Earthquakes occur when stress causes rocks within the Earth's crust to rapidly fault or fracture, releasing energy. They originate at a focus below the surface and the point directly above on the surface is called the epicenter. By measuring the arrival times of P and S waves from an earthquake at three different seismograph locations, triangulation can be used to locate the epicenter on a map. The Earth has a solid crust and mantle underlain by a liquid outer core and solid inner core, with increasing temperatures and densities toward the center.

1. Earthquakes & Earth’s
Interior

2. Earthquakes
➔ A sudden movement in the Earth’s crust that releases energy
➔ The Earth’s surface is solid rock, but beneath the surface lies a hotter
and therefore more plastic medium
➔ When the heat from the Earth’s interior radiates outward towards the
surface, the interior moves and expands
➔ As the interior moves it applies stress to the surface rocks
➔ This stress causes the surface rocks to strain or deform
➔ If this happens slowly, the surface rocks FOLD
➔ If this happens rapidly, the surface rocks FAULT resulting in an
Earthquake

3. Earthquakes
➔ Earthquakes occur along faults
◆ Most major faults are
known some are not
➔ The point IN the Earth where
the Earth fractures is called the
FOCUS
➔ The point ON the Earth’s
surface directly above the
focus is called the EPICENTER

4. 3 Types of Faults

5. How are Earthquakes measured?
➔ Seismology: the study of
Earthquakes
TWO Scales are used to Measure
➢ Mercalli Scale: based on human
impact reports
○ Measured on a scale of I-
XII
➢ Richter Scale: measures
earthquakes magnitude based
on an instrument called a
seismograph
○ Seismograph: measures
earthquake intensity
graphically based on a
weights displacement
(movement) due to
shaking

6. Mercalli Scale

7. Richter Scale

8. Earthquakes radiate energy
When an earthquake occurs the energy
radiates outwards as P-waves and S-waves
➔ P-waves (Primary-waves): vibrate back
and forth in the direction of travel
◆ They are the FASTER of the two
waves
➔ S-waves (Secondary-waves): vibrate up
and down perpendicular to the direction
of travel
◆ They are the SLOWER of the two
waves

9. Locating an Earthquake’s Center
You need
1. 3 Seismographs for P-wave, S-wave and arrival times for 3 separate
locations
2. Earthquake travel time graph
a. Determines the distance to epicenter for all 3 locations
3. Map
4. Drawing compass

10. Steps
1. Determine the difference in arrival
times of the P-waves and S-waves for all
3 locations
2. Fit those differences to the P & S wave
travel time graph. Determine the
distance to the epicenter for all 3
locations
3. Use a drawing compass to draw a circle
around the 3 locations at the proper
distances
4. The circles should meet at one location,
the EPICENTER!!!

11. The Epicenter

12. Time of earthquake origin
You must determine the exact time of origin of the earthquake
1. Use the distance you already have.
2. Go up to the P-wave curve and follow this left to the time. MARK
THIS!!!
3. Subtract the time of travel from the time of P-wave arrival
4. This is the time of origin

13. Earth’s Interior
➔ 4 distinct layers
The Crust
➢ The outermost layer
➢ Solid rock ranging in thickness from 5 to 60 km thick
○ Thicker under the continents
○ Thinner under the ocean
○ Average density is 2.7g/cm3

14. Earth’s Interior
The Mantle
➢ Beneath the crust
➢ Approximately 2,900 km thick
➢ Rocks are more plastic than solid
○ Similar to playdough
➢ Average density is 5.5g/cm3

15. Earth’s Interior
Outer Core
➢ Liquid
➢ Constant convection creating a
magnetic field
Inner Core
➢ Solid
➢ Hotter than the surface of the
sun
➔ Both are VERY HOT!!!
➔ Each are about 2,400 km thick
➔ Average density 11g/cm3

16. Earth’s Interior

17. How do we know
about the core?
➔ P-waves can travel
through solids and liquids
➔ S-waves can ONLY travel
through solids