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.