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Earth’s Interior Processes
Ch. 15 - Earthquakes & Plate
Tectonics
Earthquakes
What are earthquakes?
Earthquakes
What are earthquakes?
• Shaking of Earth’s crust caused by a release of
energy due to movement of lithospheric
plates
• Usually associated with faulting or breaking of
underground rocks
• Continuing adjustment of position results in
aftershocks
Earthquakes
• Focus: The point within the earth where the
first movement occurs during an earthquake
• Epicenter: The point on the Earth’s surface
directly above the focus (this is usually the
location reported in the media)
Earthquakes
Causes of earthquakes:
Earthquakes can occur due to many reasons:
– Volcanic eruption:
• Earthquakes can occur as rock under the earth is
moving to fill in spaces where magma is no longer
present
– Meteor impact:
• Very rarely a great meteorite will hit, causing the
ground to shake
– However most earthquakes are triggered by the
movement of lithospheric (tectonic) plates
Earthquakes
• Most of the time, friction between
the plates prevents movement
along the plate boundary.
• However, this causes strain to build
up and and cause the plates
deform or change shape
• Eventually, strain becomes great
enough to overcome frictional
forces
 Plates move suddenly, and
snap back to their original
shape but at a new location
creating an earthquake
This explanation for the cause
of earthquakes is called the
Elastic Rebound Theory
Earthquakes
Seismic Waves
• Earthquakes release energy in waves called
seismic waves.
• There are two types of seismic waves
Earthquakes
Seismic Waves
Two types of seismic waves:
1. Body waves
– travel from the focus of an
earthquake through the Earth’s
interior
2. Surface waves
– travel along the Earth’s surface
(on the ground)
Earthquakes
Seismic Waves:
Two types of BODY waves
a) Primary Waves (P-waves):
– Squeeze and stretch rocks parallel to the direction of
travel
– Also called compressional waves
– Travel through solids, rocks, and liquids
– Travel with a high velocity (5-8 km/s)
Rocks & Minerals
Seismic Waves:
Two types of BODY waves
b. Secondary waves (S-waves)
– Move rocks perpendicular (90 degree angle) to the
direction of wave movement
– Much slower than P-waves (3-6 km/s)
– Travels through solids but not liquids and gases
Rocks & Minerals
• P and S waves also help us understand the
Earth’s interior
• Both waves speed up when travelling through
more dense material and slow down through
less dense material
Earthquakes
• At 2900km deep, P waves slow down
and S waves stop
 At the core-mantle boundary
 Remember: S waves can’t travel
through liquid (outer core)
• At 5200km deep, P wave velocity
increases
 At the outer-inner core boundary
 P waves move faster through solid
than liquid
 Therefore the inner core must be
solid
Earthquakes
• When P waves cross from one layer into
another, they are refracted
• This causes a “shadow zone”, where
seismograph stations can’t detect P or S waves
from an earthquake
Earthquakes
Seismic Waves:
Two types of SURFACE waves:
a) Love waves
travel from side
to side
b) Rayleigh waves
travel in elliptical
patterns
Earthquakes
Locating an Earthquake:
• A seismograph is an instrument that detects
and record earthquake waves onto sheets
• This produces a record of seismic activity
called a seismogram
More about this next class
For now, see p. 274
Earthquakes