Lecture 8 - Reducing Seismic Risk Flashcards
Why do we say that “Earthquakes don’t kill people, buildings do”?
Building materials are important. Masonry buildings, made out of blocks such as bricks, are especially weak when subject to strong horizontal shearing, as can occur in large earthquakes.
e.g. Bam 2003 Mw 6.6 - most structures in the city collapsed as they were made of adobe brick.
Why are wood buildings much more likely to withstand earthquakes than masonry ones?
Wood is flexible, lightweight and elastic.
e.g. In the 1906 San Francisco earthquake wood-frame houses were damaged, but did not collapse. They fared better than brick buildings.
What material is the best choice when building large structures to be earthquake resistant?
Steel, as it is strong and flexible. It is expensive so is usually only used for larger buildings such as the Levi’s Stadium.
Why is it important the buildings be properly anchored to their foundations?
Small residential buildings are prone to sliding off their foundations.
What is a common failure mode in buildings during earthquakes?
“Soft first story” collapse.
What are some ways of reducing the likelihood of a building collapsing or sliding off its foundations?
Reinforcing first storeys with braces, brackets, shear walls, bolts and steel frames.
What types of building may need to undergo expensive seismic retrofits?
Historical masonry buildings in North America are of particular concern.
What is resonance?
A natural frequency of vibration.
What does the resonance of a building depend on?
size weight distribution shape materials foundations substrate (whether hard bedrock or loose sediment)
What frequency do buildings tend to resonate at?
~10 Hz divided by the number of storeys
What does seismic wave frequency depend on?
earthquake magnitude
mechanism
distance
local geology
A building will be vibrated more by seismic waves of its resonant frequency.
Why are more people being killed by earthquakes?
growth of megacities
population growth
urbanisation
What type of oscillations do tall buildings respond to?
low-frequency oscillations
What type of oscillations do medium height buildings respond to?
medium-frequency oscillations
What type of oscillations do small buildings respond to?
high-frequency oscillations
In the 2017 Mw 7.1 Puebla, Mexico earthquake, damage was focused in buildings 6 stories or shorter. What does this tell us about the frequency of the oscillations?
They were high-frequency oscillations.
What are some modern ways of reducing resonance?
Placing shock absorbers within the structure to dampen the shaking.
Base isolation where devices such as ball bearings, wheels, and shock absorbers are placed on the ground to absorb part of the earthquake energy. e.g. Vancouver’s Qube.
What must earthquake predictions specify?
The location, timing, and magnitude of a future shock.
What do earthquake predictions rely on?
Precursor signals e.g. foreshocks, electrical or magnetic disturbances, strange animal behaviour.
A successful prediction must pass statistical tests to show it improved upon random chance.
How do earthquake early warning systems work?
In an earthquake the fast p-waves are first to arrive but much of the damage is caused by the slower s-waves. Sensors detect the p-waves and immediately send data to an earthquake alert centre where the location and size of the quake is determined. A message form the alert centre is transmitted immediately to your phone or computer.
Where is the NEPTUNE cabled observatory and what types of instruments does it contain?
It spans the northern Cascadia megathrust with seismometers, pressure gauges, and other instruments.
What should you know/do when educating yourself about earthquakes?
Drop, cover, and hold on!
If at sea-level, get to higher ground and stay there until told it is safe to return.
Prepare an earthquake kit at home.
Secure tall furniture and fittings.
Consult with a structural engineer (especially for older properties, homes with cripple walls, especially masonry).
Buy earthquake insurance (but check that your insurance company is re-insured!)
What is PAGER? What does it do?
The USGS’s Prompt Assessment of Global Earthquakes for Response quickly determines the population exposed to different levels of shaking.
Based on knowledge of regional building types and standards, the PAGER algorithm estimates the number of fatalities and the economic costs.
What did the PAGER system estimate about the 2018 Mw 7.5 Palu, Indonesia earthquake?
The alert showed a 43% chance of 1,000-10,000 fatalities. Unfortunately, the Indonesian government initially downplayed the severity of the Palu earthquake.
How long did it take the Indonesian government to request outside help after the 2018 Mw 7.5 Palu, Indonesia earthquake? Why is this so unfortunate?
It took them three days to request outside help. However, people usually only survive for three days when trapped under buildings.
Unfortunately, this course of events is common in the most devastating earthquakes.
What is earthquake forecasting?
Assessing the general probability of an earthquake of a given magnitude in a given area over a long length of time.
What does earthquake forecasting rely on?
mapped locations of active faults
rates of strain accumulation on those faults
past average recurrence intervals on those faults
assumptions about the range of magnitudes those faults can generate
What are the chances of a Mw 8-9 earthquake on the Cascadia megathrust fault in the next 50 years?
~12% to ~33%
What techniques can be used for remote sensing of earthquakes?
satellite imaging
radar
What magnitude was the 1985 Michoacán, Mexico earthquake?
Mw 8.0
How far away from Mexico City was the 1985 Michoacán, Mexico earthquake?
400 km west of Mexico City
Where was damaged focused in the 1985 Michoacán, Mexico earthquake?
What does this tell us about the frequency of the oscillations?
In 5-20 storey buildings.
Low frequency.
How many fatalities did the 1985 Michoacán, Mexico earthquake cause?
~10,000 people
