1. Earthquakes
Movement of Earth’s lithosphere that occurs when rocks suddenly shift due to pressure, releasing stored energy.
Small portion of this energy is carried by vibrations called seismic waves.

2. Epicenter: Location on Earth’s surface directly above the earthquake.
Epicenter & Focus of Earthquakes
Epicenter: Location on Earth’s surface directly above the earthquake.
Some A, B, Cs of earthquakes: Earthquakes occur along faults IN the Earth, NOT on the surface of the Earth. The actual place in the Earth where an earthquake occurs is called the “Focus” also called the “hypocenter.”
This diagram shows the earthquake waves (seismic waves) moving out from the earthquake focus. The “Epicenter” is the place on the surface of the Earth directly above the earthquake. In news about earthquakes, the epicenter is often reported; this is generally where the earthquake damage will be greatest because it is the surface point closest to the earthquake. To describe where in the Earth an earthquake occurred, you must determine the “epicenter” AND the depth of the earthquake.
VIDEO LECTURE: “Earthquake focus (hypocenter) and epicenter” on TOTLE web site under “VIDEOS” under the topic “Introduction to Plate Tectonics and Earthquakes”.
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Focus (or hypocenter): Location within the Earth where the earthquake occurred. 2

3. Body waves (P and S) travel inside Earth.
Body Waves and Surface Waves
Surface waves travel along Earth’s surface.
Body waves (P and S) travel inside Earth.
Earth’s interior structure and seismic raypaths that are used to determine the Earth structure.
For large earthquakes, seismic waves travel through and around the entire Earth. This diagram is a cross section of the whole Earth showing crust, mantle, and core. “Body waves” are seismic waves that travel through the interior (the body) of the Earth. Surface waves travel around the perimeter of the Earth with oscillations that are dominantly in the upper 50 to 100 kilometers of our planet.
VIDEO LECTURE: “Seismic Waves #1: Kinds of seismic waves” by Robert Butler can be found on TOTLE web site under “VIDEOS” under the topic “Introduction to Plate Tectonics and Earthquakes”.
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The computer program “SeismicWaves” can be used to animate the body waves and surface waves from several recent large earthquakes. This is quite useful to help students understand that there are many kinds of seismic waves generated by large earthquakes and these waves can have complicated paths including bounces (reflections) and bending (refractions) at the boundaries between Earth’s crust, mantle, and core. In fact, studies of these seismic waves have provided a major portion of our knowledge about the internal structure of Earth. The SeismicWaves program is freeware developed by Alan Jones. The program runs on any PC (NOT MAC) and is downloadable from
CLASSROOM ACTIVITY: “SeismicWaves Program Student Worksheet” was developed by Roger Groom, Mt Tabor Middle School, Portland. This activity can be found on TOTLE web site under “LESSON PLANS” under the topic “Introduction to Plate Tectonics and Earthquakes”.
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Graphic from IRIS one-pager, “Exploring the Earth Using Seismology” that can be downloaded from
While P- and S- waves radiate outward in all directions, surface waves travel along the surface of the Earth and decrease in amplitude with depth. 3

4. Types of seismic waves P wave Fast S wave Intermediate Surface Slow
VIDEO LECTURE: “Seismic Waves #2: P, S, and surface waves” by Robert Butler can be found on TOTLE web site under “VIDEOS” under the topic “Introduction to Plate Tectonics and Earthquakes”.
URL
There are two classes of seismic waves: body waves, that travel at higher speeds through the deeper rocks within the Earth, and surface waves, that travel at slower speeds through rock near Earth's surface. The body waves precede the surface waves. There are two types of body waves: P-waves, that are similar to sound waves, and the slower but more damaging S-waves. Near Earth’s surface, P-waves travel about 4.8 to 8.0 km (3 to 5 miles) in one second, while S-waves travel about 3.2 to 4.8 km (2 to 3 miles) in one second. Surface waves are slower still and can cause even more damage due to their greater duration and often larger amplitudes.
Body waves can be either “P waves” or “S waves”.
P waves are also referred to as “primary waves” or “pressure waves”. These terms are accurate because P waves are the fastest seismic waves and are therefore the first to arrive from an earthquake (they are the “primary” waves). P waves travel as a series of compressions (areas where the rock is squeezed together) and expansions (areas where the rock is expanded). The diagram at the upper left shows how to make a P wave using a slinky.
S waves are also referred to as “secondary waves” or “shear waves”. These terms are accurate because S waves are the second fastest seismic waves and are therefore the second waves to arrive from an earthquake (they are the “secondary” waves). S waves travel as a series of sideways motions where the material actually moves perpendicular to the directions that the wave travels. The diagram at the center left shows how to make an S wave using a rope. You can also make an S wave using a slinky.
Surface waves have “rolling” motion kind of like waves in the ocean. They also have wave motions that decrease in amplitude downwards into the Earth just like ocean waves have water motions that decrease with depth into the ocean.
A great resource for teaching tips and animations on seismic waves can be found at
TASA Figure 219t. Graphics from Tasa Graphic Arts Inc ( : Copyright protected: The content may only be used for personal, educational or noncommercial purposes. 4

