Earthquakes 4: Earthquake hazards and mitigation

Moment magnitude scale

• quantitative measure of the maximum ground motion produced by the earthquake

EQ Energy release depends on

rock strength x fault area x fault slip

What does it mean if there is a magnitude increase?

Ground shaking: is 10x greater (ex. 10² vs 10³) 100 and 1000

Energy released: is 32x greater (ex.32²) about 1000,

Modified Mercali Scale

• qualitative (descriptive measurement

• based on perception

• measures damage & shaking

• scale 1-12. IV light, VI strong, IX severe

Earthquake intensity depends on what 3 factors?

1) earthquake magnitude

• less damaging seismic waves

2) distance from earthquake epicentre

• waves weaken with distance

3) duration

What are some reasons for variations in EQ intensity even though you are in the same region?

• soil type (ground properties)

• structural damage

• building design

• liquefaction

What kind of movement causes structural damage

• building are designed for vertical load, so horizontal shaking/acceleration is most damaging

what type of waves cause the most damage?

Surface waves

• largest amplitude

• rolling motion most destructive

• longest duration

Which buildings are more susceptible to damage?

1) taller buildings

2) poorly designed buildings

3) less cohesive/flexible materials (brick and reinforced concrete)

4) building with weak supports (ex. “soft stories will collapse)

what is the best material for earthquake proof infrastructure?

Wood, steel, reinforced concrete

• any material that is flexible and strong

What was the engineering failure on the Tacoma Narrows Bridge?

a failure to account for vibration and resonance

Structural resonance: short vs tall buildings

Short buildings:

• resonate at high frequency, fast vibrations, low amplitude

Tall buildings:

• resonate at low frequency, slow vibrations, large amplitude

Ground properties: bedrock vs well compacted sediment, vs. water saturated sediment

Bedrock: LESS intense ground motion

Well compacted sediment: MODERATE ground motion

Water saturated sediment: MORE intense ground motion

What is the relationship between soft/hard sediment vs amplitude/frequency?

soft sediment

• larger amplitude, lower frequency, longer shaking

hard ground/bedrock

• smaller amplitude, higher frequency, shorter shaking

How do waves travel through the ground?

the ground filters the waves: So if surface waves arrive, then harder sediment makes it harder for the waves to travel, thereby causing less ground motion

What are 3 Hazards triggered by earthquakes?

1) Fire

2) Tsunami

3) Landslides

What is Liquefaction?

in soft wet soil and sediments, the EQ shaking causes unconsolidated materials to liquify

• soil looses its cohesion and acts like a liquid

• soil can sink heavy objects (people, cars buildings)

• sand volcanes: cones of sediment brought to surface

What 2 factors does the degree (intensity) of liquefaction depend on?

Sediment properties and wave frequencies

Where does liquefaction happen?

• wet sea-level delta soils

• areas filled in for construction

• landfills

A tsunami is not caused by seismic waves is caused by-

reverse faulting under the sea