Earthquakes

Earthquakes

Earthquakes!

An earthquake is sudden ground movement.

This movement is caused by the sudden release of the energy
stored in rocks.

An earthquake happens when so much stress builds up in the
rocks that the rocks break and/or plates shift.

An earthquake’s energy is transmitted by seismic waves.

Plate Boundaries

Almost all earthquakes occur at plate boundaries.

All types of plate boundaries have earthquakes.

Convection (heat currents) within the Earth causes the plates to move. As
the plates move, stresses build.

When the stresses build too much, the rocks break. The break releases the
energy that was stored in the rocks. The sudden release of energy is an
earthquake.

During an earthquake the rocks usually move several centimeters. Rarely,
they may move as much as a few meters.

Elastic Rebound Theory

Elastic rebound theory. Stresses build on both
sides of a fault.

The rocks deform plastically as seen in Time 2.

When the stresses become too great, the rocks
return to their original shape.

To do this, the rocks move, as seen in Time 3.

This movement releases energy, creating an
earthquake.

Focus

The point where the rock ruptures is the earthquake’s focus.

●

Shallow earthquake has a focus less than 70 kilometers (45 miles).

○

Shallow earthquakes cause the most damage.

●

Intermediate-focus earthquake has a focus between 70 and 300
kilometers (45 to 200 miles).

●

Deep-focus earthquake is greater than 300 kilometers (200 miles).

About 75% of earthquakes have a focus in the top 10 to 15 kilometers (6
to 9 miles) of the crust.

Epicenter

The area just above the
focus, on the land surface, is
the earthquake’s epicenter

Annual earthquakes

In a single year, on average, more than 900,000 earthquakes are
recorded.

About 150,000 of them are strong enough to be felt.

Each year about 18 earthquakes are major, with a Richter magnitude
of 7.0 to 7.9, and on average one earthquake has a magnitude of 8
to 8.9.

Magnitude 9 earthquakes are rare. Only 5 from 1900 to 2000.

Multiple Choice

What is the point on Earth's surface directly above the focus?

• tsunami

• seismic wave

• epicenter

• focus

Earthquake Zones

Nearly 95% of all earthquakes take place along one of the three types of plate boundaries.

About 80% of all earthquakes strike around the Pacific Ocean basin because it is lined

with convergent and transform boundaries. The region around the Pacific is called the

Pacific Ring of Fire due to its earthquakes and volcanoes. About 15% take place in

the Mediterranean-Asiatic Belt. Convergence is causing the Indian Plate to run into the

Eurasian Plate. The remaining 5% are scattered around other plate boundaries. A few

earthquakes take place away from plate boundaries. These are intraplate

earthquakes.

Transform Plate Boundaries

Transform plate boundaries produce enormous and deadly
earthquakes.

These quakes at transform faults are shallow focus. This is because
the plates slide past each other without moving up or down.

Convergent Plate Boundaries

Earthquakes at convergent plate boundaries mark the location of
the subducting lithosphere. The motion of the lithosphere as it
plunges through the mantle causes the quakes At greater depths,
the plate heats up enough to deform plastically.

Convergent plate boundaries produce earthquakes most of the
way around the Pacific Ocean basin.In this part of the Pacific
Ocean, oceanic crust subducts beneath oceanic crust. This
creates as many as 1,500 earthquakes every year.

Intraplate Earthquakes

About 5% of earthquakes take place within a plate, away from plate
boundaries. These intraplate earthquakes are caused by stresses
within a plate. Since plates move over a spherical surface, zones of
weakness are created.

Intraplate earthquakes happen along these zones of weakness. The
earthquakes may take place along ancient faults or rift zones.

●

In recent years this has increased due to “Fracking” for oil and gas
production.2019 fracking linked to earthquakes

Seismic Waves

Seismic waves are the energy from earthquakes. Seismic waves move
outward in all directions away from their source.

Each type of seismic wave travels at different speeds in different
materials.

All seismic waves travel through rock, but not all travel through liquid or
gas.

Geologists study seismic waves to learn about earthquakes and the
Earth’s interior.

Wave structure

Seismic waves are just one type of wave.

Sound and light also travel in waves.

Every wave has a high point called a crest and a low point called a
trough.

The height of a wave from the center line to its crest is its amplitude.

The horizontal distance between waves from crest to crest (or trough to
trough) is its wavelength

Waves

Seismic Waves

There are two major types of seismic waves.

Body waves travel through the Earth’s interior.

Surface waves travel along the ground surface.

In an earthquake, body waves are responsible for the sharp jolts.
Surface waves are responsible for the rolling motions that do most of
the damage in an earthquake.

Body Waves

Primary waves (P-waves) and secondary waves (S-waves) are
the two types of body wave.

Body waves move at different speeds through different materials.

Multiple Choice

What is the name of the process occurring in the mantle that moves the tectonic plates?

• Coastal erosion

• Subduction

• Convection Currents

• Convergence

Multiple Choice

the height of the wave.

• Matter

• Wavelength

• Energy

• Amplitude

Multiple Choice

The highest point of a wave above the line of origin.

