BGA 2007 Different aspects of seismic amplitude decay in viscous magma Patrick Smith Supervisor: Jürgen Neuberg School of Earth and Environment, The University

BGA 2007

Different aspects of seismic amplitude decay in viscous magma

Patrick SmithSupervisor: Jürgen Neuberg

School of Earth and Environment,

The University of Leeds.Photo : R Herd, MVO

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Outline of Presentation

• Background: low-frequency seismicity, seismic attenuation in gas-charged magma

• Methodology: Viscoelastic finite-difference model & Coda Q analysis

• Results and Implications: plus some discussion of future work

Low frequency seismicity

High frequency onset

Coda:• harmonic, slowly decaying• low frequencies (0-5 Hz)

→ Are a result of interface waves originating at the boundary between solid

rock and fluid magma

What are low-frequency earthquakes?

Specific to volcanic environments

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Source

Propagation of seismic energyConduit Resonance • Energy travels as interface waves along conduit walls at velocity controlled by magma properties

• Top and bottom of the conduit act as reflectors and secondary sources of seismic waves

• Fundamentally different process from harmonic standing waves in the conduit

Trigger Mechanism = Brittle Failure of MeltBGA 2007

Propagation of seismic energy

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P-wave

S-wave


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Interface waves

P-wave

S-wave


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Interface waves


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Interface waves


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Interface waves


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Interface waves


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reflections


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reflections


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Low frequencies

High frequencies

FAST MODE: I1NORMALDISPERSION

SLOW MODE: I2INVERSEDISPERSION

Low frequencies

High frequencies

Acoustic velocity of fluid


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I1

I2


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I1

I2

S


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S

I1

I2


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S

I1

I2


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‘Secondary source’

I2


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Surface-wave



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Surface-wave


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I1R1


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I1R1


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I2

I1R1


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I2



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Most of energystayswithin the conduit


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R2


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R2

Events are recorded by

seismometers as surface

waves


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Why are low frequency earthquakes important?

• Have preceded most major eruptions in the past

• Correlated with the deformation and tilt - implies a close relationship with pressurisation processes (Green & Neuberg, 2006)

• Provide direct link between surface observations and internal magma processes

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Conduit Properties

seismic signals(surface)

Magma properties(internal)

Seismic parameters

Signal characteristics

Context: combining magma flow modelling & seismicity

Conduit geometry

+Properties of the magma

Attenuation via Q

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Seismic attenuation in magma

Provides information about magma properties

Why is attenuation important?

Definitions:

Apparent (coda) Intrinsic (anelastic)

Radiative (parameter contrast,

geometric spreading)

true damping amplitude decay

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Modelling Intrinsic Q

• To include anelastic ‘intrinsic’ attenuation – the finite-difference code uses a viscoelastic medium: stress depends on both strain and strain rate.

• Parameterize material using Standard Linear Solid (SLS): viscoelastic rheological model

whose mechanical analogue is as shown:

Intrinsic Q is dependent on the properties of the magma:

Viscosity (of melt & magma)Gas content

Diffusivity

Use in finite-difference code to model frequency dependent Q

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Finite-Difference Method

Domain Boundary

Solid medium(elastic)

Fluid magma(viscoelastic

)Variable Q

Damped Zone

Free surface

Seismometers

Source Signal:

1Hz Küpper wavelet

(explosive source)

ρ = 2600 kgm-3

α = 3000 ms-1

β = 1725 ms-1

•2-D O(Δt2,Δx4) scheme based on Jousset, Neuberg & Jolly (2004)

• Volcanic conduit modelled as a viscoelastic fluid-filled body embedded in homogenous elastic medium

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Determining apparent (coda) Q

Coda Q methodology:

• Decays by factor (1 Q) each cycle

Aki & Richards (2003)

Model produces harmonic, monochromatic synthetic signals

0 1 2 3 4

0

Time [number of cycles]A

mpl

itude-A0

A0

A1

A2

A3

Take ratio of successive peaks,

e.g.A1

A2

= Q

Q =A2

A1 – A2

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Calculation of coda QCalculating Q using logarithms

Gradient of the line given by:

Unfiltered data

Hence Q is given by:

0 2 4 6 8 10 12-24

-23.8

-23.6

-23.4

-23.2

-23

-22.8

-22.6

Time [cycles]

log(

Am

plitu

de)

Q value based on envelope maxima

Gradient of line =-0.10496

Q value from gradient = 31.5287

Linear Fit

Data

Results

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Apparent (coda) Intrinsic (anelastic)

An amplitude battle: competing effects

Radiative (parameter contrast,geometric spreading)

High intrinsic attenuation overcome by resonance effect – but need better understanding of how energy of interface waves is trapped

Determines behaviour at high intrinsic Q – shifts the curve vertically

0 10 20 30 40 50 60 70 80 90 1000

10

20

30

40

50

60

70

80

90

100

Intrinsic Q

Ap

pa

ren

t Q

Intrinsic Q vs. Apparent (coda) Q

2 SLS in array

0 10 20 30 40 50 60 70 80 90 1000

10

20

30

40

50

60

70

80

90

100

Intrinsic Q

Ap

pa

ren

t Q

Intrinsic Q vs. Apparent (coda) Q

2 SLS in arrayFor a fixed parameter contrast

Apparent Q greater than intrinsic Q:

Resonance dominates

Apparent Q less than intrinsic Q:Radiative energy loss dominates

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Future Work and developments• Compare attenuation of acoustic waves with interface waves, both intrinsic & radiative – aim to understand the different components of amplitude loss.

• Relate amplitudes at surface to slip at source → ‘magma flow meter’ idea

• Use flow magma models to derive viscosities – examine impact on seismic amplitude decay

• Link observables, e.g. coda decay & frequency content to magma properties such as the viscosity, gas content & pressure

Documents

BGA 2007 Different aspects of seismic amplitude decay in viscous magma Patrick Smith Supervisor: Jürgen Neuberg School of Earth and Environment, The University