Chapter 8outline

Chapter 8

Earthquakes and the Earth’s Interior

Earthquakes

Each year there are more than 30,000 earthquakes world wide.Generally, ____ are considered major quakes

Why don’t we know all this is happening?

Most earthquakes occur in remote areas with little property damage.

An earthquakes is

the vibration of the Earth produced by a rapid release of energy.Earthquakes originate at a point in the Earth called the………..

_________Energy is release in all directions from the focus in the form of waves.

The _________ is the location on the surface directly above the focus.

Faults

The earth is under constant change. It is pressed, pulled, uplifted, etc…Fractures in the crust where actual movement has occurred is called a….

________

Case Study: San Andreas

1906 – San FranciscoDuring this single event, the land on the western side of the fault shifted as much as 4.7 meters to the north when compared to the eastern side.

What causes earthquakes?Forces within the earth deform the rocks.These forces cause the rocks to bend and store _________________Eventually, this elastic energy is overcome and released at the….

_______Slippage and buildup of energy along the fault can continue until that energy is released. Rocks then snap back in place.

Elastic Rebound Theory

The spring back of the rocks into their original place is called the…

__________________

Most earthquakes are produced by the rapid release of elastic energy stored in rock that has been subjected to great forces. When the strength of the rock is ___________, it suddenly breaks, causing the __________ of an earthquake.

Aftershocks and Foreshocks

Earthquakes do not last very long.The 1906, San Francisco quake lasted 40 seconds.The movements that follow a major earthquake often produce smaller earthquakes called….

______________These aftershocks are usually less intense

or weaker than the main quake.

Small earthquakes that often occur before a major earthquake are called…..

___________They may occur days or years before a

major event.Please Note: Faults are not just one crack

in the crust. They may have numerous tributary faults much like a stream pattern.

Measuring Earthquakes

The study of seismology goes back at least 2,000 years to the ________.

Seismographs

Are instruments that record earthquake waves.

How do they work?

Weight suspended from a support attached to bedrock.

bedrock

bedrock

Earth moves

Drum records motion

Weight does not move with ground motion due to inertiav

Weight hinged to allow movement

Seismogram

This is an amplified electronic record of ground motion, producing a trace (graph) called a ____________.

Earthquake Waves

Earthquake waves spread out in all directions from the….

________.The seismogram shows these waves

in two forms1. ______________2. ______________

Surface waves

Body waves

Surface Waves

Are the seismic waves that travel along Earth’s outer layer (or ____________). These waves actually cause the ground to move.Surface waves are much like waves in the ocean moving up and down as well as side to side.These are the _____________ earthquake waves.

Body WavesThese waves travel the Earth’s interior.

There are two types:1. ___________: Primary waves, these waves

compress (push) and expand (pull) rocks in the direction the waves travel.

2. ___________: Secondary waves, shakes the particles at right angles to their direction of travel. Gases and liquids will not transmit s-waves.

Locating an Earthquake

The difference in velocities of P and S waves provides a way to locate the epicenter.P waves always reach a seismograph before S wave. The greater the interval measured on a seismogram between the arrival of the first P wave and the first S wave, the greater the distance to the earthquake source.

Earthquake DistanceTravel time graphs are

constructed from seismograms.First, find the time interval between the arrival of the first P wave and the first S wave on the seismogram.Find, on the time travel graph the equivalent time spread between the P and S wave curve.From this information, you can see the distance is 3,800 km. From the seismograph.

Earthquake Direction

We must recognize that the epicenter could be in any direction from our station. We draw a circle around our location on a map using the information from the time-travel graph.We then contact at least two (or more) stations and obtain their time travel data and draw circles around their stations.Where the _____________ intersect is the location of the earthquake’s _____________.

Triangulation

Seismic Wave SpeedsSeismographs at

five observation stations recorded the arrival times of the P and S waves produced by an earthquake. These data are shown in the graph.

Seismic Wave Speeds

X-axis––distance from the epicenter; y-axis––arrival time.

Reading Graphs:

What variable is shown on the x-axis of the graph? The y-axis?

Seismic Wave Speeds

7 minutes

Reading Graphs:

How long did it take the S waves to travel 2,000 km?

Seismic Wave Speeds

4 minutes

Estimating:

How long did it take the P waves to travel 2,000 km?

