GEO ENCN454 Flashcards
Hypocentre
- The location where the earthquake fault rupture starts, below the surface
Epicenter
- Is the projection of the hypocenter onto the ground surface
Seismic waves consist of…
- Body waves
> P-waves
> S-Waves - SV waves, SH waves
- Surface Waves
> Rayleighs waves
> Love WavesSeismic waves consist of…
- Body waves
> P-waves
> S-Waves
Body waves
Travel within body of the earth
- P waves
- S waves
P-waves
- Compressional waves
- Motion of individual particle in direction of wave motion
- Produces successive compression and dilation deformations
- Can travel through solids and fluids`
S- Waves
Secondary
- More commonly called shear waves
- Motion of individual particle is perpendicular to direction of wave motion
- Produces shearing deformation
- Cannot travel through fluids
- Vs 2-5 km/s
Surface Waves
Primarily travel along the surface of the earth
- Rayleigh waves
- Love Waves
Rayleigh Waves
- Surface wave
- Produced by interaction of P-waves and SV-waves with the earth surface
- Particle motion is more complex. Has components in the direction of motion and also perpendicular
Love Waves
- Result from interation of SH waves with a soft layer at the ground surface
- SH waves trapped within the surficial soft layer with no vertical component of particle motion
Which body waves travels faster
P-waves travel faster than S-waves
Vp
P-wave velocity
Vs
S-Wave velocity
Rhypo
distance from source to hypocenter
Repi
distance from site to epicenter
Rrup
shortest distance from site to fault supture plane
Rjb
is the closest distance to the surface projection of the fault
Magnitude measures
size of the earthquake
Seismic Moment magnitude
- a measure of work done (related to energy released) by the earthquake
Moment magnitude
- A Magnitude scale based on siesmic moment
Directionality
- when ground motion shakes stronger in one direction than others
- recorded directions not necessarily largest
- can average using the geometric mean
Natural noise in ground motion
ocean waves, animals, wind, atmospheric pressure change
human noise in ground motion
- traffic, construction, industrial process, sport events, everyday activites
Instrumental noise in ground motion
digitilisation error, incorrect installation of instrument, significant instrument rocking or rotation during grouf motion, static electricity build up
Frequency of earthquakes
- The ground motion produced by an earthquake contains a wide range of frequencies
- Frequency content characterises what frequencies are present and with what amplitudes or energy
- Frequency content is a key parameter because dynamic response of structures may be sensitive to specific frequencies (resoant effects)
Fourier Spectra
Spectra of the enerfy corresponding to each frequency present in the ground motion
Response spectra
spectra of the responce of a simple SDOF building models over a range of vibration periods
Predominant period
Vibration period corresponding to the maximum value of the fourier amplitude spectrum
Predominant Period
Vibration period corresponding to the maximum value of the Fourier Amplitude spectrum
- Indicated what frequencies have the largest amplitudes (most energy)
Duration
Duration is important as longer durations mean more energy and allows for greater resonant phenomena, cumulative damage in nonlinear structures (eg stiffness degradation) and soil response (e.g. excess pore water pressure build up)
Bracketed duration
time interval between the first and last exceedences of a specific acceleration amplitude (usually 0.05g)
Uniform duration
sum of durations for which acceleration amplitudes are larger than a specific value
Significant Duration
Duration over which a specified percentage of the ground motion energy is released
Primary factors that affect ground motions
- Magnitude
- Source to Site Distance
- Site Conditions
Secondary factors that affect ground motions
- Faulting Style
- Source Depth
- Directivity
- Basin effects
- Topographic effects
Effects of magnitude on ground motions
Larger amplitude
- because there is more work done by the earthquakes rupture
Longer duration
- Because the rupture occurs across a larger fault area and therefore over a longer duration
Stronger long period content
- Because a larger rupture area can produce longer wavelengths
Effects if source to site distance on ground motion
Amplitude decreases with increasing distance
- geometric spreading
- Anelastic attenuation
Ground motion duration increases with increasing distance due to
- larger separation between P-waves Swaves and surface waves
- move wave scattering in the earths crust
Geometric Spreading
geometric spreading reduces amplitudes due to a decrease in energy density as energy spreads out over larger volumes and areas.
Anelastic attenuation
enelastic attenuation reduces amplitudes due to loss of energy resulting from material damping (e.g. friction) which arises during cyclic motion
- the reduction in amplitude is related to a number of cycles
- For a set distance high frequency waves will go through more cycles than low frequency waves and therefore will attenuate more
Effects of local site conidtions on ground motion
- Wave amplitudes re amplified when travelling from a stiff material into a soft material
- This is due to complex interactions between transmitted and reflected waves at the boundary
- However, softer soils also have high damping which reduces amplitudes as the waves travel through the soil. Damping is even larger when shaking is strong
3 Primary factors that influence ground motion
- Magnitude
- Source-to-site
- Site conditions
3 Important characyeristics of ground motion
- Amplitude
- Frequency
- Duration
Effects of Directivity
- Superporsition of wave amplitude occur when the direction of rupture propagation and wavefront coincide because rupture propagation velocity is similar to velocity of shear waves
