Earthquakes Release Stored Potential Energy

When rocks break and an earthquake occurs a significant amount of

stored energy is released.

• The energy travels through Earth in the form of waves.

• These waves are called seismic waves.

• The waves start in the crust where the rocks break and travel

outward in all directions from this point.

• This outward traveling energy causes Earthquake damage.

Focus vs. Epicenter

• Focus:

• The focus is the starting point of an earthquake.

• It is where the rock actually snaps and motion occurs.

• Most of the time it is underground (as least 65 km down).

• Epicenter:

• The spot on Earth’s surface

directly above the focus.

• This spot experiences the

most energy during the

earthquake.

• After an earthquake, the

earthquakes position is

given by the epicenter

location.

Seismic Waves

• When there is movement at a fault, energy is

released in the form of seismic waves:

• The shaking occurs as the energy travels out from the focus.

• Like throwing a stone in a pond, the ripples of water move out in all

directions.

• There are two types of waves released

• Body Waves – travel through Earth

• Surface Waves – travel on Surface

Seismic Waves – Body Waves

• P-Waves – A fast moving compressional body wave that can travel through

both solids and liquids.

– Because they travel through liquids and solids, P-waves can travel through the liquid and solid interior of Earth and affect

locations on the other side of Earth.

Seismic Waves – Body Waves

• P-Waves (Primary – Think Slinky)

1. Primary waves: Arrive at a location before any other

wave.

2. Phastest waves: Travel at the highest speed (6km/sec.)

3. Push-Pull waves: Pushes & Pulls at rock as it travels through

it.

4. Pass Through solids & liquids – can travel through magma.

Seismic Waves – Body Waves

• S-Waves – A slower moving body wave that travels at right angles to the

primary wave and cannot travel through liquids.

– Because they travel through solids only, the S-waves do not travel

to the other side of Earth.

– When they hit the liquid magma interior of Earth, they reflectback eventually stopping.

Seismic Waves – Body Waves

• S-Waves (Secondary – Think Shaking a Rope)

1. Secondary waves: Waves that arrive second at a

location.

2. Slow waves: Travel at the slow speeds (about ½ of P wave

- 3km/sec.)

3. Shake waves: Vibrate and move up and down.

4. Solids-only waves: travel through solid rock only.

Seismic Waves – Surface Waves

• Surface Waves (Think ripples in water)

1. Waves that propagate through Earth’s surface

2. Created when P and S waves reach the surface

3. The surface rolls like an ocean wave.

4. They are the slowest type of waves.

• Surface waves are the most destructive!

Seismic Waves

We measure all of these seismic waves using a

seismograph

• It draws a zigzag pattern that depicts the seismic waves

motion.

• It creates a print out called a seismogram.

Seismic Waves

By looking at a seismogram, two conclusions can be made.

1. Speed of the waves.

2. Distance to the Earthquakes epicenter.

Seismic Waves

• P-waves are the first to arrive

• S-waves are the second to arrive

• Surface waves are the last to arrive

The greater the distance

between the S and P

waves, the further the earthquake epicenter.

Examples: Race car

racing a station

wagon…the longer the

race, the larger the distance between the

two cars

Seismic Waves

• What if you want to find the distance to an epicenter?

• Use a time-travel graph

• It shows the average travel times for P and S waves.

1. Find the time between

the P and S wave (either

from the seismogram, or

it may be given to you).

2. Use the time-travel graph

and find the spot on the

graph where the S and P

waves are separate by the

time you found in step 1

above.

3. Drop a line straight down

at the location and read

the distance off the graph.

Seismic Waves

• What if you wanted to find where the earthquake occurred?

• Once an earthquake occurs, you can determine its location.

• 3 seismograph stations must detect the seismic waves.

• Using the arrival time of the waves, the station can say that the earthquake

occurred within a certain radius (distance).

• Once 3 stations map out the radius, the exact epicenter is where the 3 circles

intercept each other.

This is referred to as

Triangulation:

Commonly used by

geologists to find the

locations of Earthquakes.

Seismic Waves

• Time lag between P-wave and S-wave indicate how far the

earthquake is from a seismograph station.

• The lag time tells us how far we are from an earthquake but not from

which direction.

Epicenter can be

any where on the

radius of the circle.

Seismic Waves

• At least 3 seismograph stations are needed to pinpoint the epicenter

of an earthquake.

Infinite number of

possible epicenters

Seismic Waves

• At least 3 seismograph stations are needed to pinpoint the epicenter

of an earthquake.

Two seismograph

stations narrow

possible epicenters

down to two.

Possible Epicenter

Possible Epicenter

Seismic Waves

• At least 3 seismograph stations are needed to pinpoint the epicenter

of an earthquake.

Epicenter

3 seismograph

stations narrow

possible epicenters

down to one.