An Earthquake is…

the shaking and trembling that results from the movement of rock beneath Earth's surface

The movement of Earth's plates produce strong forces that squeeze or pull the rock in the crust

This is an example of stress, a force that acts on rock to change its volume or shape

StressThere are three different

types of stress that occur

on the crust, shearing,

tension, and

compression

These forces cause some

rocks to become fragile

and they snap

Some other rocks tend to

bend slowly like road tar

softened by the suns

heat

Types of StressStress that pushes a mass of rock in two

opposite directions is called shearing.

- Forces in Earth’s Crust

- Forces in Earth’s Crust

Types of StressThe stress force called tension pulls on the

crust, stretching rock so that it becomes thinner in the middle.

Types of StressThe stress force called compression

squeezes rock until it folds or breaks.

- Forces in Earth’s Crust

FaultsA fault is a break in the crust where slabs of crust slip past each other. The rocks on both sides of a fault can move up or down or sideways

When enough stress builds on a rock, the rock shatters, creating faults

Faults usually occur along plate boundaries, where the forces of plate motion compress, pull, or shear the crust too much so the crust smashes

Kinds of FaultsTension in Earth’s crust pulls rock apart,

causing normal faults.

- Forces in Earth’s Crust

Kinds of FaultsA reverse fault has the same structure as a

normal fault, but the blocks move in the opposite direction.

- Forces in Earth’s Crust

Kinds of FaultsIn a strike-slip fault, the rocks on either side

of the fault slip past each other sideways, with little up and down motion.

- Forces in Earth’s Crust

Strike-Slip FaultsShearing creates this fault

In this fault, rocks on both sides of the fault slide past each other with a little up and down motion

When a strike-slip fault forms the boundary between two plates, it becomes a transform boundary

Mountains Formed by Folding

Folds are bends in rock that form when compression shortens and thickens part of Earth's crust

MOUNTAINS FORM

The crashing of two plates can cause folding and compression of crust

These plate collisions can produce earthquakes because rock folding can fracture and lead to faults

Anticlines and Synclines Geologists use the terms syncline and anticline to describe downward and upward folds in rock

An anticline is a fold in a rock that arcs upward

A syncline is a fold in a rock that arcs downward

These folds in rocks are found on many parts of the earths surface where compression forces have folded the crust

How Earthquakes FormEarthquakes will always begin in a rock beneath the surface

A lot of earthquakes begin in the lithosphere within 100 km of Earth's surface

The focus triggers an earthquake

Focus: the point beneath Earth's surface where rock that is under stress breaks

Seismic WavesSeismic Waves: vibrations that travel through Earth carrying the energy released during an earthquake

an earthquake produces vibrations called waves that carry energy while they travel out through solid material

During an earthquake, seismic waves go out in all directions to the focus

They ripple like when you through a stone into a lake or pond

Seismic Waves There are three different types of seismic waves:

P waves, Secondary or S

waves, and surface wavesAn earthquake sends out two of those waves, P and S wavesWhen they reach the top of the epicenter, surface waves form

Primary WavesAlso known as P WavesThe first waves to come are these wavesP waves are earthquake waves that compress and expand the ground like an accordion P waves cause buildings to expand and contract

Travel through BOTH liquids and solids

Secondary WavesAlso known as S Waves

After P waves, come S waves

S waves are earthquake waves that vibrate from one side to the other as well as down and up

They shake the ground back and forth

When S waves reach the surface, they shake buildings violently

Unlike P waves, which travel through both liquids and solids, S waves cannot move through any liquids

Surface WavesWhen S waves and P waves reach the top, some of them are turned into surface waves

Surface waves move slower than P waves and S waves, but they can produce violent ground movements

Some of them make the ground roll like ocean waves

Other surface waves move buildings from side to side

Detecting Seismic WavesGeologists use instruments called seismographs to measure the vibrations of seismic waves

Seismographs records the ground movements caused by seismic waves as they move through the Earth

Mechanical SeismographsUntil just recently, scientists have used a mechanical seismographa mechanical seismograph consists of a heavy weight connected to a frame by a wire or spring When the drum is not moving, the pen draws a straight line on paper wrapped around the drum Seismic waves cause the drum to vibrate during an earthquakethe pen stays in place and records the drum's vibrations The higher the jagged lines, the more severe earthquake

Instruments used to detect movementIn trying to predict earthquakes, geologists have

developed instruments to measure changes in elevation, tilting of the land surface, and ground movements along faults.

Measuring EarthquakesThere are at least 20 different types

of measures

Main 3: the Mercalli scale,

Richter scale,

and the Moment Magnitude scale

Magnitude is a measurement of earthquake strength based on seismic waves and movement along faults

Mercalli – INTENSITY, based on effects

The Richter ScaleThe 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

Does not work for big, far ones

The Moment Magnitude Scale

Geologists use this scale today

It’s a rating system that estimates the total energy released by an earthquake

Can be used for any kind of earthquakes, near or far

Some news reports may mention the Richter scale, but the magnitude number they quote is almost always the moment magnitude for that earthquake

Locating the EpicenterSince the P waves travel

faster than the S waves, scientists can use the difference in arrival times to see how far away the earthquake occurred.

It does not tell the direction however.

Determining DirectionOne station can

only learn how far away the quake occurred.

They would draw a circle at that radius.

If three stations combine their data, the quake occurred where the three circles overlap.

Locating the EpicenterTRIANGULATION is used to detect the

location of the epicenter

- Earthquakes and Seismic Waves

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, bridges, and fracture gas mains.

Earthquake RiskGeologists can determine earthquake risk by

locating where faults are active and where past earthquakes have occurred.

- Earthquake Safety

How Earthquakes Cause DamageIn addition to the DANGERS of ground

shaking, earthquakes can cause tsunamis

A tsunami spreads out from an earthquake's epicenter and speeds across the ocean.

- Earthquake Safety

Designing Safer BuildingsTo reduce

earthquake damage, new buildings must be made stronger and more flexible.

- Earthquake Safety