Chapter 8

Section 1

When you bend a stick, you notice that is changes shape while you bend it The stick will spring back if you stop applying force. But if you don’t stop bending the stick It changes permanently. If its elastic limit is passed, the stick may break As it breaks you can feel vibrations in the stick.

Rocks are like other solid materials If enough force pulls or pushes on them, they will change shape. They may even break After breaking, the ends of the broken pieces may snap back. This snapping back is called elastic rebound.

Inside Earth, pushing and pulling forces cause rocks to change shape slowly over time. As they are strained, potential energy builds up in them. This energy is released suddenly when the rocks finally break or move.

The breaking and the movement that follows causes vibrations that move through rock. If they are strong enough, the vibrations are felts as earthquakes An earthquake is a movement of the ground that occurs when rocks inside Earth pass their elastic limit, break suddenly, and experience elastic rebound.

When part of a rock breaks, rocks on either side move as a result of elastic rebound. The surface where rocks break and move is called a fault. Rocks can break in different ways, depending on the forces that cause the break.

1. Normal Fault 2. Reverse Fault 3. Strike-Slip Fault

Normal faults form where tension forces pull rocks apart The rock above the fault moves down.

Reverse faults are caused by compression Rocks pushed together or compressed When the two rocks push together, rock above the fault is pushed up.

Sections of rock move past one another in opposite directions along Earth’s surface. Also called shearing. Strike-slip faults are caused by shear forces.

Earthquakes release energy causing vibrations When this energy is released, it moves away from the fault in the form on seismic waves. The point deep inside the Earth where energy is released causing an earthquake is a focus

Some of the energy from the earthquake travels straight up to Earth’s surface where it can be felt. The epicenter is the point on Earth’s surface directly above the earthquake focus.

When seismic waves leave the focus of an earthquake, some travel through Earth’s interior, and other travel along the surface. Three Types of Waves Primary Waves Secondary Waves Surface Waves

Seismic waves that travel fastest through rock material are primary waves or P-waves. Primary waves cause the material to move from side to side, in the same direction that the wave is moving.

Other seismic waves that travel through Earth’s interior are called secondary waves. Secondary waves, or S-waves, do not move as fast as P- waves. As they move through rock material, they cause the material to vibrate at right angles to the direction of the wave.

Seismic waves that travel along Earth’s surface are called surface waves. They are the largest and slowest type of seismic wave. They cause more damage than other types of waves. Surface Waves move in different ways. They may move rock and soil in a backward rolling motion. Like waves of water Some shake or sway the rock and soil from side to side.

Scientists who study earthquakes are called seismologist They use instruments called seismographs to record seismic waves. One type of seismograph has a drum that holds a roll of paper on a frame

When seismic waves reach the station, the drum vibrates. The pen on the pendulum traces a record of the vibration

The height of the lines traced on the paper measures the magnitude of the earthquake. Magnitude is the measure of energy released by an earthquake.

The epicenter of an earthquake is the point on the surface of Earth directly above the focus Far away from the epicenter, the P-waves and S-waves arrive at different times. But close to the epicenter, the waves arrive at almost the same time.

Once scientists know the P-wave and S-wave arrival times for at least three seismograph stations, they can figure out the location of an earthquakes epicenter. They draw circles on a map. Each circle shows the distance from the seismograph station to the earthquake. The point where three or more circles intersect is the location of the epicenter.

Some earthquakes are not felt on the surface of Earth. People do not even know these small earthquakes are happening. Larger earthquakes, on the other hand, can cause major damage.

Richter magnitude is based on the measurements of heights of seismic waves as they are recorded on seismographs. Scientists use this information to determine the Richter magnitude of an earthquake. Richter magnitude describes how much energy an earthquake releases.

Very weak earthquakes have low magnitudes like 1.0 Strong earthquakes have high magnitudes in the range of 6 to 7 For every increase of 1.0 on the Richter scale, an earthquake actually releases 32 times more energy. This means that an earthquake with a magnitude of 7.5 releases 32 times more energy than an earthquake of 6.5

Another way to measure earthquakes is by the modified Mercalli intensity scale. This scale measures the intensity of an earthquake. Intensity is a measure of the amount of damage to structures and to rocks and soil in a specific area. The amount of damage depends on How strong the earthquake is Kinds of structures in an area Distance from epicenter Nature of the surface material

The Mercalli scale uses Roman numerals I through XII An earthquake with an intensity of I would be felt by few people An intensity – VI earthquake would be felt by everyone An intensity – XII would cause major damage to Earth’s surface and to human-built structures.

When an earthquake occurs on the ocean floor, powerful waves are produced These waves travel outward from the earthquake in all directions. A powerful seismic sea wave is called a tsunami

Tsunamis traveling in open ocean water are low and fast moving. But tsunamis change as they approach land. The speed of the tsunami slows and the height of the wave increases. Huge tsunami waves can be up to 30 meters high.

Giant Sea Waves "In 1992 a mild earthquake, barely noticed, hit San Juan del Sur in Nicaragua. Minutes later the peaceful harbor was drained dry as if someone had pulled a giant bath plug and let the water out. Amazed at the sight, curious people flocked to the harbor to look. As they stared, a giant tsunami rushed in and swept people and buildings far out to sea. This three-part illustration is an example of how the water is drained in a harbor, then builds up height before rushing back to the shore." - Dr. Eldridge M. Moores