cause the ground to move in an
elliptical, rolling motion.
Seismic Waves and Earth’s Interior
 The composition of the material through which P
waves and S waves travel affects the speed and
direction of the waves.
 Example: P waves travel fastest through materials that
are very rigid and are not easily compressed.
 By studying the speed and direction of seismic waves,
scientists can learn more about the makeup and
structure of the Earth’s Interior.
Shadow Zones
 Shadow Zones- are locations on Earth’s surface
where no body waves from an earthquake can be
detected.
 They exist because the materials that make up
Earth’s interior are not uniform in rigidity.
 When seismic waves travel through materials of
differing rigidities, the speed of the waves change
direction as they pass through different materials.
Earthquakes and Plate Tectonics
 Earthquakes are the result of stresses in Earth’s
lithosphere.
 Most earthquakes occur in 3 main tectonic
environments
 These are generally located at or near tectonic
plates boundaries, where stress on the rock is
greatest.
Convergent-Oceanic Environments
 At convergent plate boundaries, plates move
toward one another and collide.
 The plate that is denser subducts.
 As the plates move, the overriding plate scrapes
across the tope of the subducting plate, and
earthquakes occur.
 This can occur between continental and oceanic
plates
Divergent Oceanic Environments
 At the divergent plate boundaries that make up the
mid ocean ridges, plates are moving away from
each other.
 This spreading motion causes earthquakes along
the ocean ridges.
Continental Environments
 Earthquakes occur at locations where 2
continental plates converge, diverge or move
horizontally.
 The rock surrounding the boundary experiences
stress.
 The stress may cause mountains to form and also
causes frequent earthquakes.
Fault Zones
 Fault zones form at plate boundaries because of the
intense 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 occurs
along one or more of the individual faults in the fault
zone and sometimes causes major earthquakes.
Earthquakes away from plate
boundaries
 Not all earthquakes result from movement along
plate boundaries
 Some faults were formed millions of years ago
within 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