Faults and Earthquakes7th Grade – Chapter 7

Earthquake and Faults Vocabulary

• stress• tension• compression• shearing• normal fault• reverse fault• strike-slip fault• plateau

• earthquake• focus• epicenter• P wave• S wave• seismograph• magnitude• Richter scale

Stress in Earth’s CrustStress can push, pull, or squeeze rock in Earth’s crust. Three kinds of stress can occur in the crust.

1. Tension – when two plats pull apart

2. Compression – when two plates come together

3. Shearing – when two plates slip apart or change its shape because two plates are slipping past each other.

Forces in Earth’s Crust

Forces in Earth’s Crust

FaultsThe three main types of faults are defined by the direction in which rock moves along the fault.

1. Normal Faults – the fault cuts through rock at an angle so one block of rock sits over the fault, while the other block lied under the fault

2. Reverse Faults – when blocks move in the reverse direction of a normal fault

3. Strike-Clip Faults – when the rocks on either side of the fault slip past each other sideways, with little up or down motion.

Forces in Earth’s Crust

The low angle of a thrust fault allows rock in the hanging wall to be pushed great distances. For example, over millions of years, rock along the Lewis thrust fault in Glacier National Park has moved 80 kilometers.

Forces in Earth’s Crust

Anticlines and SynclinesCompression can cause folds in the crust. Two types of folding are anticlines, which arch up, and synclines, which dip down.

Forces in Earth’s Crust

Tension and Normal FaultsA hanging wall slips down along the faults, forming avalley, while a foot wall move outward from an area.

1. Where are the hanging wall and the two footwalls in diagram A?

2. What is the new position of the hanging wall after movement occurs in diagram B?

Forces in Earth’s Crust

Tension and Normal FaultsAs tension forces pull the crust apart, two normal faults can form a fault-block mountain range.

Forces in Earth’s Crust

The Kaibab PlateauLook at the sequence of drawings. In your own words, describe what happens in the last two diagrams.

Earthquakes and Seismic Waves

Earthquakes start below the surface of the Earth. An earthquake's seismic waves carry energy up toward the surface and down through the interior.

Seismic waves are vibrations that are similar to sounds waves. They travel through Earth carrying energy released by an earthquake.

1. Looking at the drawing showing Earth’s interior, at which point(s) can seismic waves be detected?

2. At which point do you think the seismic waves will have the most energy? Why?

Earthquakes and Seismic WavesSeismic WavesThe diagram shows how seismic waves traveled during an earthquake along the Denali fault.

A focus is under the stress that begins to break or move while the point on the surface directly above the focus becomes the epicenter of an earthquake.

Which point is the focus and which point is the epicenter?

Monitoring Earthquakes

SeismogramsWhen an earthquake’s seismic waves reach a simple seismograph, the seismograph’s drum vibrates. The vibrations are recorded by the seismograph’s pen, producing a seismogram.

Earthquakes and Seismic Waves

P, S, and Surface WavesEarthquakes release stored energy as seismic waves.

Earthquakes and Seismic Waves

Modified Mercalli ScaleThe Modified Mercalli scale uses Roman numerals to rate the damage and shaking at any given location, usually close to the earthquake.

Earthquakes and Seismic Waves

Earthquake MagnitudeThe table gives the moment magnitudes of some recent earthquakes.

Earthquakes and Seismic Waves

Seismic Wave SpeedsSeismographs at five observation stations recorded the arrival times of the P and S waves produced by an earthquake. These data were used to draw the graph.

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

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

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

Earthquakes and Seismic Waves

An Earthquake’s EpicenterThe map shows how to find the epicenter of an earthquake using data from three seismographic stations.

Seismic Data From the USArray ProjectIn 2004, scientists in the USArray project placed 400 seismographs across the western United States. Every month, 18 seismographs are picked up and moved east, “leapfrogging” the other seismographs.

Monitoring Earthquakes

Monitoring Earthquakes

Recording Seismic WavesIn a simple seismograph, a pen attached to a suspended weight records an earthquake’s seismic waves.

Monitoring Earthquakes

Earthquake RiskThe map shows areas where serious earthquakes are likely to occur, based on the location of past earthquakes across the United States.

Earthquakes Around the WorldEarthquakes are closely linked to plate tectonics. The map shows where past earthquakes have occurred in relation to plate boundaries.

Monitoring Earthquakes