1. Earthquakes & Epicenters

2. Focus: Location within the Earth where the earthquake occurred.
Epicenter & Focus of Earthquakes
Epicenter: Location on Earth’s surface directly above the earthquake.
Focus
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: Location within the Earth where the earthquake occurred.

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.

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.

5. P waves travel by COMPRESSION
Body waves
P waves travel by COMPRESSION
COMPUTER ANIMATION: “Primary (Pressure) Wave Motions” on TOTLE web site under “ANIMATIONS” under the topic “Introduction to Plate Tectonics and Earthquakes”.
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CLASSROOM ACTIVITY: “Seismic Waves and the Slinky” was developed by Professor Larry Braile, Purdue University. This activity can be found on TOTLE web site under “LESSON PLANS” under the topic “Introduction to Plate Tectonics and Earthquakes”. URL
VIDEO: A short video segment “Seismic Waves #3: Primary (Pressure) Wave in a Slinky” shows a P-wave generated in and travelling across a slinky. This video can be found on TOTLE web site under “VIDEO” under the topic “Introduction to Plate Tectonics and Earthquakes”.
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TASA Figure 219t. Graphics from Tasa Graphic Arts Inc ( : Copyright protected: The content may only be used for personal, educational or noncommercial purposes. Please do not reproduce TASA Figure 219t.

6. Body waves
S waves are shear waves
COMPUTER ANIMATION: “Secondary (Shear) Wave Motions” on TOTLE web site under “ANIMATIONS” under the topic “Introduction to Plate Tectonics and Earthquakes”.
URL
CLASSROOM ACTIVITY: “Seismic Waves and the Slinky” was developed by Professor Larry Braile, Purdue University. This activity can be found on TOTLE web site under “LESSON PLANS” under the topic “Introduction to Plate Tectonics and Earthquakes”. URL
VIDEO: “Seismic Waves #4: Shear (Secondary) Wave in a Slinky” shows a S-wave generated in and travelling across a slinky. This video can be found on TOTLE web site under “VIDEO” under the topic “Introduction to Plate Tectonics and Earthquakes”.
URL
TASA Figure 219t. Graphics from Tasa Graphic Arts Inc ( : Copyright protected: The content may only be used for personal, educational or noncommercial purposes. Please do not reproduce TASA Figure 219t.

7. A seismograph detects and records EQs.
How do scientists detect earthquakes?
When an earthquake occurs the seismic waves travel through the Earth to the seismic station where the information is transmitted to distant computers.
A seismograph detects and records EQs.
A seismogram is the EQ record.
The “seismometer” is the instrument that is supersensitive to ground motions and therefore can detect the minute ground motions produced by a distant earthquake. Some seismometers record horizontal movement (N-S or E-W) while other seismometers record vertical movement (up-down).
A “seismogram” is the graphical representation that the seismometer makes of a particular earthquake. As seen on the seismograph in the bottom figure, the order of arrival of seismic waves is: P wave, then S wave, then surface waves. When interpreted for us, a seismogram looks pretty simple. In actuality, it takes a trained eye to spot the arrivals of different kinds of seismic waves on the seismogram.

8. Seismographs Watch a computer animation of a vertical seismometer recording arrivals of the P and S waves. Watch an actual seismogram being recorded during an earthquake.
Link to Vertical Seismometer animation in Notes.
COMPUTER ANIMATION: “Vertical-Component Seismograph” on TOTLE web site under “ANIMATIONS” under the topic “Introduction to Plate Tectonics and Earthquakes”. URL
VIDEO: “Real Seismograph Recording an Earthquake” can be found on TOTLE web site under “VIDEO” under the topic “Introduction to Plate Tectonics and Earthquakes”.
URL
Link to Real Seismogram video in Notes.

9. Locating the Epicenter1
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.
URL 3

10. World Seismicity & Plate Tectonics
The computer program “SeismicEruption” can be used to animate worldwide earthquakes and volcanic eruptions from 1960 until today. This is a great activity to help students understand that earthquakes are concentrated along and near the boundaries between tectonic plates. The animation of volcanic eruptions dramatically shows the “Pacific Ring of Fire”, the belt of volcanoes surrounding the Pacific ocean.
The SeismicEruption program runs is freeware developed by Alan Jones. The program runs on any PC (NOT MAC) and is downloadable from
CLASSROOM ACTIVITY: “SeismicEruption 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”.
URL
Modified from USGS Graphics
ACTIVITY: See Notes for a link to a classroom activity on world seismicity, volcanoes, and plates.

11. INTERNET ACTIVITY: Look at current earthquakes
For an up-to-the-minute world map of earthquakes, check out the Seismic Monitor on the IRIS web site.
See notes for link.
To see seismic data from stations near you, go to the USArray Station Monitor and enter your zip code.
See notes for link.
There are several ways to look at earthquakes that just happened.
1. The US Geological Survey operates the National Earthquake Information Center. The center provides a wealth of information about recent earthquakes and archives of information about notable earthquakes worldwide.
URL
2. The Incorporated Research Institutes for Seismology (IRIS) provides a tremendous treasure chest of information about earthquakes, including educational seismology resources for a wide range of learners. The IRIS Seismic Monitor is a very effective display for recent earthquakes worldwide.
URL
3. The EarthScope Project is funded by the National Science Foundation. This project operates an array of seismometers, USArray, and makes the observations available on the Internet. You can find the USArray station nearest your school or house using the USArray Station monitor.
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12. How big was it? The Richter Scale
What is the Richter magnitude of this EQ?
S — P = 26 sec
Amplitude
= 23mm
The Richter scale is an example of a particular kind of magnitude scale for earthquakes. There are many kinds of magnitude scales but we’ll simplify this discussion. The scale illustrated in this figure uses the amplitude of S-wave ground oscillations to determine the magnitude. A seismograph from a particular earthquake is shown in the upper graph. The horizontal axis is time in seconds and the vertical axis is amplitude of ground oscillation detected by the seismometer. Obviously as the distance of the seismometer from the earthquake increases, the amplitude of ground oscillations from that earthquake decreases. So there must be an adjustment for distance of the earthquake from the seismometer. And distance is related to the difference in arrival times between the P wave and the S wave. Notice that the left hand axis in this illustration is distance of earthquake from seismometer plotted either in kilometers or in S minus P arrival time in seconds. The right hand graph is the measured amplitude of ground motion in millimeters. This is a “connect the dots” exercise. Put a dot on the left-hand graph at the measured S-P time of 24 seconds; put a dot on the right-hand graph at the measured amplitude of 23 millimeters; the line connecting the dots intersects the Magnitude scale at 5.0 so the magnitude of this earthquake is 5.0 If the amplitude of ground motion has been 2.3 millimeters for an earthquake of the same distance, the magnitude would be 4.0, emphasizing the point that the earthquake magnitude scale is “logarithmic”. A change in magnitude of ground motion by a factor of 10 results in a change in magnitude to 1.0
Magnitude = 5

14. Please put your maps and worksheets in your binder
Please put your maps and worksheets in your binder. Take out your Waves quiz.