5. Seismic Wave Travel Time Curves
COMPUTER ANIMATION: “Travel Times #3: How Travel Times Curves Were Developed” on TOTLE web site under “ANIMATIONS” under the topic “Introduction to Plate Tectonics and Earthquakes”. URL
Link to “Travel-Time Curves” animation in Notes.
By analyzing earthquake waves from thousands of earthquakes, seismologists have determined the travel times for seismic waves. 5

6. Seismic waves cont…
The path of seismic waves helped scientists understand the layers of the earth. S waves could not penetrate the core of the earth.

7. How far away was the earthquake?
VIDEO LECTURE: “Locating Earthquakes #1: Using the S - P Arrival Times to Determine the Distance From an Earthquake” by Robert Butler can be found on TOTLE web site under “VIDEOS” under the topic “Introduction to Plate Tectonics and Earthquakes”.
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P and S waves start out from the earthquake at the same time. P waves travel about 1.7 times faster, so they reach each seismic station sooner than the S waves. The farther the station is from the earthquake, the greater the time difference between the P and the S wave arrivals. This graph shows the travel time for P waves and S waves against distance from the earthquake as measured on the Earth’s surface in kilometers. For this example, the P wave arrived at the station 5 minutes and 45 seconds after the earthquake occurred; the S wave arrived at the station 10 min and 30 seconds minutes after the earthquake occurred.
Use S-arrival time minus P-arrival time to determine distance from the travel-time curves. 7

8. Locating an Earthquake
ACTIVITY:
Locating an Earthquake 1
Determine distance of EQ from three seismic stations by calculating the S minus P arrival times.
Plot them on the travel- time graph.
Intersection of the circles gives the location. 2
A classic classroom earthquake activity described in almost every textbook treatment of earthquakes is how to determine the location of an earthquake from a seismic station (= seismometer) using the travel time for P waves and S waves from that earthquake. If you have determinations of the distance of an earthquake from three seismic stations, you can triangulate the location of an earthquake. For a “regional earthquake”, you can do this problem on a flat map using a compass to draw circles around the three seismic stations with radius equal to the distance of the earthquake from those stations. The circles should intersect at the epicenter of the earthquake. For a distant earthquake (e.g. in the middle of the Atlantic Ocean), you must deal with the complexity of spherical geometry, although the principles of triangulation are the same.
In this slide an earthquake off the coast of southern Mexico is recorded by 3 stations listed in figure 1 above. If you just figure out how much time elapsed between the P and S waves, you can plot them on the lower graph (figure 2) to see how far each station was from the epicenter. You slide each seismogram along between the green and red slopes of the graph until the P-wave arrival touches the red line and the S-wave arrival touches the green line. Then you make great circles of those diameters around each station. There is only one location that it can be. If you have only 2 seismometers there are two potential earthquake locations.
CLASSROOM ACTIVITY: “Earthquake Location - Regional Triangulation with Real Data” was developed by Anne Ortiz and Tammy Baldwin and is offered through Science Education Solutions. The earthquake studies was a moderate event in northern Arizona. This activity can be found on TOTLE web site under “LESSON PLANS” under the topic “Introduction to Plate Tectonics and Earthquakes”.
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SUPPORTING POWERPOINT PRESENTATION: A short PowerPoint presentation, “EQLocation.ppt”, can be used with the classroom activity. This presentation illustrates “picking” the P- and S-wave arrivals on one seismogram and shows how to use the time difference to compute the distance of the seismic station from the earthquake. The presentation also shows the map operation of drawing circles around the stations to determine the epicenter.
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9. Measuring Earthquakes
Seismograph- record seismic waves to measure and pinpoint their epicenters.
Richter Scale- rates earthquakes based on measurements of the times and amplitudes of seismic waves by certain seismographs
Based on 10’s( a 4 is ten times greater than a 3, a 5 would be 100 times greater)
Other scales used Moment Magnitude Scale and Modified Mercalli Scale-

10. No. of earthquakes per year
Richter Scale
Richter scale no.
No. of earthquakes per year
Typical effects of this magnitude
< 3.4
Detected only by seismometers
30 000
Just about noticeable indoors
4 800
Most people notice them, windows rattle.
1400
Everyone notices them, dishes may break, open doors swing.
500
Slight damage to buildings, plaster cracks, bricks fall.
6.2  6.9
100
Much damage to buildings: chimneys fall, houses move on foundations. 15
Serious damage: bridges twist, walls fracture, buildings may collapse. 4
Great damage, most buildings collapse.
> 8.0
One every 5 to 10 years
Total damage, surface waves seen, objects thrown in the air.

