Ch. 8: Earthquakes

Question 1

Earthquake

Answer 1

Earth shaking caused by a rapid release of energy
tectonic stresses cause rocks to break
energy moves outward as an expanding sphere of waves

Question 2

What are the causes of most Seismicity?

Answer 2

• Most seismicity (earthquake activity) occurs due to…
o Motion along a fault
o Inflation of a magma chamber
o Volcanic eruptions

• Lesser causes:
o Giant landslides
o Meteorite impacts
o Nuclear explosions

Question 3

What is the focus of an earthquake?

Answer 3

the break along an earthquake fault within the

earth from which seismic waves originate

Question 4

What is the epicenter?

Answer 4

the point on the earth’s surface directly above

the focus of an earthquake

Question 5

What is the Elastic Rebound Theory?

Answer 5

Strain builds up until rock’s strength is exceeded
Stress is released, blocks are displaced across fault, then snap back to original shape.
This elastic rebound releases energy as seismic waves.

Question 6

Primary or P waves

Answer 6

Compressional, or primary (P), waves
• Push-pull (compress and expand) motion
• Fastest type of seismic waves
• Travel through solids AND liquids

Question 7

Secondary or S waves:

Answer 7

Shear, or secondary (S), waves
• Sideways motion
• Slower than P waves
• Travel only through solids, not liquids

Question 8

Surface waves:

Answer 8

Travel along Earth’s surface

These waves are the slowest and most destructive type of seismic waves.

Question 9

What are Seismographs/seismometers?

Answer 9

– Instruments that record seismic waves
– detect earthquakes anywhere on Earth
– reveal size and location of earthquakes

Question 10

What is the Richter magnitude M? How is that determined?

Answer 10

The Richter scale represents the energy released
by an earthquake.
Each increase in magnitude corresponds to 30X more energy released

Question 11

What is the Mercalli intensity? What do the Roman numerals mean?

Answer 11

• Mercalli intensity scale - Degree of shaking
damage
– Roman numerals I-XII assigned to different levels of damage.
– Damage occurs in zones related to local geology, bedrock vs. water-saturated soils

Question 12

Where do most earthquakes occur? along plate boundaries (interplate)

Answer 12

Divergent and transform: shallow
ex) San Andreas
Convergent: shallow to deep (≤700 km); Wadati-Benioff zone
ex) Japan
But some earthquakes occur within a plate (intraplate)
ex) New Madrid, MO; Charleston SC

Question 13

What are the hazards of earthquakes:

Answer 13

Ground shaking
Building collapse
Liquefaction
Landslides
Tsunami
Fire

Question 14

Earthquake prediction (short-term)

Answer 14

Precursors: clusters of foreshocks
strain build-up and deformation of crust
changes in water level, gasses in wells
animal behavior (may hear ultrasonic micro-cracking)

Question 15

stick-slip history of a fault

Answer 15

Stress builds up until rock’s strength is exceeded
• Micro-fractures form (→ foreshocks)
• When fault moves → main shock
• Minor movement later → aftershocks

Question 16

Forecasting (long-term)

Answer 16

: identifying recurrence intervals between seismic events

Seismic gaps: regions along a plate boundary where no recent large earthquake

Question 17

Preparedness:

Answer 17

seismic engineering; 
- bracing
- cross beam
- anchor bolts
- rollers
community earthquake drill

Question 18

The Japan Earthquake video

Answer 18

cracks and liquefaction
two tsunamis
thousands of aftershocks