Earthquakes

How and Where Earthquakes Occur

PPT Modified from Troy HS

Is there such thing as

“earthquake weather?” Absolutely NOT!!!

Geologists believe that there is no connection between

weather and earthquakes. They are the result of geologic

processes within the earth and can happen in any weather and

at any time during the year. Earthquakes originate miles

underground. Wind, precipitation, temperature, and barometric

pressure changes affect only the surface and shallow

subsurface of the Earth. Earthquakes are focused at depths

well out of the reach of weather, and the forces that cause

earthquakes are much larger than the weather forces.

Earthquakes occur in all types of weather, in all climate zones,

in all seasons of the year, and at any time of day.

When do earthquakes occur?

• Earthquakes happen

once every 10 seconds

on Earth.

• About 3 million

earthquakes happen per

year.

• On average there is one

8 magnitude, 10 7

magnitudes, 100 6

magnitudes and so on

per year.

– Most earthquakes are

of very low

magnitudes

What is an earthquake?

• Movement or trembling of the ground that

is caused by a sudden release of energy

when rocks along faults move

• Reasons Earthquakes happen:

– Plates Moving

– Faults (cracks in Earth’s surface)

– Caverns collapsing

– Meteor Impacts

– Volcanic Eruptions

Where do most earthquakes

occur? • Plate boundaries or

• Faults

What is a Fault?

• a fracture along which the blocks of crust

on either side have moved.

• Plate boundaries are always faults, but not

all faults are plate boundaries.

• Fault zones or systems – region of

numerous, closely spaced faults

In California earthquakes are

caused by

• The San Andreas

Fault System

– San Andreas + Faults

= EARTHQUAKES

Three Types of Faults

• Strike-slip

• Dip-slip

– Normal

– Reverse

• (Thrust is a reverse fault with a small

dip angle)

Click here to see the faults in action

Strike-Slip Fault

• Also called transform faults or boundaries

• are vertical (or nearly vertical) fractures

where the blocks have mostly moved

horizontally.

• If the block opposite an observer looking

across the fault moves to the right, the slip

style is termed right lateral; if the block

moves to the left, the motion is termed left

lateral.

Dip-Slip Faults

• Dip-slip faults are inclined fractures where

the blocks have mostly shifted vertically.

• If the rock mass above an inclined fault moves

down, the fault is termed normal

• if the rock above the fault moves up, the fault

is termed reverse.

• A thrust fault is a reverse fault with a dip of 45

degrees or less.

Dip-slip Faults Animations

San Andreas Fault In Action

Draw California and the San

Andreas

Terms used to describe an

earthquake

Epicenter

Focus

Fault

Seismic Waves

Focus Vs. Epicenter

• Focus – the location

within Earth along a

fault at which the first

motion of the

earthquake occurs

• Epicenter- point on

Earth’s surface directly

above an earthquake’s

starting point, or focus

How does energy travel in an

earthquake?

• Energy travels in waves from the focus

• The focus is the part of the fault that

moved where energy was released

• The epicenter is located directly above

the focus on the surface

Elastic Rebound Theory

• The elastic rebound theory is an explanation for how energy is spread during earthquakes.

• As plates on opposite sides of a fault are subjected to force and shift, they accumulate energy and slowly deform until their internal strength is exceeded.

• At that time, a sudden movement occurs along the fault, releasing the accumulated energy, and the rocks snap back to their original undeformed shape.

How does energy travel in an earthquake?

Waves

When P and S waves reach the

surface they make surface waves

Body Waves Surface Waves

Travel Through

the Earth

P Waves

S Waves

Raleigh Waves

Love Waves

Travel Through

the Earth

P Waves

Primary Waves

Reach an area first Can travel through

solid, liquid and air

Compress

and Expand

Body Waves

Travel Through

the Earth

S Waves

Shear Waves

Reach an area

second

Can only travel

through

solid

Move at right

angles to the

direction they

are traveling

Body Waves

P-Waves Vs. S-Waves

P-Waves (Compression)

S-Waves (Right angles)

Interior Of Earth

Interior Of Earth

Surface Waves

Raleigh

Love

Rolling Waves Jolting Waves

Like a ripple in a pond Move at right angles

Travel Along the

Surface

Love Waves Vs. Rayleigh Waves

Surface

Surface

MythBusters

• https://www.youtube.com/watch?v=SsABn

1zH_Js

Locating and Measuring Earthquakes

Loma Prieta, Ca 1989

Kobe, Japan 1995

How are earthquakes detected? • Earthquakes are detected

by seismographs (detects

and records EQ)

• Seismographs record the

information on a

seismogram (the EQ

recording)

• Seismology is the study of

earthquakes and their

seismic waves

Locating the Epicenter

In order to determine the location of an earthquake, the earthquake needs to be recorded on three different seismographs that are at significantly different locations.

The other piece of information needed is the time it takes for P-waves and S-waves to travel through the Earth and arrive at a seismographic station.

The Triangulation Method

Triangulation

A mathematical method for locating the

epicenter of an earthquake using three

or more data sets from seismic stations.

Data is collected using earthquake

monitoring instruments called

seismographs which record the seismic

waves of the earthquake.

Triangulation

If three arrival times are available at three

different seismic stations then triangulation

can be used to find the location of the focus or

epicenter and the time of occurrence of the

earthquake.

The distance between the beginning of the

first P wave and the first S wave tells you how

many seconds the waves are apart.

Triangulation

• Since the P waves travel faster than the S

waves, P waves will arrive at a given

seismograph station sooner than S waves.

• In other words, the S waves lag behind the P

waves.

• In fact, the time difference between when the

P waves arrive at a seismograph station and

when the S waves arrive at the same station

is called Time Lag.

The difference in

arrival times can tell us

the distance from the

earthquake

The amplitude of the

s-wave can be

compared with the

time to determine the

magnitude

How do you find the epicenter of an

earthquake?

• You must have at least 3 seismograms from different stations

• You determine the distance each station is by determining the lag time between the p wave and the s wave – The farther away the station the greater lag

• You draw circles out at those distances on a map

• Where the circles intersect is the location of the epicenter

** Three stations does not provide exact data the

data from many stations is compiled to determine the

exact epicenter

How is the strength of the

earthquake determined? • Intensity

– The effect of the Earthquake on the area or strength of shaking

– changes with distance and type of ground the location is on

– Measured by the Mercalli Scale

• Magnitude

– The actual energy released by the Earthquake

– This number does not change with distance

– Measured by the Richter Scale

How does the Richter Scale show

magnitude?

• Every number on the Richter scale shows an increase of 31 times in energy released

• What is the difference in energy between a 3 and a 4?

– 31 times

• What is the difference between a 7 and a 9?

– 7 8 9 - 31 x’s 31 = 961 times more powerful!

31x 31

Bigger Faults Make Bigger Earthquakes

1

10

100

1000

5.5 6 6.5 7 7.5Magnitude

Kilo

mete

rs

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