1. Forces inside Earth Information Scale and Forecasting
19.2 Earthquakes
Forces inside Earth
Information
Scale and Forecasting

2. What are earthquakes? Earthquakes occur at plate boundaries.
As plate move their rock edge experience immense pressure and this causes waves.
Pressure is so great it breaks the rock.
Rock breaks = energy is released (seismic waves)

3. Requirements for Earthquakes
Fault – boundary of two tectonic plates
Active Stress along the fault
Failure
Rocks “snapping” back elastically
Reach Elastic Limit
Elastic Rebound Theory
Phenomenon whereby stressed rocks snapback elastically after an earthquake to their pre-stress condition
Stress exceeds rupture strength of rock

4. General Definitions
Focus- The point beneath the Earth’s surface where the rocks break and move.
Epicenter- The point on the Earth’s surface directly above the focus.
Seismic Waves- Waves produced by earthquakes

5. Focus vs. Epicenter Focus Epicenter
Point in Earth energy is released from
P-waves
S-waves
Move outward in all directions, not just toward surface
Epicenter
Point directly above focus on the surface
Energy that reaches surface generate surface waves
Move along surface
Elliptical motion & back and forth
Most destruction

6. Seismic Waves (ENERGY)
Primary (P-waves)
Travel fastest
Longitudinal wave
Through solids, liquids, and gasses
Faster through denser material
Originate at Focus
Secondary (S-waves)
Slower that P-waves
Transverse waves
Perpendicular to flow
Can not pass through liquid or gasses
Faster through denser material
Originate at Focus

7. Surface Waves Moves only along Earth Surface
Combination of up and down motion and back and forth motion
Cause more destruction than P or S waves.

8. Locating an Epicenter Based on different speed of seismic waves
Need at least 3 seismograph stations
Determine arrival times of p and s waves
Calculate difference between p and s waves
Use the graph from Earthquake information work sheet
Compare radius with 2 other stations
Draw radius from all three stations
Intersection point is EPICENTER

9. Using Seismic Waves to Map Earth’s Interior
At certain depths, speed and path of waves change
Through lithosphere – speed up
Asthenosphere – slow down
Lower Mantle – speed up
Outer core – s waves stop
Inner core – speed up (p waves)

10. Normal Faults Tension Force – causes stretching and elongation
Pulls rocks apart (divergent boundary)
Fracture
Rock above fault moves downward in relation to rock below fault line
Formed Sierra Nevada Mountains

11. Reverse Fault Compression force – Convergent boundaries
Pushes on rocks causing to bend and sometimes break
Rocks above fault are forced up and over rocks below fault line
Rocky Mountains

12. Strike-slip Fault (Transform)
Shear Force
No upward or down movement
Surfaces snag and rocks are twisted and strained due to friction
Short and sudden but can be slow and steady
San Andreas Fault

13. Magnitude & Intensity Damage caused by Earthquakes depends on…
Distance to Epicenter
Depth of Focus
Bedrock/Soil Types
# and types of structures
Magnitude vs. Intensity
Magnitude – measured force based on seismic waves
Intensity – What was felt? What occurred? Observer specific

14. Modified Mercalli Scale
Arbitrary set of definitions based upon…
What people in area feel
Observations of damage to buildings
Advantages
Great if in inhabited area of developed country
Easy to use
NO Specific equipment
Disadvantages
Difficult in places without buildings or trees (desert)
Observer specific – what appears to be “heavy” to one may not be for someone else

16. Richter Magnitude Scale
Based on Seismic waves
Deals with strength of break (fault line)
Each step is 30 fold higher increase in magnitude
Advantages
Allows for comparison regardless of location
Good for small, nearby quakes (seismograph)
Used from 1930’s – 1980’s

18. Moment Magnitude Scale
Estimates Total energy released by quake
Good for ALL earthquakes
Measures size and type of waves and movement along faults
When you hear “Richter” on the news, they are actually referring to Moment Magnitude Scale as of late 1980’s

19. Forecasting vs. Predicting
Forecasting – General statement about what will happen in future (not specific)
Predicting – Specific statement with specific date
Animal behavior – ground animals come out of ground
Radon Levels increase in ground water
Why it is difficult to predict
Too many factors
Movement, stresses, rock mechanical property, past movement, seismic gaps