Engineering Geology and Geohazards Lecture 10: Introduction to seismology and seismic waves Flashcards
What causes earthquakes
Plate tectonic movements, which are
themselves the result of the Earth transporting
heat outwards from its centre by convection.
Define an earthquake
Sudden release of accumulated elastic
strain energy (producing seismic waves)
in a specific area of the Earth’s
lithosphere, normally through
reactivation of a fault.
Describe Stress
Stress is directed force, measured in pascals
(Pa). 1 pascal = 1 N/m2 (Newton =
gravitational force of 102 grams, an average
apple)
Often measured in megapascals (MPa), Pa
x 106
Describe Strain
Strain is the result of stress, ie., deformation.
- Three types: elastic, plastic, brittle
Describe the two concepts of the Elastic rebound theory of earthquakes
Clock spring analogy
Jelly analogyD
Define a fault
A fault is a fracture in the Earth’s crust where two blocks of rock move relative to each other. Faults can be active or inactive, and can range in size from millimeters to thousands of kilometers.
What are the two types of deformation caused when a fault ruptures
Static deformation is the permanent displacement of the ground due to the event.
Dynamic motion, represented by seismic (elastic) waves radiated from the rupture site (hypocentre).
most of the plate-tectonic energy driving fault rupture is taken up by static deformation, up to 10% may dissipate immediately in the form of seismic waves.
Describe dynamic motion
Dynamic motion, represented by seismic (elastic) waves radiated from the rupture site (hypocentre).
- Rupture propagates along the fault plane from the hypocentre, and may reach the Earth’s surface.
Both body waves and surface waves are generated.
- Dissipation of stored elastic strain energy likely to be incomplete (earthquake cycle). Earthquake prediction.
List all the types of seismic waves
Body waves
* P- Primary
* S- Secondary
* SH- Horizontal
* SV- Vertical
- Surface waves
* Rayleigh waves
* Love waves
What are the body wave category
Body waves travel through the “body” of the Earth.
Velocity changes are gradual (except for major
discontinuities)
Define P waves
P waves arrive first.
The fastest, they travel at velocity between 1.5 and 6 kilometres per second in the Earth’s crust.
P waves can travel
through solids, liquids
or gases.
▪Primary, pressure
waves. Analogous to
sound waves.
▪Particle motion is along the direction of travel (propagation) of the wave, i.e.,
longitudinal waves.
Define S waves
Secondary, shear
waves.
▪Arrive second slower than P. Velocity usually
at 60% to 70% of P waves
▪Vibration perpendicular to the direction of propagation: can be split into horizontal(SH)
and vertical(SV), components.
▪S waves do not propagate through liquids or gases, since these lack shear strength.
Describe the surface wave category
Travel across the surface of the Earth.
Usually larger amplitudes, longer wavelengths and lower velocity than body waves.
- Principal cause of earthquake damage.
- Surface wave amplitude
decays exponentially
with depth.D
Define Raleigh waves
Similar to swell in the open ocean, but note
the retrograde ellipse of particle motion.
If a surface wave has a vertical component it
must be a Rayleigh wave.
Define Love waves
Only have a horizontal component of oscillation,
perpendicular to direction of propagation.
● Equivalent to trapped SH waves.
● Only occur when there are distinct subsurface layers
Describe the travel of the waves around and through the earth
Body waves travel through body of the earth, effectively taking a shortcut in comparison to surface waves which have to go along the actual surface or ‘circumference’ of the Earth.
P waves travel through the Earth, through mantle, they are the fastest wave and reach seismographs first.
S waves also travel through the mantle but are not as fast as P waves and therefore reach the seismograph second.
Surface waves are the slowest reaching the seismograph last. They travel along the surface of the Earth.
Describe the distinctive properties and physical laws of seismic waves
They follow Snell’s law (concerning angles of
incidence and refraction/reflection as a wave
impinges on the interface between two
contrasting media)
- They transform
- They disperse
SNELL’S LAW
Light travelling from lower velocity medium, water, to higher velocity medium, air.D
Describe snells law in seismic waves
The ray is refracted according to Snell’s Law at seismic discontinuities within the Earth normal
The reflected and refracted rays lie in the
plane formed by the incident and the
normal to the surface sin i1/ v1 = sin i2/ v2 = constant
Angle of incidence = angle of reflection
So if v2 > v1 then i2 > i1 and the ray
trajectory flattens out, i.e. is refracted
away from the normal on entering a
higher velocity medium. This is generally
the case on the descending trajectory.
Similarly, upgoing rays are generally
entering a lower velocity layer, so
refracted towards the normal.
Seismic wave velocity contrasts between the crust
and mantle: the Moho
Define wave transformation
Similarly, when SV hits a boundary obliquely it gets
reflected and refracted into a P and SV.
- When SH hits boundary obliquely only get
reflected and refracted SH. - Several transformations can occur on one ray path, leading to a complicated picture. This complexity can be useful in determining a more detailed picture of the internal structure of the Earth.
Describe the various terminology for different types of refracted seismic waves
PKP - Refracted through the core
PcP - Reflected off core
PP - Reflected off surface
Describe other various terminology for seismic wave phases in the Earth
P - Primary wave
K - P wave through outer core
I - P wave through inner core
PP - Reflected P wave with 2 legs
pP - P wave with leg from focus to
surface
SP - S wave reflected as P wave
S - Secondary wave
J - S wave through inner core
SSS - Reflected S wave with 3 legs
sS - S wave with leg from focus to
surface
PS - P wave reflected as S wave
c - Wave reflected at outside boundary of outer core (e.g., ScS)
i - Wave reflected at outside boundary of inner core (e.g., PKiKP)
LQ - Love waves
LR - Rayleigh waves
