EARTHQUAKES

DEFINITION

Vibrations in the ground caused by a release of energy along faults, (breaks), in the Earth’s

lithosphere as a result of plate tectonics.

MAIN CAUSES OF EARTHQUAKES

1. Plate movements

2. Volcanic Eruptions

3. Mining

4. Meteor Impacts

ROCK DEFORMATION

When a force is applied to a body of a rock at plate boundaries, the rock may bend or break.

FAULTS

As stress builds in the deformed rock, a fault

can form.

Definition: Faults are a break in the Earth’s

lithosphere where one block of rock

moves toward, away from, or past

another.

TYPES OF FAULTS

1. Strike-slip

2. Normal

3. Reverse

STRIKE-SLIP

A strike-slip fault occurs when two blocks of rock slide horizontally past each other in opposite directions.

Location: Transform Plate Boundaries

STRIKE-SLIP FAULT

NORMAL FAULT

Tectonic forces pull two blocks of rock apart. The block of rock above the fault moves down relative to the block of rock below the fault.

Location: Divergent Plate Boundary

NORMAL FAULT

REVERSE FAULT

Tectonic forces push two blocks of rock together.

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

Location: Convergent Plate Boundaries.

REVERSE FAULT

FOCUS & EPICENTER

Focus: The location inside the Earth where the

rock first begins to move or break.

Epicenter: The location on the earth’s surface

which is directly above the focus.

FOCUS & EPICENTER

SEISMIC (BODY) WAVES

When rocks move along a fault, a release of energy travels out in all directions as waves from

the focus.

TYPES OF SEISMIC WAVES

1. Primary Waves: P-waves

2. Secondary Waves: S-waves

3. Surface Waves

PRIMARY WAVESP-waves cause rock in the ground to move in a push-pull

motion similar to spring. P-waves are the fastest moving of all seismic waves.

SECONDARY (SHEAR) WAVES

S-waves cause the rocks to move up and down at right angles relative to the direction the wave is

traveling.

SURFACE WAVES

Surface waves travel along the Earth’s surface moving rocks up and down in a rolling motion.

Surface waves are the slowest type of seismic waves.

TYPES OF SURFACE WAVES

1. Love Waves

2. Rayleigh Waves

LOVE WAVES

Love waves causes the rock to move side to side in a directions perpendicular to the waves’ direction of travel.

RAYLEIGH WAVES

Rayleigh waves are slow moving waves that travel along the surface in a rolling motion similar to ocean waves.

LOCATING THE EPICENTER

An instrument called a seismometer measures and records ground motion and can be used to determine the

distance seismic waves have traveled.

SEISMOMETER

SEISMOGRAM

ANALYZING THE DATA

Step 1:

Determine the number of seconds between the

arrival time of the first P-wave and the first S-

wave. This time difference is know as

“Lag Time”. The lag time must be calculated from

at least 3 different recording stations.

ANALYZING THE DATA CONTINUEDStep 2: Plot the lag times found from the three recording

stations on a (S-P) Lag Time Graph in order to

determine the distance to the epicenter.

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ANALYZING THE DATA CONTINUED

Step 3: Using a ruler and a map scale to measure the

distance between the seismometer and the

earthquakes epicenter.

Step 4: Place the point of a compass on the seismometer

location and the pencil on the distance away from

the epicenter. Next, draw a circle around the

seismometer. Repeat this step for the other two

recording stations.

THE EPICENTER

Where the three triangulated circles intersect is the location of the epicenter of the earthquake.

DETERMINING AN EARTHQUAKES MAGNITUDE

Scientists use three different scales to measure and describe an earthquakes magnitude. Magnitude is a measure of the amount of energy released during an earthquake.

1. Richter Magnitude Scale

2. Moment Magnitude Scale

3. Modified Mercalli Scale

RICHTER SCALE

The Richter scale is used to interpret two different values recorded by the seismometer.

1. The amount of ground motion.

2. The amount of energy released by the earthquake

GROUND MOTION

The Richter scale begins at zero but has no upper limits. Each increase of 1 unit represents ten times the amount of ground motion.

Example: A magnitude 8 produces ten times greater ground shaking than a magnitude 7.

The ground motion value is the measure used to alert the general public.

ENERGY RELEASE

The Richter scale also measures the energy released by the earthquake. On this scale each increase of one whole number, is represented by a 31-fold increase in energy.

Example: A magnitude 8 is 31 times more powerful than a

magnitude 7

MOMENT MAGNITUDE SCALE

The moment magnitude scale measures the total amount of energy released by an earthquake. The energy released depends on three factors:

1. The size of the fault

2. The amount of motion that occurs along the fault

3. The strength of the rocks that break during the earthquake.

MOMENT MAGNITUDE SCALE CONTINUED

On this scale for each increase of one unit the earthquake releases 31.5 times more energy.

Example: A magnitude 8 earthquake releases more than

992 times more energy than a magnitude 6.

MODIFIED MERCALLI SCALE

On the Modified Mercalli scale, an earthquakes magnitude is measured by the amount of damage caused by ground motion. This scale ranges from 1 – 12. 1 being minimal

and 12 being total destruction.

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LARGEST RECORDED EARTHQUAKE

The largest ever recorded earthquake occurred in Chile. May 22nd 1960 at 11:19:14 UTC a magnitude 9.5 earthquake was recorded. Approximately 1,655 killed, 3,000 injured, 2,000,000 homeless, and $550 million damage in southern Chile; tsunami caused 61 deaths, $75 million damage in Hawaii; 138 deaths and $50 million damage in Japan; 32 dead and missing in the Philippines; and $500,000 damage to the west coast of the United States.

ASSOCIATED EARTHQUAKE HAZARDS

1. Aftershocks

2. Fires

3. Tsunamis

4. Liquefaction

LIQUEFACTION

Liquefaction occurs when loose soil temporarily takes on some properties of a liquid. (Quicksand)

RESULTS OF LIQUEFACTION

HOW CAN EARTHQUAKES BE USED

Seismologists have used earthquakes to map the Earth’s interior. P and S waves change speed and direction

depending on the density of different materials. Scientists have found that S-waves cannot travel through the outer

core, proving it is liquid. By studying the P-waves scientists were also able to determine that the inner core

is composed of iron and nickel.

EARTH’S INTERIOR USING SEISMOLOGY