Week 4 Flashcards
What is ground shaking
- most important
- intensity of the source is proportional to the ground shaking
- fault ruptures generate earthquakes, which consist of several different kinds of waves
- Body waves (P wave and S wave)
- Surface waves (Rayleigh wave and Love wave)
Characteristic of ground shaking
- shaking greatest near earthquake source
- maximum shaking and its duration, scale roughly to magnitude
- shaking may locally exceed 1g and 1m/s
- last a few seconds to several minutes
Resonance of Building with different heights
Different frequency can generate different damage to different frequency bonds
Why do some buildings fall in earthquakes
- All building have a natural period, or resonance which is the number of seconds it takes for the building to naturally vibrate back and forth
- The ground also has a specific resonant frequency
- Hard bedrock has a higher frequency than softer sediments
- If the period of ground motion matches the natural resonance of a building, it will undergo the largest oscillations possible and suffer the greatest damage
What is frequency
the number of waves that pass through a point in one second
What is period
the amount of time it takes one wave cycle to pass the given point
What is resonance
the tendency of a system to oscillate with greater amplitude at some frequencies than at others
Resonant frequency
the frequency at which the maximum-amplitude oscillation occurs
Seismic design
_ in poor countries or countries without good seismic design, seismic shaking can produce horrific results
- EQ don’t kill people, buildings kill people - more so in poorer countries than rich one
How do we mitigate the impact of ground shaking
One has to know:
1. Where are the active faults
2. How fast are they slipping
3. What is their very long term history of producing big earthquakes
4. How does shaking decrease away from the fault
Earthquake early warning basics
- In an earthquake, a rupturing fault sends out different types of waves. The fast moving P waves is first to arrive, but damage is caused by the slower S waves and later arriving surface waves
- Sensors detect the P wave and immediately transmit data to an earthquake alert center where the location and size of the quake are determined and updated as more data becomes available
- A message from the alert center is immediately transmitted to your phone or computer which calculates the expected intensity and arrival time of shaking at your location
Surface faulting
- Building that are not bulit on faults may not be damaged but those built on fault are confirm damaged
- dependent on building quality
- Infrastructure design could accommodate to fault lines, e.g trans-alaska pipeline
What is the California’s Alquist-Priolo Fault zone act
- every active fault zone is restricted with respect to land use
- doesn’t apply to new construction of less than 4 single family dwellings, nor to buildings constructed before the law
- sellers are required to notify buyers, but not renters of the hazard
Landslides
Ground shaking can dislodge large masses of unstable rocks and soils especially areas that have experienced liquefaction and have steep slopes
Where not to live
- Close to the surface expression of the fault
- On a steep slope near the river
- On a slope that faces towards the fault
- In an area that has just suffered heavy rainfall
- On a susceptible rock type