• Trough

• Crest

• Wavelength

• Amplitude

Multiple Choice

The distance between two consecutive crests.

• Wavelength

• Medium

• Frequency

• LIne of Origin

Multiple Choice

What happens when tectonic plate shift suddenly?

• Thunderstorm

• Volcano

• Earthquake

• Landslide

Multiple Choice

Moving in a rolling direction is what type of earthquake wave?

• love wave

• P wave

• S wave

• Rayleigh wave

Multiple Choice

Where do earthquakes generally originate?

• At plate boundaries and fault lines

• Near the center of tectonic plates

• Above sea level

• Next to mountain ranges

Primary Waves

P-waves are faster. They travel at about 6 to 7 kilometers (about 4 miles)
per second.

Primary waves are so named because they are the first waves to reach a
seismometer.

P-waves squeeze and release rocks as they travel. The material returns
to its original size and shape after the P-wave goes by.

For this reason, P-waves are not the most damaging earthquake waves.
P-waves travel through solids, liquids, and gases.

Secondary Waves

S-waves are slower than P-waves.

They are the second waves to reach a seismometer. S-waves move
up and down.

They change the rock’s shape as they travel. S-waves are about half
as fast as P-waves, at about 3.5 km (2 miles) per second. S-waves
can only move through solids.

This is because liquids and gases don’t resist changing shape.

Surface Waves

Surface waves travel along the ground outward from an
earthquake’s epicenter.

Surface waves are the slowest of all seismic waves. They travel
at 2.5 km (1.5 miles) per second.

There are two types of surface waves.

Love waves move side-to-side, much like a snake.

Rayleigh waves produce a rolling motion as they move up

down.

Tsunami

Japanese word meaning "harbor wave."

Tsunami are deadly ocean waves from the sharp jolt of an undersea
earthquake.

Tsunami waves have small wave heights relative to their long
wavelengths, so they are usually unnoticed at sea. When traveling
up a slope onto a shoreline, the wave is pushed upward.

Since tsunami are long-wavelength waves, a long time can pass
between crests or troughs. Any part of the wave can make landfall
first.

When the trough hit land first, the water will recede into the ocean
as it builds to the crest of the wave.

Measuring Seismic Waves

A seismograph is a machine that records seismic
waves. In the past, seismographs produced a
seismogram.

A seismogram is a paper record of the seismic waves
the seismograph received. Seismographs have a
weighted pen suspended from a stationary frame. A
drum of paper is attached to the ground. As the
ground shakes in an earthquake, the pen remains
stationary but the drum moves beneath it. This creates
the squiggly lines that make up a seismogram

Seismograms contain a lot of information about an earthquake: its strength,
length, and distance.

Wave height is used to determine the magnitude of the earthquake. T

he seismogram shows the different arrival times of the seismic waves.

The first waves are P-waves since they are the fastest.

S-waves come in next and are usually larger than P-waves.

The surface waves arrive just after the S-waves. If the earthquake has a shallow
focus, the surface waves are the largest ones recorded.

The Richter Magnitude Scale

Charles Richter developed the Richter magnitude scale in 1935.
The Richter scale measures the magnitude of an earthquake's
largest jolt of energy. This is determined by using the height of the
waves recorded on a seismograph.

The Richter scale is logarithmic. The magnitudes jump from one
level to the next. The height of the largest wave increases 10 times
with each level. So the height of the largest seismic wave of a
magnitude 5 quake is 10 times that of a magnitude 4 quake.

Moment Magnitude Scale

The moment magnitude scale is the favored method of
measuring earthquake magnitudes. It measures the total energy
released by an earthquake.

Moment magnitude is calculated by two things.

● One is the length of the fault break.
● The other is the distance the ground moves along the fault.

Multiple Choice

Moves back and forth in a zigzag pattern is what type of earthquake wave?

• body wave

• surface wave

• P wave

• love wave

Multiple Choice

The strength of an earthquake's seismic waves...

• epicenter

• magnitude

• tsunami

• focus

Multiple Choice

Slower destructive waves that occur on the surface of the earth are what type of earthquake waves?

• love wave

• surface waves

• P wave

• body waves

Multiple Choice

Fast-moving waves that occur beneath the surface of the earth are what type of earthquake waves?

• love wave

• surface waves

• P wave

• Body waves

Multiple Choice

How do scientists know that the outer core is a liquid?

• P waves do not travel through the outer core but S waves do.

• S waves do not travel through the outer core but P waves do.

• P waves and S waves do not travel through the outer core.

• P waves and S waves travel through the outer core.

Multiple Choice

What is the beginning point of an earthquake?

• focus

• epicenter

• tsunami

• lithosphere

Multiple Choice

What are the vibrations of energy flowing from the focus?

• epicenter

• tsunami

• focus

• seismic waves

Multiple Choice

Waves transfer _________.

• Energy

• Matter

• Candy

• Boats

Multiple Choice

What is the crust and upper area of the mantle called?

• focus

• tsunami

• epicenter

• lithosphere

Multiple Choice

Which earthquake wave has the highest velocity?

• P wave

• S wave

• X wave

• R wave