Earthquakes Notes
(Glencoe Chapter 19)
E1.2E Evaluate and the future career and occupational prospects of science fields
E3.2B Explain how scientists infer that the Earth has interior layers with discernable properties
using patterns of Primary (P) wave and Secondary (S) wave arrivals.
E3.2d Explain the uncertainties associated with models of the interior of the Earth and how
these models are validated.
E3.3A Explain how plate tectonics accounts for earthquakes.
E3.3d Distinguish plate boundaries by the pattern of depth and magnitude of earthquakes.
E3.4A Use the distribution of earthquakes and volcanoes to locate and determine the types of
plate boundaries.
E3.4B Describe how the sizes of earthquakes are measured or characterized.
E3.4C Describe the effects of earthquakes on humans.
E3.4f Explain why fences are offset after an earthquake using the elastic rebound theory.
Earthquake- shaking and trembling that results from movement of rock beneath earth’s surface;
caused by stress
Stress- forces that act on rock (pulling or squeezing) to change its shape and volume
Energy stored in rock until rock breaks or changes shape; energy is released
during earthquake
3 Types of Stress- work over millions of years to change shape and volume of rock
A. Shearing- pushes mass of rock in two opposite directions
- demo: tear paper in half
B. Tension- pulls on rock, stretching it and causing it to become thin in the middle
- demo: pull on both ends of paper to tear
C. Compression- squeezes rock together, bunching it up in the middle; decreases volume
- demo: crumble paper up
Breaking of Rock at Faults (E3.3A) (E3.4A)
When enough stress builds up in rock, the rock breaks, creating a fault
Fault- a break in earth’s crust where slabs of crust is moving
- Usually occurs (90%) along plate boundaries, where forces of plate motion compress,
pull, or shear the crust so much that it breaks
- Can also occur away from plate boundaries (where old plate boundaries used to exist
but these faults have been buried by rock material over millions of years)
Scientists use the locations of earthquakes and volcanoes to help determine the location and
types of tectonic plate boundaries.
- Map out where earthquakes (and volcanoes) have occurred (historically) and “connect
the dots” to plot out where plate boundaries are.
- Also use geologic info to determine type of plate boundary
Lots of Earthquakes on Pacific Coast because of movement of Pacific Plate and the North
American Plate. The Pacific Plate is moving northward, sliding past it.
- San Andreas Fault (CA) is 650 miles long and extends to depths of 10 miles deep
Kinds of Faults (E3.3A) (E3.3d) (E3.4A)
A. Strike-Slip Faults- rocks slide past each other (transform plate boundary)
- shallow earthquakes (25 km or less)
- magnitude less than 8.5
- caused by shearing
Right-Lateral Strike-Slip Fault- when standing on one side of a fault, the opposite side
appears to be moving to the right
Left-Lateral Strike Slip Fault- when standing on one side of a fault, the opposite side appears
to be moving to the left
Ex: San Andreas Fault (CA)
B. Normal Faults- rocks pull apart, or diverge (divergent plate boundary)
- shallow earthquakes (70 km or less)
- magnitude less than 8
- caused by tension
- fault is at an angle, so one block lies above the other
block wide on top = hanging wall
block wide on bottom = footwall
- hanging wall slips downward below footwall (moves away)
Ex: Rio Grande Rift Valley (NM)- two pieces of crust are diverging
C. Reverse Faults- rocks push together, or converge (convergent plate boundary)
- deep earthquakes (greater than 200 km)
- high magnitude (typically greater than 8.5)
- caused by compression
- reverse action of a normal fault
- hanging wall moves upward above footwall (toward each other)
Ex: Glacier National Park (MT)
Friction Along Faults
Friction- force that opposes motion of one surface against another
- Friction low- low chance of earthquake
- Friction high- high chance of earthquake
Ex: San Andreas Fault- friction is high in most places- earthquakes!!!
Mountain Building
Forces of plate movement can change a flat plain to a towering mountain range
A. Mountains formed by faulting
- normal faults uplift a block of rock
- series of mountains and valleys (basins and ranges)- seen in NV and UT
B. Mountains formed by folding
- compression causes folds and bends in rock
- can form towering mountains (Himalayas, Alps) or earthquakes (fractures)
C. Anticlines and Synclines- caused by compression
- Anticline- fold in rock that bends upward into an arch (Black Hills, SD)
- Syncline- fold in rock that bends downward to form a bowl (Illinois Basin)
D. Plateaus- large flat land elevated above sea level
- form when a vertical fault pushes up rock that is wider than it is tall
- Ex: Colorado Plateau (Four Corners- AZ, UT, CO, NM)- used to be part of sea floor
The Study of Seismic Waves (E3.2B)
Scientists who study earthquakes are a type of geologist known as seismologists.
Everyday, there are nearly 8,000 earthquakes
Seismic Waves- vibrations produced by movements in the earth (earthquakes)
Carry energy released by earthquake through the earth and to the surface
(Surface) energy is greatest at epicenter
Types of Seismic Waves
A. Primary Waves (P-Waves)
- first to arrive
- compress and expand, like an accordion or slinky – longitudinal waves
- can move through solids and liquids
B. Secondary Waves (S-Waves)
- second to arrive
- up and down, side to side motion
- can ONLY move through solids (NOT liquids)
C. Surface Waves (extension of P and S waves)
- waves that reach the surface
- move slower than P and S waves
- produce the most severe damage
Using Seismic Waves to Determine the Earth’s Interior Layers and Properties (E3.2B)
Study the change in speed and direction of seismic waves as they travel through the earth
Seismic waves travel faster through solids than liquids
S-waves cannot travel through liquids and therefore cannot be detected on the other side
of the earth. (Image on Glencoe page 503)
This creates a shadow zone
An occurrence of S- wave shadow zones indicate a fluid core.
