Earthquakes 4 Flashcards
list 5 hazards related to earthquakes
- Ground shaking
-> structural damage to buildings - Liquefaction
- Fires
- Tsunami
- Landslides / subsidence
which direction of shaking damages buildings the most?
horizontal shaking
-> buildings are designed to take vertical load
Why are ground conditions important?
the ground filters our the frequencies of EQ
- affects duration of shaking
- influences stability of structures
What ground material is more problematic?
soft and wet ground
intensity of ground motion in bedrock
less intense
intensity of ground motion in well-compacted sediment
less intense
intensity of ground motion in water saturated sediment
more intense!
What kinds of buildings leads to more shaking?
- tall
- poorly designed
- less cohesive or flexible material
What are some good material for building design and why?
wood
steel
reinforced concrete
-> flexible, strong
What are some bad material for building design and why?
brick
stone
concrete (without steel)
-> brittle failure, poor shear strength
What is an example of retrofitting?
bad material can sometimes be retrofit
ex/ putting in a steel frame
soft first storey
weight on weak supports
shear failure = collapse
What is resonance?
- all objects vibrate at characteristic frequencies
- controlled by size, composition, structure
a key design factor to control
avoid critical resonance at earthquake frequencies
Resonance disasters happen when
the forcing vibrations are tuned to one of the primary resonance frequencies of the structure
Short buildings (stiff) tend to resonate at ____
high frequencies
- fast vibrations with low amplitudes
Tall buildings (flexible) tend to resonate at ____
low frequencies
- slow vibrations with big amplitudes
Some examples of tuning buildings to avoid resonance
- mass dampers
(pendulum in building that counters sway) - seismic base isolation system
(decouple building from its base)
What is liquefaction?
unconsolidated materials liquefy in soft wet sediments
- loses cohesion and flows
- dense objects can sink
- sand volcanoes form
Sand volcanoes
cones of sediment brought to surface with water as it flows
What happens to friction force and water pressure before and after liquefaction?
Before: strong sediment
- high friction force
- low water pressure in pore space
After: grains vibrate
- friction forces decrease
- increase water pressure in pore space push grains apart
Liquefaction: why do objects like cars get stuck in the ground when shaking stops?
- water pressure drops
- grains stop vibrating
= ground becomes firm again
Liquefaction most hazardous in
Richmond, Delta
«refer to map on slides»
Describe our secondary system of fire hydrants
connected to seawater and generators
How do fires occur as a result of EQ?
buildings collapse and burn
water, electric, gas lines break
roads inaccessible
EQ generated tsunami often occur at
convergent margins
-> not usually part of divergent or transform unless caused by landslide
Landslides happen when
unstable ground fail when shook
