Earthquakes

Chapter 15 Lesson 1P530-540

Vocabulary• Earthquake(531) – the vibrations in the ground that

result from movement along breaks in Earth’s lithosphere

• Fault (533) – a break in Earth’s lithosphere where one block of rock moves toward, away from, or past another

• Seismic wave (534) – energy that travels as vibrations on and in Earth

• Focus (534) – These waves originate where rocks first move along the fault, at a location inside Earth

• Epicenter (534) – the location on Earth’s surface directly above the earthquake’s focus

Vocabulary• Primary wave (535) – Also called p-waves, it causes particles in

the ground to move in a push-pull motion similar to a coiled spring

• Secondary wave (535) – Also called S-waves, it causes particles to move up and down at right angles relative to the direction the wave travels

• Surface wave (535) – causes particles in the ground to move up and down in a rolling motion

• Seismologist (536) – scientists that study earthquakes• Seismometer (537) – an instrument that measures and records

ground motion and can be used to determine the distance seismic waves travel

• Seismogram (537) – a graphical illustration of seismic waves

What are Earthquakes?• Earthquakes are the vibrations in the ground that

result from movement along breaks in Earth’s lithosphere

• These breaks are called faults• The forces that move tectonic plates also push and

pull on rocks along the fault and if these become big enough the blocks of rock on either side of the fault can move horizontally or vertically

• The greater the force the larger and more disastrous the earthquake

Where do Earthquakes Occur?

Earthquakes and Plate Boundaries• Earthquakes result from the build up and release

of stress along active plate boundaries• Some earthquakes occur more than 100km below

Earth’s surface• The deepest earthquakes occur at convergent

plate boundaries– Here the denser oceanic plate subducts into the

mantle– These earthquakes release tremendous amounts of

energy

Earthquakes and Plate Boundaries

• Shallow earthquakes occur along divergent plate boundaries, like mid-ocean ridges

• Shallow earthquakes also occur along transform boundaries

• Along continental convergent boundaries, earthquakes of varying depths occur

Rock Deformation

• When a force such as pressure is applied to rock along plate boundaries, the rock can change shape. This is called rock deformation.

• Eventually rocks can be deformed so much that they break and move.

• This is a lot like bending a stick until it breaks

Faults

• When stress builds in places like a plate boundary, rock can form faults.

• A fault is a break in Earth’s lithosphere where one block of rock moves toward, away from, or past another

• When rocks move in any direction along a fault, an earthquake occurs

• The direction depends on the force applied

Types of Faults

Reverse Fault

• Forces push two blocks of rock together.

• The rock above the fault moves up relative to the block of rock below the fault

• Occurs at convergent boundaries

Normal Fault

• Forces pull two blocks of rock apart.

• The rock above the fault moves down relative to the rock below the fault

• Occurs at divergent plate boundaries

Strike-Slip

• Two blocks of rock slide horizontally past each other in opposite directions

• Occurs at transform plate boundaries

Earthquake Focus and Epicenter• When rock moves along a fault, they release energy that

travels as vibrations on and in Earth called seismic waves• These waves originate where rocks first move along the

fault, at a location inside Earth called the focus– An earthquakes focus can occur anywhere between Earth’s

surface and depths of greater than 600km• Earthquakes are often referred to by their epicenter

– The epicenter is the location on Earth’s surface directly above the earthquake’s focus

Seismic Waves• During an earthquake, a rapid release

of energy along a fault produces seismic waves

• Seismic waves travel outward in all directions through rock– Similar to a stone being dropped in

water, seismic waves move outward in circles

– Seismic waves transfer energy through the ground and produce the motion that you feel during an earthquake

– The energy released is stronger near the epicenter and decrease in energy and intensity as you move outward

Types of Seismic Waves

Primary Waves (P-Waves)• Causes rock particles to vibrate in the same

direction that waves travel• Fastest seismic wave• First to be detected and recorded• Travels through solids and liquids

Secondary Waves (S-Waves)• Causes rock particles to vibrate perpendicular to the

direction that waves travel• Slower than P-waves, but faster than surface waves• Detected and recorded after P-waves• Only travels through solids

Surface Waves • Cause rock particles to move in a rolling or

elliptical motion in the same direction that waves travel

• Slowest seismic wave• Generally causes the most damage

Mapping Earth’s Interior

• Scientists that study earthquakes are called seismologists

• They use the properties of seismic waves to map Earth’s interior

• P-waves and S-waves change speed and direction depending on the material they travel through

Inner and Outer Core• Through extensive earthquake studies,

seismologists have discovered that S-waves cannot travel through the outer core

• This discovery proved that Earth’s outer core is liquid unlike the solid inner core

• By analyzing speed of P-waves traveling through the core, seismologists also discovered that the inner and outer cores are composed of mostly iron and nickel

The Mantle• Seismologists also have used seismic waves to

model convection currents in the mantle• The speeds of seismic waves depend on the

temperature, pressure, and chemistry of the rocks that the seismic waves travel through.

