Geology 5640/6640Introduction to Seismology

9 Feb 2015

© A.R. Lowry 2015

Last time: Data: Raypaths and Phase Arrivals• Phase arrivals are commonly denoted by a shorthand describing their propagation path (e.g. SKS travels first as an S-wave through the mantle, then P-wave through outer core, then S through the mantle).

• Ray paths are normals to wavefronts and (skeletally) represent propagation paths from a source to receiver.

• Different phases can be recognized in a seismogram by their differing expressions in vertical and radial/transverse horizontal components, and their relative arrival times (a function of distance to the source).• Seismology is the primary tool for investigation of physical properties, structure and processes in the Earth’s interior!

Probe: Earthquakes

www.iris.edu

• Many each year strong enough to generate signal at antipodes10 major (magnitudes 7-8) 32 megaton ~ Largest test100 large (6-7) 1 megaton1000 damaging (5-6) 32 kiloton ~ Trinity

Lars Stixrude

Detector: seismograph

Lars Stixrude

Global Seismic Network

USArray

Lars Stixrude

Seismicnetworks

To zeroth order, radially homogeneous & isotropic

Monotonic and smooth increase with depth except:

• Core-mantle boundary• Smaller discontinuities• Near surface

Observable: elastic wave velocitiesand density

Types of Seismic Analysis*

• Normal modes • Relative arrival times (“precursors”) • Waveform modeling • Impulse response (“receiver functions”) • Tomography • Anisotropy • Noise?

(*used for research into Earth’s physical properties and structure… Earthquake source analysis we’ll examine later; industrial & applied seismology is another class!)

Generally speaking, the frontiers in Earth Science lie where the Earth deviates from that first-order (radially symmetric, isotropic) picture of Earth structure

Types of Seismic Analysis:

Normal modes

What is a free oscillation?What is a free oscillation?

3rd

Harmonicetc.

2nd Harmonic

1st Harmonic

)cos(),(),(

ioneigenfunct :),(sin

encyeigenfrequor eigenvalue :

)cos(sin~

2 :mode

)cos(sin~

0

txUAtxd

xUL

xjL

cj

L

ctj

L

xjd

jc

LTj

ic

L

tc

xd

jjjj

j

jj

j

th

ωω

ωπ

πω

ππ

πω

ωω

∑∞

=

=⇒

=⎟⎠

⎞⎜⎝

⎛

=

⎟⎠

⎞⎜⎝

⎛

=

=

⎟⎠

⎞⎜⎝

⎛FundamentalL

x

Free oscillation = stationary waveInterference of two counter propagating waves

(see e.g. http://www2.biglobe.ne.jp/~norimari/science/JavaEd/e-wave4.html)

Michel Van Camp

For 2004 Sumatra-Andaman,static (permanent) deformationwas recorded to 1000s of km…

The static is not instantaneous but propagates at ~Rayleigh-wave velocity. After Park et al, 2005After Park et al, 2005

Seismic normal modesSeismic normal modes

Periods < 54 min, amplitudes < 1 mm

Observable months after great earthquakes (e.g. Sumatra, Dec 2004)

Few hours after the earthquake (0S20)

(Duck from Théocrite, © J.-L. & P. Coudray)

Michel Van Camp

Few minutes after the earthquakeConstructive interferences free oscillations (or stationary waves)

0S0 : « balloon » or

« breathing » :

radial only

(20.5 minutes)

0S2 : « football » mode

(Fundamental, 53.9 minutes)

0S3 :

(25.7 minutes)

Spheroidal normal modes: examples:Spheroidal normal modes: examples:

Animation 0S2 from Hein Haakhttp://www.knmi.nl/kenniscentrum/eigentrillingen-sumatra.html

Animation 0S0/3 from Lucien Saviothttp://www.u-bourgogne.fr/REACTIVITE/manapi/saviot/deform/

0S29 from:http://wwwsoc.nii.ac.jp/geod-soc/web-text/part3/nawa/nawa-1_files/Fig1.jpg

0S29 :

(4.5 minutes)

... ...

Rem: 0S1= translation

...

Michel Van Camp