17- Earthquake Hazard

Question 1

What causes earthquakes

Answer 1

Along a fault, large rock masses are moving past each other pushed by tectonic forces. When stress builds up to a critical point (10-100s of years), and the accumulated stress is greater than the friction forces, there is enough force to cause a sudden slip along the fault, releasing stored strain energy. This produces shock waves into surrounding rock.

Question 2

Focus/ Hypocenter

Answer 2

Originating point of an earthquake on the fault line

Question 3

Epicenter

Answer 3

Point on surface directly above focus

Question 4

Body waves

Answer 4

Transmit energy through the earths interior in all directions from the earthquakes focus
Divided into:

P waves:
-travel through solids and liquids
-compressional wave, material pushed parallel to travel path
- ~5km/s

S-waves:
-travel only through solids
-up, down motion that pushed material at right angle to wave path
- ~2-3km/s

Question 5

Surface waves

Answer 5

Transmit energy along earths surface, causing surface to vibrate.
Includes 2 waves: (both cause extensive damage to rigid structures)
Love waves:
-restricted to earths surface
-cause sideways shaking of the ground(think snake), causing extensive damage to rigid structures
-speed slightly less than s waves
Rayleigh waves:
-produce rolling motions of the ground (imagine water ripple)
-slightly lower velocity than S waves

Question 6

How are earthquakes measured

Answer 6

1. Intensity
   - subjective measure based on damage and people’s reaction
   -measured using modified mercalli intensity scale
   -factors that affect intensity; distance from epicenter, focal depth, population density, local geology, duration and building types
2. Magnitude
   - quantitative measure of energy released at its source using Richter scale
   - base 10 logarithmic scale, converts largest recorded seismic wave to numerical magnitude
   - each number increase = 10 fold increase in shaking amplitude and 30 fold increase in energy release

IMPORTANT: earthquake has single magnitude regardless of distance, but intensity differs with distance

Question 7

Geology affect on earthquake intensity

Answer 7

Ground shaking: amplitude, duration and damage increases in poorly consolidated soil
Ex. Solid bedrock will inhibit large damage, thick soils will settle, increasing potential for damage(liquefaction)