• Seismic waves tend to slow down as they travel through hot material– For example, seismic waves are slower in areas of the

mantle beneath mid-ocean ridges or near hotspots– Seismic waves are faster in cool areas of the mantle

near subduction zones

Locating an Earthquakes Epicenter• An instrument called a seismometer measures and records

ground motion and can be used to determine the distance seismic waves travel

• Ground motion is recorded as a seismogram, a graphical illustration of seismic waves

• Seismologists use a methods involving the speed and travel times of the waves to determine the distance to the earthquake epicenter from at least three different seismometers, this is called triangulation

How to Find an Epicenter?Step 1: Determine the number of seconds between the arrival of the first P-

wave and the first S-wave on the seismogram.This is called lag time.

Lag time = (arrival time of 1st S-wave) – (arrival time of 1st P-wave)

How to Find an Epicenter?

Step 2:Use a graph showing lag time versus distance.

Use the lag time to find the distance.

How to Find an Epicenter?Step 3:

Using a ruler and a map scale, measure the distance between the seismometer and the earthquakes epicenter.Draw a circle with a radius equal to the distance.When three circles are plotted, the epicenter will be where the three circles intersect.

Lab

• http://www.glencoe.com/sites/common_assets/science/virtual_labs/ES09/ES09.html

Determining Earthquake Magnitude

• Scientists use 3 different scales to measure and describe earthquakes:– The Richter Scale– The Modified Mercalli Scale– Moment magnitude scale

The Richter Scale• The Richter magnitude scale uses the amount of

ground motion at a given distance from an earthquake to determine magnitude

• It begins at zero, but there is no upper limit to the scale• Each 1 unit increase represents ten times the amount

of ground motion recorded on a seismogram.– For example, a magnitude 8 earthquake produces 10 times

greater shaking than a magnitude 7• The largest earthquake ever recorded was a magnitude

9.5 in Chile in 1960. The earthquake and following tsunamis left nearly 2000 people dead and 2 million people homeless.

Moment Magnitude Scale• Seismologists use the moment magnitude scale to measure

the total amount of energy released by the earthquake.• The energy released depends on the size of the fault that

breaks, the motion that occurs along the fault, and the strength of the rocks that break during an earthquake.

• The units in this scale are exponential• For each increase of one unit on the scale, the earthquake

releases 31.5 times more energy– That means a magnitude 8 earthquake releases more than 992

times the amount of energy than that of a magnitude 6 earthquake.

The Modified Mercalli scale• Another way to measure and describe an earthquake is

to evaluate the damage that results from shaking– Shaking is directly related to earthquake intensity

• The Modified Mercalli scale measures earthquake intensity based on descriptions of the earth’s effects on people and structures– The Mercalli scale ranges from I, when shaking is not

notice able, to XII, when everything is destroyed.• Local geology also contributes to earthquake damage.

– In an area covered by loose sediment, ground motion is exaggerated.

• http://elearning.niu.edu/simulations/images/S_portfolio/Mercalli/Mercalli_Scale.swf

*Side Notes: Roman Numerals• V=Values: X = 10 ; V = 5 ; I = 1• Add similar values that are next to one another such as III (1+1+1

= 3)• Add a smaller value that comes after a larger value, such as XV (10

+ 5 = 15)• Subtract a smaller value that precedes a larger value, such as IX

(10 – 1 = 9)• Use the fewest possible numerals to express the value (X rather

than VV)• Counting goes like

– I, II, III, IV, V, VI, VII, VIII, IX, X, XI, XII, XIII

*Practice: What is the value of Roman numeral XVI? XIV?

Earthquake Risk• Not all earthquakes occur near plate boundaries• Seismologist study the probability of an earthquake at

a given location• Areas that experienced earthquakes in the past will

likely experience earthquakes again• The New Madrid Fault in the central United States

has a history of severe earthquakes in 1811-1812 registering magnitudes of 7.8 and 8.1

• However, on average only about 10 earthquakes with magnitudes greater than 7.0 occur worldwide each year.