11. Modified Mercalli Intensity Scale
Mercalli Intensity Equivalent Richter Magnitude Witness Observations
I 1.0 to 2.0 Felt by very few people; barely noticeable.
II 2.0 to 3.0 Felt by a few people, especially on upper floors.
III 3.0 to 4.0 Noticeable indoors, especially on upperfloors, but may not be recognized as an earthquake.
IV 4.0 Felt by many indoors, few outdoors. May feel like heavy truck passing by.
V 4.0 to 5.0 Felt by almost everyone, some people awakened. Small objects moved. trees and poles may shake.
VI 5.0 to 6.0 Felt by everyone. Difficult to stand. Some heavy furniture moved, some plaster falls. Chimneys may be slightly damaged.
VII 6.0 Slight to moderate damage in well built, ordinary structures. Considerable damage to poorly built structures. Some walls may fall.
VIII 6.0 to 7.0 Little damage in specially built structures. Considerable damage to ordinary buildings, severe damage to poorly built structures. Some walls collapse.
IX 7.0 Considerable damage to specially built structures, buildings shifted off foundations. Ground cracked noticeably. Wholesale destruction. Landslides.
X 7.0 to 8.0 Most masonry and frame structures and their foundations destroyed. Ground badly cracked. Landslides. Wholesale destruction.
XI 8.0 Total damage. Few, if any, structures standing. Bridges destroyed. Wide cracks in ground. Waves seen on ground.
XII 8.0 or greater Total damage. Waves seen on ground. Objects thrown up into air.

13. How earthquakes effect humans
Destroys buildings, roads, bridges
May signal upcoming volcanic activity
Causes landslides
Causes fires
Death

14. Volcano A mountain that forms when magma reaches the surface.
Magma rises because it is less dense than the solid rock around and above it.

15. A hotspot is a location on the Earth's surface that has experienced active volcanism for a long period of time
Firstly, what are hotspot volcanoes and how do they form?
A hotspot is a location on the Earth's surface that has experienced active volcanism for a long period of time.
The source of this volcanism is a mantle plume of hot mantle material rising up from near the core-mantle boundary through the crust to the surface (see left diagram).
A mantle plume may rise at any location in the mantle, and this is why hotspot volcanoes are independent from tectonic plate boundaries.
The Hawaiian island chain are an example of hotspot volcanoes (see right photograph).
The Hawaiian island chain are examples of hotspot volcanoes.
Photo: Tom Pfeiffer /

16. The volcanoes get younger from one end to the other.
The tectonic plate moves over a fixed hotspot forming a chain of volcanoes.
Hotspot’s commonly form volcanic island chains (like the Hawaiian islands). These result from the slow movement of a tectonic plate over a FIXED hotspot.
Persistent volcanic activity at a hotspot will create new islands as the plate moves the position of the “old” volcanic island from over the hotspot.
Therefore at one end of the island chain you see the youngest, most active volcanic islands (directly over the hotspot) and along the island chain the extinct volcanoes become older and more eroded (see diagram).
This way geologists can use hotspot volcano chains to track the movement of the tectonic plate through time.
The volcanoes get younger from one end to the other.

17. Volcanism is mostly focused at plate boundaries
Pacific Ring of Fire
This map shows the margins of the Pacific tectonic plate and surrounding region. The red dots show the location of active volcanism. Notice how the majority of the volcanism is focused in lines along the plate boundaries? For this region this area is known as the “Pacific Ring of Fire”.
But why are all of the volcanoes located at the plate margins?
Volcanism is mostly focused at plate boundaries

18. Mt. St. Helen

19. How volcanoes affect humans
Collapses roofs
Brings down power lines
Kills plants
Contaminates water supplies
Respiratory hazard for humans and animals
Buries cities/land
Burns
Moving on from pyroclastic flows, there are other hazards associated with volcanic eruptions. Such as Pyroclastic Fall:
An explosive eruption will produce an eruption column of hot gas, ash and debris ejected kilometres into the air. As this debris falls back down to the ground it can cause a lot of damage.
Like too much snow on a roof, too much ash raining down from an eruption column can cause the roof to collapse.
Ash loading on power lines will cause them to fall.
As little as 1 centimetre of ash accumulated on the leaves of a plant will stop it from being able to photosynthesize and therefore the plant will die.
Lots of fine ash falling in lakes, rivers and water reservoirs will cause contamination making it unfit to drink, or to live in if you are a fish etc.
Very fine ash particles, if inhaled by humans, can cause extensive damage to the lungs causing a respiratory disease called silicosis.

20. Tsunamis
Large sea wave generated by an underwater earthquake, volcano, or landslide.