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GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Earthquake Waves
Lecture 2
GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Earthquake SeismogramsTraining in observational seismology and seismological
observatory practiceHow 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
Diametral plane
↓ v increasing
r
SeismometerEarthquake
∆oEpicentral distance
(degrees) Centre of the Earth
Body waves follow a curved path with velocity increasing with depth as rock becomes more dense
GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Snell’s Law
The ray is refracted according to Snell’s Lawnormal
ascending
descending
v1
v2
i1
i2
1. 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.
GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD
Multi-layered medium
When there are several layers the paths are curvedv1v2
vn
v1 < v2
v gradually increasing
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 waves1. 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
SeismometerGreat circle
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
amplitude dies away
with depth exponentially
Retrograde ellipse
forward-up-back-down
surfaceOccurs because of boundary conditions at interface
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.