Earthquakes
Section 1
What Are Earthquakes? earthquake the shaking of the ground. seismology the study of earthquakes. seismologists, the scientists who study earthquakes.
Where Do Earthquakes Occur?
Most earthquakes take place near the edges of tectonic plates. Tectonic plates are giant pieces of
Earth’s thin, outermost layer. Tectonic plates move around on top of a layer of plastic rock
Tectonic plates move in different directions and at different speeds. Two plates can push toward or pull away from each other. They can also slip slowly past each other. As a result of these movements, numerous features called faults exist in the Earth’s crust. fault a break in the Earth’s crust along which blocks of the crust slide relative to one another.
Earthquakes occur along faults because of this sliding.
What Causes Earthquakes?
As tectonic plates push, pull, or slip past each other, stress increases along faults near the plates’ edges. In response to this stress, rock in the plates deforms.
Deformation
the change in the shape of rock in response to stress. Rock along a fault deforms in mainly two ways. It deforms in a plastic manner, like a piece of molded clay, or in an elastic manner, like a rubber band. Plastic deformation does not lead to earthquakes.
Elastic deformation, does lead to earthquakes.
Rock can stretch farther without breaking than steel can, but rock will break at some point.
Think of elastically deformed rock as a stretched rubber band. You can stretch a rubber band only so far before it breaks. When the rubber band breaks, it releases energy. Then, the broken pieces return to their unstretched shape.
Faults at Tectonic Plate Boundaries
Earthquake Zones
Earthquakes can happen both near Earth’s surface or far below it. Most earthquakes happen in the earthquake zones along tectonic plate boundaries. Earthquake zones are places where a large number of faults are located. The San Andreas Fault Zone in California is an example of an earthquake zone. But not all faults are located at tectonic plate boundaries.
Sometimes, earthquakes happen along faults in the middle of tectonic plates.

How Do Earthquake Waves Travel?
Seismic Waves
Waves of energy that travel through the Earth .
Body Waves
Seismic waves that travel through the Earth’s interior.
There are two types of body waves: P waves and S waves.
Surface Waves
Seismic waves that travel along the Earth’s surface.
Each type of seismic wave travels through Earth’s layers in a different way and at a different speed. Also, the speed of a seismic wave depends on the kind of material the wave travels through.
Pressure Waves
Waves that travel through solids, liquids, and gases
They are the fastest seismic waves, so P waves always travel ahead of other seismic waves.
P waves are also called primary waves, because they are always the first waves of an earthquake to be detected.
S Waves
Rock can also be deformed from side to side. After being deformed from side to side, the rock springs back to its original position and S waves are created. S waves , or shear waves, are the second-fastest seismic waves.
S waves shear rock side to side, which means they stretch the rock sideways.
Unlike P waves, S waves cannot travel through parts of the Earth that are completely liquid. Also, S waves are slower than P waves and always arrive later. Thus, another name for S waves is secondary waves.
Surface Waves
Surface waves move along the Earth’s surface and produce motion mostly in the upper few kilometers of Earth’s crust.
There are two types of surface waves.
One type of surface wave produces motion up, down, and around,
The other type produces back-and-forth motion like the motion produced by S waves.
Surface waves are different from body waves in that surface waves travel more slowly and are more destructive.

Section 2
Locating Earthquakes
Seismographs are instruments located at or near the surface of the Earth that record seismic waves.
When the waves reach a seismograph, the seismograph creates a seismo-gram. seismogram is a tracing of earthquake motion and is created by a seismograph.
Determining Time and Location of Earthquakes
Seismologists use seismograms to calculate when an earthquake began.
Seismologists find an earthquake’s start time by com paring seismograms and noting the differences in arrival times of P waves and S waves.
Seismologists also use seismograms to find an earthquake’s epicenter. epicenter is the point on the Earth’s surface directly above an earthquake’s starting point. focus
is the point inside the Earth where an earthquake begins.
The S-P Time Method
The first step in this method is to collect several seismograms of the same earthquake from different locations. Then, the seismograms are placed on a time-distance graph.
The seismogram tracing of the first P wave is lined up with the P-wave time-distance curve, and the tracing of the first S wave is lined up with the S-wave curve
The distance of each station from the earthquake can be found by reading the horizontal axis. After finding out the distances, a seismologist can locate an earthquake’s epicenter
Measuring Earthquake Strength and Intensity
The Richter scale
Seismologist Charles Richter created the scale in the 1930s.
Richter wanted to compare earthquakes by measuring ground motion recorded by seismograms at seismograph stations.
Earthquake Ground Motion
Magnitude
A measure of the strength of an earthquake
The Richter scale measures the ground motion from an earthquake and adjusts for distance to find its strength.