Seismic Wave Speeds

2,000 = 3.5 minutes4,000 = 4.5 minutes

Calculating:

What is the difference in the arrival times of the P waves and the S waves at 2,000 km? At 4,000 km?

Locating the Epicenter

Earthquake Zones

About 95% of the major earthquakes occur in a few narrow zones.Most fall on the _______________Another active zone is along the Mediterranean Sea.A third belt coincides with the mid-ocean ridge system.

Measuring EarthquakesScientists have used two different measurements to describe the size of an earthquake – intensity and magnitude._________ is the amount of shaking at a given location based on earthquake damage. _____________ are quantitative measurements that rely on calculations using seismographs. Magnitudes are a measure of the size of seismic waves or the amount of energy released at the source of the earthquake.

Richter Scale

This is an outdated method of measuring the magnitude of an earthquake. The Richter scale is based on the ____________________________ (P,S, or surface wave) recorded on a seismogram. A ________________ in wave amplitude equals an increase of 1 on the magnitude scale.

Example: An earthquake with a magnitude of 5.0 is ___________ greater than an earthquake with a magnitude of 4.0. Since earthquake waves weaken with distance, The Richter scale is only useful for small, shallow earthquakes within 500 km of the epicenter. ______________ use the Richter scale, but scientists no longer use it.

Moment Magnitude

________________ is derived from the amount of displacement that occurs along a fault zoneFactors used to calculate the Moment Magnitude:

1. ______________________2. ______________________3. ______________________

(surface area of fault) X ( average displacement along fault) X (rigidity of rock)

  These factors together provide a

measure of ____________________ rock can store before it suddenly slips and releases its energy during an earthquake.

Moment magnitude is the _______________________ measurement for earthquakes because it is the only magnitude scale the estimates the energy released by earthquakes.

Destruction from Earthquakes Good Friday, 1964, AlaskaThis is the scene of the most violent earthquake in North America during the 20th century. The moment magnitude was 9.2 and the event lasted 3-4 minutes. The quake left 131 dead and thousands homeless. The state’s economy was badly damaged. However, if this quake had happened on an average weekday, many more lives would have been lost.

Seismic Vibrations

The damage to buildings and other structures from earthquake waves depends on several factors:1. ___________________________2. ______________________ on which the structure is built3. ____________ of the structure

Building Design

What type of building would best survive an earthquake?

__________________Why?

They are more _________

Multi-story buildings suffer the most. Steel reinforced buildings survive the best. Many new construction techniques place hydraulics and springs under buildings to allow more movement. The frequency of earthquake vibrations has much to do with which buildings are damaged.

Liquefaction

Where loosely consolidated sediments are saturated with water, earthquakes can cause a process call ______________. Under these conditions normally stable soil turns to liquid that is not able to support a buildings. Structures may settle or collapse. Underground storage tanks and sewer lines may float toward the surface.

Tsunamis

are destructive ocean waves.

Causes of Tsunamis

A tsunami triggered by an earthquake occurs where a slab of the __________________________ vertically along a fault. A tsunamis can also occur when the vibration of a quake sets an underwater landslide into motion.

A tsunami travels across the ocean at speeds of 500 to 950 kilometers per hour. In the open ocean a tsunami can pass unnoticed. How is this possible?

Its height is usually less than 1 meter and the distance between wave crests can range from 100 to 700 kilometers.When the tsunami enters shallower coastal water, the waves are slowed and the water begins to pile up to heights that’s sometimes are greater than 30 meters.

Tsunami Warning System

Large earthquakes are reported to the Tsunami Warning System in Honolulu from seismic stations around the Pacific. Scientists use water levels in tidal gauges to determine whether a tsunami has formed. A warning is then issued so evacuation of coastlines near the epicenter can proceed.On average, only one or two tsunamis are generated each year. Only about one tsunamis in every 10 years causes major damage and loss of life.

Other Dangers

________________ – unconsolidated rock and soil on slopes loosen and fall because of earthquake vibrations. They can also cause ground to collapse, liquefy, or subside.

__________ – 1906, San Francisco…..many wood structures burned when gas and electric lines where cut. Many water lines were broken crippling firefighters. In 1923, an earthquake in Japan caused an estimated 250 fires. These fires destroyed half of Tokyo and killed over 100,000 people.

