parts of the earth
crust: surface (5-75km)
mantle: below crust (2850km)
core: liquid outer, solid inner
seismology
study of seismic waves through the earth
three distinctive types of plate boundary
- spreading ridge
- subduction zone
- transform fault
spreading ridge
not generally regions of high earthquake activity
subduction zone
potential to generate mega earthquakes
transform fault
one of the biggest risks to society as they are hard to detect
plate rebound theory
where there is a gap in recorded earthquakes along a plate boundary, is the likely location of a BIG earthquake
NZ earthquake faults
i) subduction of Pacific under Australian along east cost of north island
ii) subduction of Australian beneath Pacific in south west of south island
iii) numerous transform faults in axial tectonic belt, largest is Alpine fault (650km)
Richter Scale
traditional scale for earthquake magnitude
- local magnitude: based on ground motion 100 km from earths epicentre
- upper limit of around 7, due to failure of near surface soil layer limiting maximum ground motion to reach surface
Mw
Moment magnitude
- related to total energy released by fault due to its displacement
- not affected by local ground failure problem
- generally, larger fault area ruptured = larger magnitude
three component waves
- s waves
- p waves
- surface waves
s waves
shear waves - slower but more damaging
p waves
dilational - travel fast, first to reach surface
response due to earthquake loading
depends only on the damping ratio and natural frequency of the building, w(n)
structures with same damping ratio varying Tn
longer Tn gives a larger peak displacement than shorter period structures
structures with same Tn but varying damping ratio
as damping ratio increases, peak displacement observed decreases
Sv
pseudo velocity
Sv = w(n) Sd
- directly related to peak strain energy Es
Sa
pseudo acceleration
Sa = w(n)^2 Sd
- directly proportional to peak base shear and moment
Es
strain energy
Es = mSv^2 / 2
Response Spectrum
defined as the peak value of this quantity as a function of the natural vibration period, Tn
- gives an indication of the severity of a ground motion on structures of various frequencies
