4.1 Forces in Earth’s Crust Chapter 4: Earthquakes 4.1 Forces in Earth’s Crust

Stress Stress is a force that acts on rock to change its shape or volume. Energy caused by stress is stored in rocks until it changes shape or breaks.

Stress Tension: stress force that pulls on the crust and thins rocks in the middle; occurs when plates pull apart. Compression: stress force that squeezes rock until it folds or bends; occurs when plate comes together. Shearing: stress that pushes a mass of rock in opposite directions; occurs when plates slide past each other.

Normal Faults formed from rocks being pulled apart at divergent boundaries one block, called the hanging wall, sits over the fault and the other block, called the footwall, sits beneath the fault. The hanging wall slides down and the footwall moves up.

Reverse Faults formed from rocks being pushed together at convergent boundaries The hanging wall moves up and the footwall moves down.

Strike-slip Faults The rocks on either side of the fault slip past each other at a transform boundary. There is little up and down movement.

Folding Earth’s Crust Folds are bends in rock that form from compressions shortening and thickening the crust. Upward fold into an arch – anticline Downward fold into a V shape – syncline

Forming Mountains Compression from convergent plates colliding can build mountain ranges. Fault block mountains form from tension in Earth’s crust and faulting. If there are two normal faults near each other, the land between could eventually drop down creating a valley.

Plateaus Plateau: large area of flat land elevated high above sea level. Forces push up large, flat blocks of rock.

4.2 Earthquakes and Seismic Waves Chapter 4: Earthquakes 4.2 Earthquakes and Seismic Waves

Seismic Waves Earthquake: shaking and trembling that results from movement of rock beneath Earth’s surface. Plate movements and stress in Earth’s crust can lead to earthquakes, releasing great amounts of energy.

Seismic Waves Seismic waves: vibrations that are similar to sound waves; travel through Earth carrying energy from earthquakes. The focus of an earthquake is the area beneath Earth’s surface where the rocks are shifted (action that causes Earthquake). The epicenter is the point on Earth’s surface above the focus.

Seismic Waves P waves (primary waves) compress and expand the ground first waves/fastest moving travel through solid, liquid, and gas

Seismic Waves S waves (secondary waves) travel at half the speed of P waves and arrive second travel through solids only strong enough to shake structures

Seismic Waves Surface waves move most slowly produce severe ground movements; can cause ground to roll or buildings to shake back and forth.

Measuring Earthquakes Seismograph: instrument that records and measures the seismic waves.

Measuring Earthquakes Modified Mercalli Scale Rates the amount of shaking Rated by observations; useful in areas where there aren’t instruments available.

Measuring Earthquakes Richter Scale Finds the magnitude; a number assigned to an earthquake based on size. Depend on the size of the waves recorded by a seismograph.

Earthquake Magnitude Bill Nye!

Measuring Earthquakes Moment Magnitude Scale Rates the total energy an earthquake releases. Geologists use data from seismographs and other sources.

Comparing Magnitudes A 1-point increase on a scale represents an increase of 32-times more energy. The higher an earthquake is rated, the more energy it has and the more damage it can do.

Locating an Earthquake You need seismogram info from 3 places: Find the difference of arrival times between the P and the S waves. The difference is used to find the distance to the epicenter – the longer it takes, the farther away the epicenter. A circle is drawn around each point (location). The point where they all cross is the epicenter.

4.3 Monitoring Earthquakes Chapter 4: Earthquakes 4.3 Monitoring Earthquakes

Earthquake Risk Largely depends on how close a given location is to a plate boundary. In N. America, there are plate boundaries in California, Washington, and Alaska. Around the world, the Ring of Fire contains boundaries where several volcanoes and earthquakes occur.