Predicting Earthquakes

________________ Predictions – The goal of short-range predictions are to provide an early warning of the location and magnitude of a large earthquake.

Researchers monitor precursors:1. __________2. __________3. __________4. __________5. __________

Short range predictions have not been successful.

Long-Range Predictions

These forecasts give the probability of certain magnitude earthquakes occurring within 30 – 100+ years.

Predictions are based on the idea that earthquakes are repetitive or cyclic. Scientists study historical records looking for patterns of occurrence. They also study, seismic gaps, which are areas along faults where there has not been any earthquake activity for a long period of time.

There has only been __________in long-term forecasting. Scientists just don’t yet understand enough to about earthquakes to make accurate predictions.

Earth’s Layered Structure

How can we study the internal structure of the Earth? Lets come up with some ideas…….

__________

Layers Defined by Composition

If the Earth were made of the same materials throughout, seismic waves would……?Spread through it in a straight lines at constant speed.

What we know……

Seismic waves reaching seismographs located farther from the earthquake travel at faster average speeds than those recorded at locations closer to the event. Seismic increase in speed with depth is due to increased pressure.As a result, seismic waves are __________ as they travel.

Earth’s interior consists of three major zones defined by its chemical composition – __________, __________, and __________.

CrustThe crust is the thin, rocky outer layer of the Earth. It is divided into the oceanic and continental crust. __________– composed of mostly basaltic (basalt) rock averaging 7 km thick. __________– 8-75 km thick and is composed mostly of granitic (granite) rock.

Oceanic rocks are generally younger than continental rocks and __________.

lithosphere

asthenosphere

mantle

Mantle

Over 82% of the Earth’s volume is contained in the mantle. The mantle is a rocky solid with __________properties. Rocks here are composed of mostly peridotite and denser than the crust.

Core

Composed of an __________alloy. The core is the most dense layer about 13 times denser than water.

Layers Defined by Physical Properties

Earth’s interior gradually increases in __________, __________, and __________ with depth.

__________– when a substance is heated, the transfer of energy increases the vibration of the particles. If the temperature exceeds the melting point, the forces between the particles are overcome and melting begins.If temperature were the only factor that determined whether a substance melted, the Earth would be a molten ball covered with a thin, solid outer shell.

As temperature increases with depth, so does pressure which increases rock strength. This can cause a substance to become brittle, putty-like, or liquid.

Earth can be divided into layers based on physical properties – __________, __________, __________, and __________.

Lithosphere and Asthenosphere

__________ – crust (both oceanic and continental) and the uppermost mantle. __________ – under the lithosphere, it is slightly weaker than the crust. Its temperature and pressure results in a small amount of melting. Rocks here are close enough to their melting point and therefore __________.

Lower Mantle – The lower mantle is more rigid than the upper mantle. Rocks here are very hot and capable of flow (much like molten plastic). Rocks at the edge of the outer core are softer and more flowing.

Inner and Outer Core

__________– a liquid layer __________– Despite its high temperature, the material in the inner core is compressed into a solid state by the immense pressure.

So, how does the Earth generate its magnetic field?

The outer core rotates at a different speed then does the inner core and the mantle. __________ is created __________. This in turn is our gravity.

Discovering Earth’s Layers

In 1909, a Croatian seismologist discovered that the velocity of seismic waves increases abruptly below 50 kilometers of depth. This boundary separates the crust from the underlying mantle and is known as the Mohorovicic discontinuity or __________.

Another boundary was found between the mantle and the outer core. P waves are bent around the outer core. The outer core causes P waves that travel through the core to arrive several minutes later than expected. This region is called the __________.

Shadow zone

S waves do not travel through the outer core; therefore geologists concluded that this region is __________.

Discovering Earth’s Composition

1. __________through drilling in the continental crust. Mostly granite2. __________– Mostly basalt3. __________– practically melted asthenosphere yields peridotite, which is a lava like material in Hawaii. 4. __________ show us a peridotite substance as well as iron-nickel.

Earth’s inner core is like the iron-nickel meteorites The mantle is like the peridotite meteorites.Time to think about the description in chapter 1 of how the Earth formed. Do you remember?