CHAPTER 5

EARTHQUAKES

SECTION 1: WHAT ARE EARTHQUAKES

http://video.nationalgeographic.com/video/environment/environment-natural-disasters/earthquakes/inside-earthquake/

• Seismology is a branch of science devoted to studying earthquakes

Where Do Earthquakes occur?

• Most earthquakes take place near tectonic plates.

• Remember that a fault is 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.

The largest and most active earthquake zone lies along the plate boundaries surrounding

the Pacific Ocean.

What Causes Earthquakes?

• As the 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

• If it deforms in a plastic manner, like a piece of molded clay, earthquakes do not occur.

• If deformation occurs in an elastic manner, like a rubber band, then earthquakes will occur.

• Rock can stretch farther without breaking than steel can, but rock will break at some point.

This road cut is adjacent to the San Andreas Fault in southern California. The rocks in the cut have undergone deformation

because of the continuous motion of the fault.

• Elastic Rebound – the sudden return of elastically deformed rock to its original shape.

– Occurs when more stress is applies to the rock than it can withstand

– During elastic rebound, energy is released and some of it travels as seismic waves, which cause an earthquake

Faults at Tectonic Plate Boundaries

• Transform motion occurs where two plates slip past each other and creates strike-slip faults; blocks of crust slide horizontally past each other

• Convergent motion occurs where two plates push each other creating reverse faults; blocks of crust that are pushed together slide along reverse faults

• Divergent Motion occurs where two plates pull away from each other creating normal faults; blocks of crust that are pulled away from each other slide along normal faults.

Earthquake Zones

• Earthquakes can happen both near Earth’s surface or far below it.

• Earthquake zones are places where a large number of faults are located

• Sometimes instead of occurring at tectonic plate boundaries, earthquakes can happen along faults in the middle of tectonic plates.

How Do Earthquake Waves Travel?

• Waves of energy that travel through the Earth are called seismic waves.

• Seismic waves that travel through the Earth’s interior are called body waves.

P waves – travel through solids, liquids, and gases

• P stands for “pressure”

• Are the fastest waves, so travel ahead of other waves

• Also called “Primary Waves” because they are the fist waves of an earthquake to be detected

S waves

• Also called “shear waves”

• Second-fastest seismic waves

• Stretch the rock sideways

• Cannot travel through parts of Earth that are completely liquid

• Slower than P waves and arrive later, so are also called “secondary waves”

• Seismic waves that travel along Earth’s surface are called surface waves.

– Produce motion mostly in the upper few kilometers of Earth’s crust

– One type produces motion up, down, and around

– Other type produces back-and-forth motion

– Surface waves travel more slowly and are more destructive

SECTION 2: EARTHQUAKE MEASUREMENT

Locating Earthquakes

• Seismographs are instruments located at or near the surface of the Earth that record seismic waves.

• A seismogram is a tracing of earthquake motion and is created by a seismograph

• Seismologists find an earthquakes start time by comparing seismograms and noting the differences in arrival times of P and S waves.

• An epicenter is the point on the Earth’s surface directly above an earthquakes starting point

• A focus is the point inside the Earth where an earthquake begins

• The S-P time method is the simplest method used to find an earthquakes epicenter.

Measuring Earthquake Strength and Intensity

• The Richter Magnitude Scale – commonly called the Richter scale – developed by Charles Richter in the 1930’s – compares earthquakes by measuring ground motion recorded by seismograms at seismograph stations

• A measure of the strength of an earthquake is called magnitude

• Every time the Richter scale increases by one, the measured ground motion becomes 10 times larger

• A measure of the degree to which an earthquake is felt by people and the amount of damage caused by the earthquake, if any, is called intensity.

Currently in the US we use the Modified Mercalli Intensity Scale to measure earthquake intensity.

• Roman numeral scale of I to XII

• A level I describes an earthquake not felt by most people

• A level XII indicated total damage of an area

Intensity values for the 1906 San Francisco earthquake varied from place to place. The

maximum intensity level was XI.

April 18, 1906

Epicenter was near San

Francisco, but was felt from

Southern Oregon to

South LA, felt inland as far as

central Nevada!

March 11, 2011 “The Great East Japan Earthquake”

• Magnitude of 8.9

• Focus was 20 miles underwater

• 1 of 5 most powerful quakes in the world since 1900

• Caused a tsunami with waves of 133 ft!!

• 15,850 deaths• 6,011 injured• 3,287 missing

August 23, 2011

• 5.8 Magnitude Earthquake with an epicenter in Virginia

• Felt here in Western PA

• Did anyone feel the tremors???

Review of 5-2

1. Seismic Waves are recorded how?– Seismograms

2. What is the method used to find the earthquakes epicenter?– S-P Method

3. What does the Richter Scale measure?– Strength

4. What does the Modified Mercalli Intensity Scale Measure?– Intensity

SECTION 3: EARTHQUAKE AND SOCIETY

Earthquake Hazard

• Earthquake hazard is a measurement of how likely an area is to have damaging earthquakes in the future.

• Level is determined by past and present seismic activity.

This is an earthquake-hazard map of the continental United States. It shows various levels of earthquake hazard for different areas of the

country.

Earthquake Forecasting

• Scientists have discovered some patterns in earthquakes that allow them to make some general predictions.

• Earthquakes’ strength is related to how often they occur. See Table 1 on page 141!

• Another method of forecasting an earthquake’s strength, location, and frequency is based on 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 sites of strong earthquakes in the future.

• The areas along a fault where relatively few earthquakes have occurred are called seismic gaps.

• Not all scientists believe that the Gap Hypothesis is an accurate way of forecasting earthquakes, but scientists used the method to accurately predict the Loma Prieta earthquake in the San Francisco Bay area, which filled in the seismic gap in 1989, measuring a 6.9 of the Richter scale.

Earthquakes and Buildings

• The process of making older buildings/structures more earthquake resistant is called retrofitting.

During the January 17, 1995, earthquake, the fronts of entire buildings collapsed into the streets of Kobe, Japan

Earthquake-Resistant Building Technology:

• Mass Damper

• Active Tendon System

• Base Isolators

• Cross Braces

• Flexible Pipes

America’s Fault Lines

• http://www.youtube.com/watch?v=o0tVbjrbkp8

(4:19)

Are you Prepared for an Earthquake?

• Safeguard your home by putting heavier objects on lower shelves,

• have your home strengthened,

• find safe places in and out of your home,

• make a plan with others to meet in the safe place when the quake is over,

• store nonperishable food and water, a fire extinguisher, flashlight with batteries, a portable radio, medicines, and a first-aid kit in a place you can access after the earthquake.

• If you are indoors when the shaking starts, crouch or lie face down under a table or desk in the center of the room

• If you are outside when the shaking starts, lie face down away from buildings, power lines, and trees and cover your head with your hands

• If you are in a car when the shaking starts, stop the car and remain inside.