Earthquake Waves
Lecture 2
GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Earthquake Seismograms
Training in observational seismology and seismological
observatory practice
How to interpret
a seismogram:analyse both old
ink trace and
modern digital
records (using
SUDS)
analysing earth
structure
GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Body waves
Body waves travel through the body of the Earth
Earthquake
Seismometer
↓ v increasing
Body waves follow a
curved path with velocity
increasing with depth as
rock becomes more dense
Epicentral distance
(degrees)
Diametral plane
r
∆o
Centre of the Earth
GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Snell’s Law
The ray is refracted according to Snell’s Law
1.
normal
The reflected and refracted rays lie in
the plane formed by the incident and the
normal to the surface
2.
sin i1/ v1 = sin i2/ v2 = constant
3.
Angle of incidence = angle of reflection
So if v2 > v1 then i2 > i1 and the ray trajectory
flattens out. This is generally the case on the
descending part trajectory. Similarly, upgoing
rays are generally bent towards the vertical.
ascending
v1
v2
i1
i2
descending
GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Multi-layered medium
When there are several layers the paths are curved
v1
v2
v1 < v2
v gradually
increasing
vn
When i = 90o the ray path is horizontal and bottoms out.
The deepest part is the turning point
GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Types of body waves
There are two types of body waves
1. P waves arrive first. Primary, pressure waves. Analogous to sound
waves. Particle motion is along the direction of travel (propagation)
of the wave, i.e., longitudinal waves.
P waves can travel through solids, liquids or gases.
2. S waves arrive second (Oldham, 1900). Secondary, shear waves.
Slower than P.
S waves vibrate perpendicular to the direction of propagation.
A shear wave can be split into orthogonal, i.e., horizontal and
vertical, components.
S waves do not propagate through liquids or gases, since these
don’t have any shear strength.
GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Types of body waves
P wave
SV wave
GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Surface waves
Great circle
Seismometer
Earthquake
The path of a surface waves is a great circle.
Because surface wave velocities are low, they arrive after body waves.
Two types of surface waves:
1.
Love waves
2.
Rayleigh waves
GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Types of surface waves
Love wave
Rayleigh wave
GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Love waves
Love waves only have horizontal component perpendicular to
direction of propagation. Equivalent to trapped SH waves.
Only occur when there are distinct layers.
amplitude
dies away
with depth
exponentially
GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Rayleigh waves
Akin to rolling sea waves – layering not necessary
surface
amplitude
dies away
with depth
exponentially
Occurs because of
boundary conditions
at interface
Retrograde ellipse
forward-up-back-down
If a surface wave has a vertical
component it must be a Rayleigh wave.
Vertical and longitudinal component.
(Combination of P and SV)
GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Summary
1) Seismology is difficult: have to deal with P & S waves,
Rayleigh and Love waves. (Other phases: Stoneley waves
and T phases would be covered in advanced seismology.)
2) However this very complexity necessitates the use of
seismology in determining Earth structure and earthquake
mechanics. Seismology has the highest resolution of any of
our geophysical probes in mapping out Earth structure and
composition.
3) Seismology has had the biggest impact of any discipline on
the Earth sciences and is predominant in geophysics.
GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD