Earthquakes are caused by the sudden movement or breakage of the Earth's crust along plate boundaries and faults. Plate tectonic stresses build up as plates stick together over time, storing energy. When the stresses exceed the strength of the rocks, the plates slip rapidly, releasing stored energy in the form of seismic waves. There are three main types of seismic waves - P waves, which move rock back and forth; S waves, which move rock sideways; and surface waves, which cause the most earthquake damage as they travel along the Earth's surface. Seismograms record these wave arrivals and amplitudes, which can provide information about the distance to the earthquake epicenter.

1. EARTHQUAKES:
WHY? AND HOW?

2. EARTHQUAKES
• Caused by plate tectonic stresses
sudden movement or shaking of the Earth
• Located at plate boundaries
• Resulting in breakage of the Earth’s brittle crust

3. PLATE TECTONIC STRESSES
• Plate boundaries and faults (= cracks where plate sections
are moving in different directions) cause friction as plates move
• Plates in a fault zone have STICK-SLIP motion
– Periods of no movement (stick)
and fast movement (slip)
– Energy stored as plates stick,
– Energy released as plates slip

4. EARTHQUAKE DAMAGE
• Landslides
• Building damage
• Liquefaction

5. EARTHQUAKE DAMAGE
Most caused by SURFACE waves (arrive last)

6. EARTHQUAKE WAVES
• FOCUS = place deep within the Earth and along the fault where
rupture occurs
• EPICENTER = geographic point
on surface directly above focus
• SEISMIC WAVES produced by the release of energy
– move out in circles from the point of rupture (focus)
– 2 types: surface & body (travel inside & through earth’s layers)
• P waves: back and forth movement of rock; travel through
solid/liquid/gas
• S waves: sideways movement of rock; travel through solids only

7. EARTHQUAKE WAVES
• 3 types of seismic waves show up on seismogram
– P waves: shake earth in same direction as wave;
travel through solid, liquid, gas
– S waves: Shake earth sideways to wave direction;
travel through solids only
– Surface waves: circular movement of rock;
travel on surface – cause most damage!!

8. EARTHQUAKE WAVES
P waves move through solids & liquids
S waves move through solids only!!!

9. EARTHQUAKE WAVES
Body P waves S waves
waves
AKA
Moves
through
Movement
of rock
Primary (1st to arrive)
Longitudinal, Compression
all states of matter
(solid, liquid, gas)
• back and forth movement of rock
• push/pull or compression/stretch out
• Vibration is same as the direction of
travel
Secondary (2nd to arrive - larger)
Transverse, Shear
Can go through solids only
• Move sideways
• perpendicular to direction of wave
travel
• Like snake

10. EARTHQUAKE WAVES
Lets test your understanding!!
Is this a P or an S wave?
P wave!
S Wave

11. EARTHQUAKE WAVES
Seismographs record earthquake waves
Seismograms show:
• Amplitude of seismic waves (how much rock
moves or vibrates)
• Distance to the epicenter
(indirectly)

12. DISTANCE TO THE EPICENTER
Difference in time of arrival between S&P
wave  distance to the epicenter
Time-Travel Chart (in ESRT): matching time
difference provides the distance
• Small time difference 
close to epicenter
• Large time difference 
far from epicenter
What you need:
• Estimate the time of arrival of the P and S wave
• Subtract to get time difference

13. HOW TO READ SEISMOGRAMS
P & S (body waves) move through earth & arrive first
• P & S waves used to calculate magnitude of earthquake and distance
to the epicenter
• Examples (you try it!)