Each time the magnitude increases by one unit, the measured ground motion becomes 10 times larger.
For example, an earthquake with a magnitude of 5.0 on the Richter scale will produce 10 times as much ground motion as an earthquake with a magnitude of 4.0. Furthermore, an earthquake with a magnitude of 6.0 will produce 100 times as much ground motion (10 ×
10) as an earthquake with a magnitude of 4.0.
Modified Mercalli Intensity Scale
Intensity
A measure of the degree to which an earthquake is felt by people and the amount of damage caused by the earthquake, if any
Currently, seismologists in the United States use the Modified Mercalli Intensity Scale to measure earthquake intensity.
This scale is a numerical scale that uses Roman numerals from I to XII to describe increasing earthquake intensity levels.
An intensity level of I describes an earthquake that is not felt by most people. An intensity level of XII indicates total damage of an area.

Section 3
Seismologists are not able to predict the exact time when and place where an earthquake will occur.
They can, at best, make forecasts based on the frequency with which earthquakes take place.
Therefore, seismologists are always looking for better ways to forecast when and where earthquakes will happen.
In the meantime, it is important for people in earthquake zones to be prepared before an earthquake strikes.
Earthquake Hazard
Earthquake hazard is a measurement of how likely an area is to have damaging earthquakes in the future.
An area’s earthquake-hazard level is determined by past and present seismic activity.
The greater the seismic activity, the higher the earthquake-hazard level.
The West Coast, for example, has a very high earthquake-hazard level because it has a lot of seismic activity.
Earthquake Forecasting
Forecasting when and where earthquakes will occur and their strength is difficult. By looking carefully at areas of seismic activity, seismologists have discovered some patterns in earthquakes that allow them to make some general predictions.
Strength and Frequency
Earthquakes vary in strength. And you can probably guess that earthquakes don’t occur on a set schedule. But what you may not know is that the strength of earthquakes is related to how often they occur.
The relationship between earthquake strength and frequency is also at work on a local scale. For example, each year approximately 1.6 earthquakes with a magnitude of 4.0 on the Richter scale occur in the Puget Sound area of Washington State. Over this same time

period, approximately 10 times as many earthquakes with a magnitude of 3.0 occur in this area. Scientists use these statistics to make forecasts about the strength, location, and frequency of future earthquakes.
The Gap Hypothesis
The gap hypothesis
is a hypothesis that states that sections of active faults that have had relatively few earthquakes are likely to be the sites of strong earthquakes in the future.
Seismic Gaps
The areas along a fault where relatively few earthquakes have occurred
Earthquakes and Buildings
Today, older structures in seismically active places, such as California, are being made more earthquake resistant.
Retrofitting
The process of making older structures more earthquake resistant.
A common way to retrofit an older home is to securely fasten it to its foundation. Steel can be used to strengthen structures made of brick.
Earthquake-Resistant Buildings
A lot has been learned from building failure during earthquakes. Armed with this knowledge, architects and engineers use the newest technology to design and construct buildings and bridges to better withstand earthquakes. Carefully study Figure 4 to learn more about this modern technology.
Are You Prepared for an Earthquake?
Before the Shaking Starts
The first thing you should do is safeguard your home against earthquakes.
You can do so by:
1)putting heavier objects on lower shelves so that they do not fall during the earthquake.
2) talk to a parent about having your home strengthened.
3) find safe places within each room of your home and outside of your home.
4) make a plan with others (your family, neighbors, or friends) to meet in a safe place after the earthquake is over. This plan ensures that you will all know who is safe.
During the earthquake, waterlines, power lines, and roadways may be damaged.
5) store water, nonperishable food, a fire extinguisher, a flashlight with batteries, a portable radio, medicines, and a first-aid kit in a place you can access after the earthquake.
When the Shaking Starts
The best thing to do if you are indoors when an earthquake begins is to crouch or lie face down under a table or desk in the center of a room
If you are outside, lie face down away from buildings, power lines, and trees and cover your head with your hands.

If you are in a car on an open road, you should stop the car and remain inside.
After the Shaking Stops
You should try to calm down and get your bearings as quickly as possible.
Then, remove yourself from immediate danger, such as downed power lines, broken glass, and fire hazards.
Always stay out of damaged buildings, and return home only when you are told that it is safe to do so by someone in authority.
Be aware that there may be aftershocks, which may cause more damage to structures.
Recall your earthquake plan, and follow it.