2. The average length of an earthquake is from 30 to 60 seconds.
An earthquake is the shaking and trembling that results from the movement of rock beneath the Earth’s surface.
The average length of an earthquake is from 30 to 60 seconds.
If friction between two blocks of rock is low, the rocks slide constantly without sticking.
If the friction is high, the blocks lock together.
When the amount of stress is greater than the amount of friction, the blocks unlock suddenly, setting off an earthquake!

3. What is a fault?
When stress builds up in rocks, it fractures. This fracture in the Earth’s crust is known as a fault. (The Earth is cracking under so much stress!)

5. What is a focus? What is an epicenter?
or hypocenter , is the point in the earth where the earthquake rupture or fault movement actually occurred
 the point on the Earth's surface that is directly above the focus

6. The farther the distance between the focus and the epicenter, the weaker the earthquake.
Distance from Focus to Epicenter

7. What are seismic waves?
Vibrations released from the focus in all directions that carry the energy of an earthquake away from the focus

10. What are the three types of seismic waves?
Primary waves
(P waves)
Secondary Waves
(S waves)
Surface waves (L waves)

11. Describe a P wave. Travel the fastest
Travel solids, liquids, and gases
Move through the Earth at different speeds, depending on the density
Called “push-pull” waves (compress and expand the ground like an accordion)

13. Describe an S wave. Arrive at a given point after the P waves
Travel only through solids
Move through solids at different speeds depending on the density
Cause rock particles to move from side to side & up and down

15. Describe a L wave. Slowest of the seismic waves
Move along the Earth’s surface like waves travel in the ocean
Cause most of the damage during an earthquake

17. What do scientists use to detect and measure earthquakes?
Seismographs are used by scientist to detect, measure & record the vibrations of seismic waves.
What do scientists use to detect and measure earthquakes?
An earthquake’s magnitude is a
measurement of its strength based
on seismic waves and movement
along faults.

20. Label the Seismogram

21. How do scientists determine the location of an earthquake’s epicenter?
Seismologists use data from seismograms to locate an epicenter.

22. After an earthquake, P waves arrive at seismograph first then the S waves.
This difference in time between the P & the S waves is called the S-P Interval

23. This difference in time tells scientists how far the epicenter is from the station.
The greater the difference, the farther away the epicenter.

24. . This information is then used to plot a circle on a map.
The radius of the circle represents the distance from the station to all points where the epicenter could be.

25. To determine the epicenter, at least 3 stations are needed.
The epicenter is found where the 3 circles intersect!

26. Let’s Practice finding S-P intervals…
Geology Labs:
Determining The Earthquake Epicenter