Parts of an Earthquake
Focus (or hypocenter)- point beneath earth’s surface where earthquake “started”
Epicenter- point on earth’s surface directly above the focus where most energy is
released and damage occurs
Locating the Epicenter
Measure difference between time arrival of P and S waves
The greater the difference, the further away the earthquake is
Draw (at least) 3 circles on a map of the distance from the epicenter
Where the 3 circles intersect is the epicenter
Look at page 509 of the Glencoe Text OR page 69 of the Inside Earth Text
Monitoring Faults and Potential Earthquakes
Predicting earthquakes is very difficult
Scientists continue to look for way to accurately predict when and where earthquakes will occur
Devices that Monitor Faults (E1.2E)
Scientists observe changes in stress and crust formation. NOTE that these instruments simply
MONITOR faults. They do NOT measure an actual earthquake.
A. Creep Meters- measures horizontal movement of the ground by use of a wire and a
B. Laser-Ranging Devices- measures fault movement by detecting change in distance
between receiver and reflector
C. Tiltmeters- measures tilting of the ground
D. Satellite Monitors- use radio waves to record changes in the ground (elevation) due to
Monitoring Risk in the United States (E1.2E)
- Geologists can determine earthquake risk by locating where faults are active and where past
earthquakes have occurred
- High Risk in U.S. is along Pacific (West) Coast- CA, WA, AK- this is where Pacific and North
American Plates meet
- Michigan is at minor risk of experiencing an earthquake
Measuring and Characterizing Earthquakes (E1.2E) (E3.4B)
Detecting Seismic Waves
Seismologists study earthquakes
Seismometer- instrument that records ground movements
Magnitude- measurement of how much energy is released during an earthquake; how much
ground movement was caused
Measure of Earthquake Magnitude
A. Moment Magnitude Scale- estimates total energy released by earthquake
- those with a 5.0 or above start to produce damage
- most scientists find this the most accurate measurement of earthquakes
B. Richter Scale- rates size of the (largest) seismic waves
Intensity- measurement of how strong the ground shakes during earthquake (and the damage that
typically results)
Measure of Earthquake Intensity
C. Modified Mercalli Scale- rates the intensity and damage of an earthquake
Earthquake Hazards and Safety (E3.4C)
STORY- Jan. 1995, earthquake struck Japan. In 20 seconds, thousands of buildings collapsed,
numerous freeways toppled, and more than 130 fires were started. Over 5,000 people died.
Many of the houses were top-heavy and collapsed because they weren’t well supported. It
would be like trying to hold up a thick textbook with four pencils.
How Earthquakes Cause Damage (E3.4C)
Seismic waves cause a lot of shaking, which can destroy buildings/bridges, topple utility poles,
and snap gas and water pipes.
- Can also cause: 1) landslides, 2) avalanches, and/or 3) tsunamis
- S waves are the ones that cause the most damage
Soil Conditions- the “tighter” the soil, the less shaking will be experienced
- loose, sandy soil will result in lots of shaking
Liquefaction- when soil that holds water is shaken so much that soil turns to mud
- triggers landslides; buildings sink and slide
Aftershocks- usually weaker earthquake(s) that occurs after a major one
Tsunamis- HUGE, destructive waves caused by underwater earthquakes
- part of sea-floor rises, causing waves to spread out
- waves start out low, but get taller and closer together as they reach shallow areas
- can travel as fast as 500 mph and get as tall as a six story building!!
Making Buildings Safer
To better withstand earthquakes, buildings must be: 1) strong, and 2) flexible
A. Choice of Location
- avoid steep slopes and loose soil
- build far away from fault lines (further away means less shaking)
B. Construction Methods
- anchor buildings to solid rock
- base-isolated building- building rests on shock-absorbing rubber pads, to reduce the violent
- use flexible joints and automatic shut-off valves on water and gas pipes
Protecting Yourself During an Earthquake
Main danger indoors are falling objects and flying glass
You need to drop, cover, and hold
- crouch beneath a desk or table, while holding the legs
- if that’s not available, sit down against an inner wall (away from outer walls, windows,
and wall hangings), duck your head into your knees, and cover your head with your arms
- also may stand in an inner doorway (not an outside door)
- May wish to make an earthquake kit- includes bottled water, canned food, first aid
- get to an open area, away from buildings, power lines, trees, etc.
- sit or lie down to avoid falling or being thrown
Elastic Rebound Theory (E3.4f)
When stress builds up, rock is deformed and bends. All things associated with that rock also
moves with the deformed rock. Eventually, the stress is too much and the rock snaps back into
This is often why fences (as well as roads and other structures along a fault line) are often
offset, or broken after an earthquake occurs.
General Info
Earthquake Info with some pics and animation
Earthquake Animations
E3.2B Using Seismic Waves to Determine Earth’s Interior
E3.3A Plate Tectonics Accounts for Earthquakes
Animations of fault movement
Lots of pics and Notes- VERY Good
Normal Fault
Reverse Fault
Good Geology Site
E3.3d Earthquake Patterns Determine Plate Boundaries
Maps on Earthquake Depth
E3.4A Distribution of earthquakes to locate and determine plate boundaries
E3.4A Distribution of Earthquakes
Global Pic
E3.4B Measuring and Characterizing Earthquakes- Intensity and Magnitude Scales
E3.4C Earthquake Hazards and Safety
Earthquake Stories
Earthquake Activity
…in Michigan
E3.4f Elastic Rebound Theory
Explanation and animation
Explanation and Images
10.5 Movie Earthquake Myths/Fallacies