# Earthquakes.

## Presentation on theme: "Earthquakes."— Presentation transcript:

Earthquakes

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

Stress There 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

- Forces in Earth’s Crust
Types of Stress Stress that pushes a mass of rock in two opposite directions is called shearing.

- Forces in Earth’s Crust
Types of Stress The stress force called tension pulls on the crust, stretching rock so that it becomes thinner in the middle.

- Forces in Earth’s Crust
Types of Stress The stress force called compression squeezes rock until it folds or breaks.

Faults A 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 Faults - Forces in Earth’s Crust Tension in Earth’s crust pulls rock apart, causing normal faults.

Kinds of Faults - Forces in Earth’s Crust A reverse fault has the same structure as a normal fault, but the blocks move in the opposite direction.

Kinds of Faults - Forces in Earth’s Crust In a strike-slip fault, the rocks on either side of the fault slip past each other sideways, with little up and down motion.

Strike-Slip Faults Shearing 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 Form Earthquakes 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 Waves Seismic 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 waves An earthquake sends out two of those waves, P and S waves When they reach the top of the epicenter, surface waves form

Primary Waves Also known as P Waves
The first waves to come are these waves P 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 Waves Also 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 Waves When 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 Waves
Geologists 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 Seismographs
Until just recently, scientists have used a mechanical seismograph a 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 earthquake the pen stays in place and records the drum's vibrations The higher the jagged lines, the more severe earthquake

Instruments used to detect movement
In trying to predict earthquakes, geologists have developed instruments to measure changes in elevation, tilting of the land surface, and ground movements along faults.

Measuring Earthquakes
There 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 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 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 Epicenter
Since 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 Direction
One 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 Epicenter
- Earthquakes and Seismic Waves TRIANGULATION is used to detect the location of the epicenter

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 Risk - Earthquake Safety Geologists can determine earthquake risk by locating where faults are active and where past earthquakes have occurred.

How Earthquakes Cause Damage
- Earthquake Safety How Earthquakes Cause Damage In 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.

Designing Safer Buildings
- Earthquake Safety To reduce earthquake damage, new buildings must be made stronger and more flexible.