Download EARTHQUAKES AND PLATE TECTONICS

Earthquakes
HOW AND WHERE
EARTHQUAKES HAPPEN
Why Earthquakes Happen?
Rocks along faults pressed tightly together.
 Stress occurs but friction prevents them from
moving – fault is said to be locked.
 Rocks under stress suddenly shift along a fault
 Fault – break in body of rock where one block
slides relative to another
 When rocks along a fault move, a sudden
release of energy occurs causing movements
on the ground...

….This is an
Earthquake
FOCUS

Area along a fault where the slippage first
occurs

Focus is beneath the earth’s surface

The epicenter is directly above

The depth of the focus varies – 90% of
continental earthquakes have shallow focus

Shallow foci – within 70km from surface

Deep foci – subduction zones farther from
plate boundaries than shallow quakes
FOCUS DEPTH - 1975-95
FOCUS DEPTH OF
EARTHQUAKES IN
WASHINGTON 1977-1996
EPICENTER

A point on the earth’s surface directly
above the focus
What happens when rocks in
motion release energy in the
form of vibrations?
Seismic Waves
travel outward in all directions from focus
Seismic Waves

Two types of waves produced from
earthquakes :
Body waves – travel through body of
a medium
Surface waves – travel along surface
of body rather than through middle
Surface Waves
Form from motion along shallow fault or by
energy transfer from P and S waves
reaching earth’s surface.
 Slowest moving seismic waves
 Cause greatest damage at surface
 Two types : Love and Rayleigh

Love and Rayleigh Waves
Love waves – rock
moves side-to-side
and perpendicular
to wave direction
 Rayleigh waves –
move ground in an
elliptical rolling
motion

Earthquakes and Plate Tectonics

Most earthquakes are located at or near
tectonic plate boundaries
Convergent Oceanic Environments
Plates move toward one another and collide
 Subduction occurs as denser plate sinks under
overriding plate - earthquakes occur
 Oceanic-oceanic or oceanic-continental

Divergent Oceanic Environments
Plates moving away from each other and
mid-ocean ridge
 Spreading motion causes earthquakes

Continental Environments
Two continental plates converge, diverge,
or move horizontally to one another
 Rocks build up stress
 Mountains form and earthquakes occur

MAJOR EARTHQUAKE ZONES

Pacific Ring of Fire

Mid-ocean ridges

Eurasian-Melanesian Mountain Belt
MAJOR EARTHQUAKES
AREAS
Fault Zones
Intense stress along plate boundaries create
fault zones
 Plates separate, collide, subduct or slide past
one another
 Movement from stress buildup causes major
earthquakes

PUGET SOUND FAULT
ZONES
Studying Earthquakes
Ancient Chinese Method of Studying Earthquakes
Magnitude

The measure of the
strength of an
earthquake,
determined by
measuring the
amount of ground
motion.
Richter Scale

Used for most of the
20th century. Each
number represents a
factor of 30 (a 5
earthquake has 30
times the energy of a
4). We now prefer to
use the……
Moment Magnitude

Based on the size of the area of fault movement, the
average distance that the fault blocks move, and the
rigidity of the rocks in the fault zone. More accurate
for large earthquakes.
Intensity

Based on an earthquake’s
effects. The Mercalli
scale (p. 304) uses Roman
numerals I-XII. Depends
on magnitude, distance
from the epicenter, local
geology, earthquake
duration, and human
infrastructure.
Charleston, SC - 1886
I & II

Not felt except by a
very few.

Felt by a few persons
on the upper floors
of buildings.
III & IV

Felt indoors.

Hanging objects
swing.
V & VI

Felt outdoors.

Felt by all, many
scared and run
outdoors.
VII & VIII

Difficult to stand.

Damage slight in
specially designed
structures.
IX & X

Damage considerable in
specially designed
structures.

Ground cracked, rails
bent.
XI & XII

Bridges destroyed,
broad fissures in
ground.

Total destruction.
Earthquakes and Society
Effects on Humans, Property & Infrastructure
How to PREPARE for an Earthquake
MAJOR TSUNAMIS
Caused by an earthquake whose
epicenter is on or below the ocean
floor that causing:
A rapid drop or rise in the ocean
floor
OR
An underwater landslide that
displaces a large volume of water
Much Safer To Be Out In The Open

Most injuries result
from the collapse
of buildings, falling
objects, and flying
glass.
Other Dangers Include
Landslides
 Fires
 Explosions (broken electric and gas lines)
 Floods (from collapsing dams)

Ground Type Is Important

Loose soil and rock
can vibrate like
Jell-O.
KOBE SEISMOGRAPH
Top - Station on solid ground
Right - Station on water
saturated,
soft ground
LIQUEFACTION
Kobe
Quake Duration

A long duration moderate earthquake can
cause more damage than a short one at a
higher magnitude.
FIRES
Kobe
Destruction to Buildings and
Property
STRUCTURAL DAMAGE
Kobe earthquake
CHILE – 1960
Largest Recorded 9.5
ALASKA- 1964
9.2
EARTHQUAKE SAFETY
People who live near active faults should be
ready to follow a few simple earthquake
safety rules to help prevent death, injury,
and property damage
BEFORE
- Be prepared
DURING - Stay calm
AFTER - Stay cautious
Before an Earthquake
 Be
prepared. Keep an adequate supply
of food, water, batteries, flashlights and
a radio.
 Prepare
an earthquake plan and
discuss it with your family.
 Learn
how to turn off the gas, water,
and electricity in your home.
During an Earthquake
 Protect
yourself by moving to a safe
position.
 Best way is the “Triangle of Life” (next slide--)
 Indoors – get under a table or if not
possible in a inside door frame away
from heavy furniture
– get
away from tall
buildings, tunnels, power lines or
bridges out in the open
 Outdoors
Triangle of Life during a Quake



Newest research shows your first best position
to get into during a quake is next to and below
the level of a heavy object – table or furniture
Cover your head in bracing position (like shown on a plane)
Any falling debris will hit table or furniture and
arc over you and you will be protected
Bracing Under a Table
Especially at School
Earthquake Warnings and
Forecasts
Scientists study past earthquakes to
predict where future earthquakes will
occur.
There is currently no way to predict
exactly when an earthquake will occur.
Seismic Gaps – Areas along a fault where
stress has built up due to a decrease in
seismic activity recently but where strong
earthquakes occurred in the past
Don’t be Scared, Be Prepared!
3 Days, 3 Ways
Earthquake Preparedness
Starts NOW!
3days3ways.org
Mr. K will show you how starting Tuesday!