Earth Science – Chapter 5

Earthquakes

Plate Boundaries

Pacific-North American Plate Boundary

Section 5-1 Earth’s Crust in Motion

• An earthquake is the shaking and trembling that results from movement of rock beneath Earth’s surface. Powerful forces caused by movement of Earth’s plates squeeze rock and pull it in different directions – force that acts on rock to change it shape or volume is called stress.

5-1 Stresses

• There are 3 types of stresses:1. Shearing: pushes a mass of rock in two

opposite directions. (Strike-Slip Fault) – blocks move sideways in opposite direction.

Strike Slip Fault

Tension

• 2. Tension: pulls on the crust, stretching rock. (Normal Faults) - Hanging wall moves down.

Footwall

Hanging Wall

Tension

Compression

• 3. Compression squeezes rock until it folds or breaks. (Reverse Faults) -– hanging wall moves up.

Compression

Faults - CA

Faults – St. Andreas Fault, Ca.

Faults

• When stress builds up in rock, it fractures along a fault. A fault is a break in Earth’s crust where slabs of crust slip past each other.

• 1. Strike-slip fault: the rocks on either side of the fault slide past each other sideways with little up or down motion. Strike-slip faults are caused by shearing.

Faults

• Normal Fault – one block of rock lies above the fault while the other block lies below below it. Normal Faults are caused by tension.

• Reverse Fault: the hanging wall slides upward past the footwall. Reverse Faults are caused by compression.

Fault Animations

http://www.iris.edu/gifs/animations/faults.htm

Earthquake

• How blocks of rock move along a fault depends on how much friction there is between them. If friction is low, the blocks slide constantly without sticking. If friction is high, the blocks lock together. When the amount of stress is greater than the amount of friction, the blocks unlock, setting off an earthquake.

Movement Along Faults

• When normal faults uplift a block of rock, a fault-block mountain forms.

• A fold is a bend in a rock that forms when compression shortens and thickens part of the crust.

• A fold that bends upward in an arch is an anticline.

• A fold that bends downward in the middle to form a bowl is a syncline.

• When a fault pushes up a large, flat block of rock, a plateau can form.

5-2 Measuring Earthquakes

• The point beneath Earth’s surface where rock under stress breaks to cause an earthquake is called the focus.

• The point on the surface directly above the focus is called the epicenter. During an earthquake, vibrations called seismic waves move out from the focus in all directions. Seismic waves carry the energy of the earthquake away from the focus through Earth’s interior, and across the surface.

Epicenter

3 Types of Seismic Waves

• P Waves – compress and expand the ground like an accordion.

• S Waves – vibrate from side to side and up and down.

• Surface Waves – when P and S waves reach the surface, some are changed into Surface waves. Surface waves move more slowly, but produce the most severe ground movements.

Tsunamis

Sumatra Island in Indonesia - Epicenter

Sumatra Island in Indonesia

Sumatra Island in Indonesia

Sumatra Island in Indonesia

Sumatra Island

Instruments to Measure Earthquakes.

• Seismograph – measure and record the vibrations of seismic waves.

• An earthquakes magnitude is a measurement of its strength bases on seismic waves. Three scales:– 1. Mercalli Scale – rates earthquakes by describing

their effects on people, buildings, and land surfaces.– 2. The Richter Scale – rates earthquakes according to

the size of the seismic waves as measured by the seismograph.

– 3. The Moment Magnitude Scale – rates earthquakes by estimating the total energy they release.

Earthquake Magnitude

Earthquake Magnitude

Seismic Waves

Seismograph Lab

Alaska – Seismic Map

5-3 Earthquake Safety

• The severe shaking produced by seismic waves can damage or destroy buildings, bridges, topple utility poles, and fracture gas and water mains. Earthquakes can trigger landslides, mud slides, avalanches, and huge water waves –called??

• Aftershocks are smaller earthquakes that occur after a large earthquake in the same area.

Safety

• Most deaths and injuries occur from damaged and toppled buildings.

• The main danger to people during an earthquake is from falling objects and flying glass. The best way to protect yourself is to drop, cover, and hold.

• If you are indoors when an earthquakes strikes, never run outside. Drop, cover, and hold. Leaving a building can result in death.

• If you are outdoors, stay in an open area. Do not go under trees, buildings, etc. Drop, Cover, and hold.

5-4 Monitoring Faults

• Geologists try to predict when and where an earthquake will occur. To observe changes in the elevation or tilts of the land, geologists put in place instruments that measure stress and deformation in the crust.

• Four instruments are used:

1. Creep meter – measure sideways movement of the ground.

2. Laser-ranging devices –measures slight fault movements

3. Tiltmeters – measures how much the ground has tipped along a fault.

4. Satellites – measures radio waves of the ground to measure changes in elevation.

Loma Prieta, CA 1989

Kobe, Japan - 1995

Landers, CA 1992

Niigata, Japan 1964

Turnnagin Heights,Alaska,1964

http://earthquake.usgs.gov/recenteqsww/world_moll.map