Part 4. Earthquakes Explained
Earthquakes occur when energy is released, in the form of seismic waves, as a result of fault formation or movement along an existing fault. Many earthquakes occur along the faults between tectonic plates. Earthquakes occur below the surface at the focus. The area on the surface directly above the focus is the epicenter. Seismic waves emanate from the focus in all directions.
There are four types of seismic waves. The first waves to reach the surface are P waves (primary/pressure waves). They are created by alternating compression (pushing) and dilation (pulling) of rock. The second waves to reach the surface are S waves (secondary/shear waves). These waves shear the rock back and forth at right angles to the direction of travel. Both P and S waves originate from the focus. L waves (Love waves) and R waves (Rayleigh waves) originate from the epicenter. These are often the most damaging to surface structures.
Imagine you are holding a slinky attached to a wall. Your hand is the focus and the wall is the epicenter. The image on the top represents the motion of P waves. The image on the bottom represents the motion of S waves.
- What is the difference between an epicenter and a focus?
- During an earthquake, why will P waves hit the Earth’s surface first?
Part 5. Seismometer Animations
On your iPad, using the Photon web browser (or any web browser on a laptop), go to http://www.wwnorton.com/college/geo/egeo2/content/index/animations.htm
Watch animations:
(8.3) Seismic Wave Motion – two videos
(8.4) How a Seismograph Works
- Describe the difference between how S-waves and P-waves move through the Earth.
- The seismograph shown in the video is only measuring horizontal ground movement. This is only one of three axes along which the ground can move. Describe these three axes.
Part 6. How a Seismometer Works
On your iPad, open the iSeismometer app.
This application mimics a three-component seismograph. Each graph measures movement along one of three possible axes (east-west, north-south, up-down). These three axes are represented in math as X, Y, and Z axes.
Hold your iPad flat on the table, then experiment with different ways to “shake” your iPad and observe the movements on the graphs.
- Describe the direction (i.e. up/down) of movement that each of the three graphs is measuring.
- What does the height of each wave (the amplitude) represent?
- When you place the iPad on the lab bench and pound the table close to the iPad, what is the resulting amplitude in comparison to just tapping the table?
- Is this analogous to earthquakes with different epicenters or earthquakes with the same epicenter, but of different magnitudes?
Sample Solution