Transcript Ch. 11 Earthquakes

Earthquakes
Chapter 12
What’s an Earthquake?



Earthquakes are movements of the ground
that are caused by a sudden release of
energy when along a fault move.
Occur when rocks under stress shift
suddenly along a fault.
Stress is a force that can change the size
& shape of rocks.
Elastic Rebound Theory




This theory states --- sudden return of elastically
deformed rock to its undeformed shape.
If the fault is locked, stress increases.
When stress reaches passed a certain point, the
rocks fracture, separate at weakest point, and
spring back or rebound, to original shape.
As they fracture and slip the rocks along the
fault release energy in the form of an
earthquake.
Anatomy of an Earthquake



Focus --- The area along the fault where
slippage first occurs.
Epicenter – The point on the Earth’s
surface directly above the focus.
2 main types of seismic waves:


Body waves – travel through the body of a
medium.
Surface waves – travel along the surface of a
body rather than through the middle.
Types of Body Waves
1.
2.
Primary waves or P waves – move the
fastest; first to be recorded; can travel
through solids and liquids; cause rock
particles to move together and apart along
the direction of waves.
Secondary waves or S waves --- second to
be recorded; only travel through solids;
cause rock particles to move at right angles
to the direction in which waves are traveling.
Cause the greatest damage.
2 Types of Surface Waves
1.
2.
Surface waves or
Love waves or L
waves – cause rock
to move side to
side &
Perpendicular to
direction waves are
traveling.
Rayleigh waves -cause the ground
to move with an
elliptical, rolling
motion.
Major Earthquake Zones



Most earthquakes occur along or near edges of
the earth’s lithospheric plates
Stress is greatest along these moving plate
boundaries.
Earth has 3 major earthquake zones:
1.
2.
3.
Convergent Oceanic environments – move toward each
other and collide; as the plates move, the overriding
plate scrapes across the top of the subducting plate.
Divergent Oceanic environments – Make up the midocean ridges, plates are moving away from each other.
Spreading causes earthquakes.
Continental environments – 2 continental plates
converge, diverge, or move horizontally in opposite
directions.
Fault zones --- Form at plate boundaries because of
intense stress that results when plates separate,
collide, subduct, or slide past each other.
Recording Earthquakes


Seismograph – records & detects vibrations in
the ground.
Consists of 3 separate devices:
1.
2.

One device records the vertical motion of the
ground.
The other 2 devices record horizontal motion in the
east-west and north-south.
The seismograph records motion by tracing
wave-shaped lines on paper and translating
the motion into electronic signals (known as a
seismogram).
Measuring an Earthquake



Richter scale – measures the ground motion
from an earthquake to find its strength.
Moment magnitude is a measurement of a
quakes strength based on size of area the fault
moves, the average distance that the fault
blocks move, and the rigidity of the rocks in the
fault zone.
Mercalli scale – expresses the intensity of an
earthquake or the amount of damage it causes.
Expressed by a roman numeral and a
description.