earthquakes1.ppt

Dead - More than 3,000

•A report of U.S. Army

relief operations recorded:

•498 deaths in San Francisco

•64 deaths in Santa Rosa

•_{102 deaths in and near}

San Jose

Earthquakes

• Vibration of the Earth produced by the rapid release of energy.

• ….. Massive energy!

• Earthquakes occur along plate boundaries at points called faults.

• Energy is stored in the rocks which

Earthquakes

Energy radiates out from the

focus of the

earthquake

.

The

focus

is the place within the Earth

where the rock breaks, producing an

earthquake. Energy moving outward

Focus and Epicenter

• The

focus

is the earthquake's

underground point of origin or

hypocenter.

• The

epicenter

is the point on the

Earth’s surface that is directly above

the point where an earthquake

Stress and Strain on Rock

As we are dealing with a generally solid rigid plates, we can expect tremendous

stress, strain and tension to build up as the plates bend, especially in the areas around the boundary of the plates. These stresses, strains and tensions produce stress and

Stress and Strain: Rock Behavior

Strain - the result of stress or deformation

elastic deformation - when stresses are removed, rock returns to original shape

plastic deformation - permanent deformation. when stresses are removed, rock stays bent

rupture - breakage and fracturing of the rock, causing an earthquake.

Stress and Strain: the forces of

the earthquake

• Tectonic forces apply stress to rock in three basic forms

1. Compression: pushing together or compression

2. Tension/Extensional : Stress that acts to lengthen an object or pull it apart.

Stress and Strain –Energy

Released

 When stress and strain on certain parts of the

plates exceed the threshold that can be sustained by the elasticity of the rocks in

that area, the plate ruptures, releasing strain energy which is called an earthquake.

 This energy is transmitted through the plates

in the form of seismic waves, heat and

Faults: Origin of the Earthquake

A fault is a large crack in the Earth's crust where one part of the crust has moved against another part.

The parts of a fault are (1) the fault plane,

(2) the fault trace,

The fault plane is

where the action is. It is a flat surface that may be vertical or sloping. The line it makes on the Earth's surface is the fault trace.

Where the fault

plane is sloping, the upper side is the

hanging wall and the lower side is the footwall. When the fault plane is

vertical, there is no hanging wall or

footwall.

Faults: Origin of the Earthquake

Fault Types

There are three basic fault types

1. Normal faults form when the hanging wall drops down.

The forces that create normal faults are pulling the sides apart, or

Fault Types

There are three basic fault types

2. Reverse faults form when the hanging wall moves up.

Fault Type

3. Strike-slip faults

have walls that move sideways, not up or down.

That is, the slip occurs along the strike, not up or down the dip.

Normal Fault

Seismic Waves

 When an earthquake fault ruptures, it causes two types of deformation: static; and

dynamic. Static deformation is the

permanent displacement of the ground due to the event.

 After the earthquake, the formerly straight line is distorted into a shape having

Seismic Waves

2. Seismic Waves

The second type of deformation, dynamic motions, are essentially sound waves

radiated from the earthquake as it ruptures. While most of the plate-tectonic energy

driving fault ruptures is taken up by static deformation, up to 10% may dissipate

Seismic Waves: Body Waves

There are two types of body waves



P-Waves or Primary Waves

P-Waves

1. P waves arrive first. Primary, pressure waves.

 _{Analogous to sound waves.}

 _{Particle motion is along the direction of}

travel (propagation) of the wave, i.e., longitudinal waves.

P-Wave Motion

P-Wave Motion

P waves are compression waves - the wave pulse or pulses travels through the rock in a series of compression pulses. On either side of the compression the rock is stretched.

The stretching and compression of the rock is relatively small, allowing the wave to

travel very quickly.

S-Wave Motion

S-Wave Motion

S waves are characterized by a sideways

movement. The rock materials are moved from side to side as the wave passes.

S waves are like water waves, the wave pulses travel along by moving the medium from

side to side. As the pulse moves along, each section of rope moves to the side then back again in succession. Rocks are more

Surface Waves

The surface waves are the slowest of the three earthquake wave types.

Two basic types of surface waves

1. Love Waves

1. L-waves or long waves.Complex

motion. Up-and-down and side-to-side.

Slowest. Causes damage to structures

during an earthquake.

2. Rayleigh waves involve orbital

motions, like water waves. A surface

Using Seismic Waves to Study

Earth's Interior

Seismic Waves travel through the entire

Earth

Seismic waves as “x-rays” to look

inside the earth

• However, it turns out that S waves cannot travel

through the core, and only P waves are recorded in some places:

• Seismic waves travel faster through denser material.

• Because of this, the path traveled by a seismic wave is bent towards the surface.

Seismometers

large mass, freely suspended.

• In the example on the left, a rotating drum records a

red line on a sheet of paper. If the earth moves

(in this case from left to right) the whole machine

will vibrate too.

Seismograph: the record of the

Earthquake

The record of an earthquake, a seismograph, as recorded by a seismometer, will be a plot of vibrations versus time. On the

seismograph, time is marked at regular

Measuring Earthquakes

There are at least 20 different types of

measures

3 of them are the

Mercalli scale, Richter scale, and the Moment Magnitude scale

Magnitude is a measurement of

earthquake strength

Earthquake Strength

The intensity or strength of an earthquake is measured by seismologist in two main

ways:

1.The Richter Scale

• measures the amount of energy that an earthquake releases

The Richter Scale

The Richter scale is a rating of the size of seismic waves as measured by a particular type of mechanical seismograph Developed in the 1930’s

All over the world, geologists used this for about 50 years Electric seismographs

eventually replaced the

mechanical ones used in this scale

Provides accurate

measurements for small, nearby earthquakes

Earthquake Strength

2. The Mercalli Scale

• Measures the amount of damage from an earthquake

• Ranges from I to XII

• Based on common earthquake occurrences such as "noticeable by people" "damage to buildings" chimneys collapse" "fissures

The Mercalli Scale

Developed in the twentieth century to rate earthquakes according to their intensity The intensity of an

earthquake is the strength of ground motion in a given place Is not a precise measurement But, the 12 steps explain the damage given to people, land surface, and buildings

The same earthquake could have different Mercalli ratings because of the

different amount of damage in different spots

The Moment Magnitude

Scale

Geologists use this scale today

How Earthquakes Cause Damage

The severe shaking provided by seismic waves can damage or destroy buildings and bridges, topple utility poles, and damage gas and water mains

With their side to side, up and down movement, S waves can damage or

destroy buildings,