Seismology Orals

8/2/2019 Seismology Orals

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INTRODUCTION to

SEISMOLOGY

Ashley Shuler

Orals Prep 2008-2009

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What is Seismology?

The study of wave propagation and what it tells usabout the structure of the Earth and the physics ofEarths deep interior.

We study earthquakes which radiate seismicwaves that travel throughout the Earth, causingground motion that is measured on sensitiveinstruments. Sometimes we use explosions (activevs. passive source).

Seismology has many applications includinghazards, exploration and nuclear monitoring.


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Where Do Earthquakes Occur?

Earthquakes occur on faults and along plateboundaries.

The earths surface is composed of a smallnumber of rigid plates that drift slowly over

geologic time. Their relative motions along theirboundaries give rise to earthquakes.

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Types of Faults

Just as there are 3 kinds ofplate boundaries, there are 3types of faults.

Normal faults occur in

extensional environments(rifts, mid-ocean ridges).Thrust faults occur in

compressive environments(subduction zones), andstrike-slip faults occur intransform environments.

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Earthquakes are not point sources, but occur

along planes. Earthquakes have a finiterupture velocity, which is generally .8*shear

velocity.

There hypocenter is where the earthquake

begins. The epicenter is the surface

projection of the hypocenter.

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Earthquake Sources

Earthquakes are double-couple sources.

Different types of earthquakes produce differentradiation patterns and have different momenttensors. These differences can be visually

displayed with focal mechanisms.

They are made from the radiation pattern of P-waves. They are plotted on the lower hemisphereof an imaginary sphere surrounding the source.

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Focalmechanisms are

made from thefirst motions ofearthquakes.

There is some

ambiguityconcerningwhich plane isthe true fault

plane. Moreinformation isneeded.

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Elastic Rebound Theory

Earthquakes arecaused by the slowaccumulation ofshear stress and

strain that issuddenly released bymotion along a fault

(producingpermanentdisplacement).

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Stress and Strain

Stress is the internalforces between partsof a medium. Strainis related todeformation. Thesequantities are linkedby a constitutive

relationship, andthrough elasticparameters.

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Some math

Homogeneous

equation of motion

Vector equation

Solutions give the P

and S wavevelocities (dependent

on density and

elastic parameters)

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Body Waves

Body waves travel through volumes of the Earth(P and S waves).

P waves have displacement in the direction ofpropagation (compressional). S waves have

displacement perpendicular to the direction ofpropagation (shear) - can be broken down into SVand SH.

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Surface Waves

Surface waves are confined to the free surface. Rayleigh waves are radially polarized and are

caused by the interaction of P and SV waves at thefree surface.

Love waves are transversely polarized and aretrapped in a surface low-velocity layer.

Compared to body waves, they travel slower andhave less amplitude decay.

Surface wave velocities are frequency dependent(dispersion). In general, longer period wavestravel faster because they feel deeper.


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Phase Identification on

Seismograms P wave first,

mostly onvertial.

S wave later,on horizontals.

Love wavelater on

transverse. Rayleigh wave

last on verticaland radial.

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Waveform Notes

In a seismogram, we are seeing a convolution ofthe source-time function with the Earth responseand the instrument response.

We see body wave phases which travel at certainvelocities, and surface waves which travel atdifferent speeds depending on frequency.

Phase velocity - speed of single phase (frequency)

Group velocity - speed of wave packet (envelope)




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Normal Modes

Any finite solid willresonate at certainvibration frequencies.

The Earth hasspheroidal andtoroidal modes.

Any wave motionwithin the Earth maybe expressed as a sumof normal modes withdifferent excitations.This representation isunique.

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Ray Theory (good approximation, but doesnt fit everything)

We often consider rays

instead of wavefronts.Rays are perpendicularto wavefronts and obeySnells Law (sin

theta/v=constant) Because compressional

and shear velocitiesincrease as a function of

depth, rays turn. If there is a low-velocity

zone, we can getshadows.

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Travel-time curves

Traveltime curvesare created by

picking phases from

seismograms

recorded at varying

distances.

These tell us about

Earth structure.

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What happens at boundaries?

Waves can be transmitted or reflected atboundaries. The reflection and transmission

coefficients depend on density, wave speed,

and angle of incidence. Waves can also be converted. P can convert to

SV and vice versa.

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The amplitudes of waves change as they

propagate. If the area increases, the

amplitudes decrease (geometrical

spreading).

The amplitudes also depend on the

impedance (product of density and

wavespeeds).

Energy is also lost to attenuation through

anelastic processes such as internal friction.


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Magnitude Scales

First done in 1935 by Richter. The Richter scalewas empirical, and not related to the source. It

saturated at high magnitudes.

Today we use the moment scale (moment is the

product of rigidity, displacement, and fault area).

A M7 earthquake releases 32 times more energy

than a M6 earthquake, and 1000 times more

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Gutenberg-Richter Relationship

There is a relationshipbetween the magnitudeand the total number ofearthquakes in any

given region and time. Log N = A -bM

B is typically 1. Thismeans that there are 10

M3 earthquakes forevery M4 earthquake,etc.

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Seismic Intensity

Another measure isintensity, whichdescribes localshaking as determined

by damage tostructures andperceptions of peoplewho experienced theearthquake.

This is measured onthe Mercalli scalefrom I to XII.

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Earth Structure

Seismology allowed us to determine thepresence of the core (absence of direct Pand S arrivals at source-receiver distancesgreater than 100 degrees).

It also allowed us to determine the presenceof a solid inner core (reflections from innercore boundary in shadow zone).

We know the outer core is liquid because itcasts shadows in S-waves (shear waves donot propagate in fluids).


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P and S wave velocities Rapid velocity increases in

upper mantle, transition zonewith mineralogical phasechanges (410 and 660 km),gradual increase to CMB

At CMB, P wave velocitydrops and S velocity goes tozero. P velocity increases withdepth due to increased

pressure/temperatue P and S wave velocities

increase with depth in innercore

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Reflection Seismology

Mostly used in oceans andindustry.

Large numbers of sources

and receivers at short,regularly spaced intervals.

Looking for changes in

velocity or density

Will be discussed by Milena!

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Tomography

We often want to inverttraveltimes or full waveforms toget 3-D velocity structures.

This is complicated by noise,

limited data, nonuniformdistribution of sources/receivers,location/timing errors.

In tomography, we set up a grid,

and the travel time is the sum ofthe product of distance and1/velocity through each gridpointon the raypath from source to

receiver.

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Earthquake Location

Earthquakes are treated as pointsources during location.

A location can be approximatedusing S-P traveltimes at a few

stations. From this guess of location,

depth and origin time, wecompare the predicted arrival

times to observed. Perturb andIterate.

There are tradeoffs betweendepth and origin time.

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Instrumentation

Early seismographs were pendulums sensitive toground motion (velocity).

Modern seismographs are electromagnetic masseson a spring. The voltage required to keep the massin place is proportional to velocity.

We use broadband seismometers with a broaddynamic range.

Earthquakes are detected and located within a fewminutes of occurring in many cases. Digital data isfreely shared among many nations. There isexcellent global coverage that is always improving(both permanent and temporary stations).


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Can we predict earthquakes?

Only in a long-term statistical sense (ie 30%

chance of having M7 or greater in the next

10 years) Many seismologists doubt we will ever be

able to predict earthquakes within a

potential evacuation period - too complex,too many variables

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