Earthquake intensity Flashcards
What is the MMI?
Mercalli Intensity scale
How is the MMI split?
Into 11 levels
The low intensity (1-6) are public observations
High intensity (7-11) are based off structural damage
shown as roman numerals - many depend where observer is
What part of earthquake waves cause damage?
Acceleration, Velocity or Displacement?
Acceleration (rate of change of velocity)
What is the physical eqiuvalent of intensity?
peak (horizontal) acceleration
What equation did richter produce for intensity?
Log (A) = (X/3) - 0.5
A = Peak ground acceleration
X = Intensity
Why are deep earthquakes less dangerous?
They do not generate high amplitude surface waves
How does the magnitude of an earthquake affect the shaking?
Higher magnitude will increases the amplitude of waves and thus the amount of shaking
How does the length of earthquake affect shaking?
Longer shaking increasing damage and the perceived intensity
What is the relationship between magnitude and intensity and how are the intensity lines shown on the graph?
As magnitude increases so does intensity (with a distance dependant factor) - intensity lines displayed as isoseimals (lines of the same intensity)
What is the effect of distance on earthquake waves?
Body waves pass through earth like expanding hemisphere (energy fall = 1/R^2)
Surface waves as along surface expand as circle (energy loss =1/R)
As a result surface waves are more damaging over distance as less energy is lost
What is the effect of regional geology on earth quake intensity?
If material favours energy proporgation (i.e. layer cake straigigraphy) usually simple stratigraphy intensity will be greater over larger area but if the regional geology is more tectonically complex energy will be scattered in smaller area
What is the effect of local geology on intensity?
Using San fransico 1906 as an example
Areas associated with high intensity were situated either near rupture or on areas of bay mud or fill
Marina bay not high intensity from 06 as rubble from event was filler
What are some factors relating to local geology that can effect intensity?
Amplification of ground motion by soft sediments
Liquefaction of soft sediments
Choice of building materials
Why will shaking tend to be greater in unconsolidate or loose soils?
As wave energy is proportional to wave velocity X density X Amplitude^3/2
So when a unconsolidated soil is reaches density and wave velocity decrease but as energy stays the same there is a vast increase in Amplitude (shaking)
What was the reason for the Cypress highway collapse?
Was on area of soft mud so heavy shaking
biggest killer as cars squashed on lower level and cars fell from top
